Marijuana: Much More Than You Wanted To Know

This month I work on my hospital’s Substance Abuse Team, which means we treat people who have been hospitalized for alcohol or drug-related problems and then gingerly suggest that maybe they should use drugs a little less.

The two doctors leading the team are both very experienced and have kind of seen it all, so it’s interesting to get a perspective on drug issues from people on the front line. In particular, one of my attendings is an Obama-loving long-haired hippie who nevertheless vehemently opposes medical marijuana or any relaxation on marijuana’s status at all. He says that “just because I’m a Democrat doesn’t mean I have to support stupid policies I know are wrong” and he’s able to back up his opinion with an impressive variety of studies.

To be honest, I had kind of forgotten that the Universe was allowed to contain negative consequences for legalizing drugs. What with all the mental energy it took protesting the the Drug War and getting outraged at police brutality and celebrating Colorado’s recently permitting recreational cannabis use and so on, it had completely slipped my mind that the legalization of marijuana might have negative consequences and that I couldn’t reject it out of hand until I had done some research.

So I’ve been doing the research. Not to try to convince my attending of anything – as the old saying goes, do not meddle in the affairs of attendings, because you are crunchy and taste good with ketchup – but just to figure out where exactly things stand.

I. Would Relaxation Of Penalties On Marijuana Increase Marijuana Use?

Starting in the 1970s, several states decriminalized possession of marijuana – that is, possession could not be penalized by jail time. It could still be penalized by fines and other smaller penalties, and manufacture and sale could still be punished by jail time.

Starting in the 1990s, several states legalized medical marijuana. People with medical marijuana cards, which in many cases were laughably easy to get with or without good evidence of disease, were allowed to grow and use marijuana, despite concerns that some of this would end up on the illegal market.

Starting last week, Colorado legalized recreational use of marijuana, as well as cultivation and sale (subject to heavy regulations). Washington will follow later this year, and other states will be placing measures on their ballots to do the same.

One should be able to evaluate to what degree marijuana use rose after these policy changes, and indeed, many people have tried – with greater or lesser levels of statistical sophistication.

The worst arguments in favor of this proposition are those like this CADCA paper, which note that states with more liberal marijuana laws have higher rates of marijuana use among teenagers than states that do not. The proper counterspell to such nonsense is Reverse Causal Arrows – could it not be that states with more marijuana users are more likely to pass proposals liberalizing marijuana laws? Yes it could. Even more likely, some third variable – let’s call it “hippie attitudes” – could be behind both high rates of marijuana use and support for liberal marijuana regimes. The states involved are places like Colorado, California, Washington, and Oregon. I think that speaks for itself. In case it doesn’t, someone went through the statistics and found that these states had the highest rates of marijuana use among teens since well before they relaxed drug-related punishments. Argument successfully debunked.

A slightly more sophisticated version – used by the DEA here – takes the teenage marijuana use in a state one year before legalization of medical marijuana and compares it to the teenage marijuana use in a state one (or several years) after such legalization. They often find that it has increased, and blame the increase on the new laws. For example, 28% of Californians used marijuana before it was decriminalized in the 70s, compared to 35% a few years after. This falls victim to a different confounder – marijuana use has undergone some very large swings nationwide, so the rate of increase in medical marijuana states may be the same as the rate anywhere else. Indeed, this is what was going on in California – its marijuana use actually rose slightly less than the national average.

What we want is a study that compares the average marijuana use in a set of states before liberalization to the average marijuana use in the country as a whole, and then does the same after liberalization to see if the ratio has increased. There are several studies that purport to try this, of which by far the best is Johnston, O’Malley & Bachman 1981, which monitored the effect of the decriminalization campaigns of the 70s. They survey thousand of high school seniors on marijuana use in seven states that decriminalize marijuana both before and for five years after the decriminalization, and find absolutely no sign of increased marijuana use (in fact, there is a negative trend). Several other studies (eg Thies & Register 1993) confirm this finding.

There is only a hint of some different results. Saffer and Chaloukpa 1999 and Chaloupka, Grossman & Tauras 1999 try to use complicated econometric simulations to estimate the way marijuana demand will respond to different variables. They simulate (as opposed to detecting in real evidence) that marijuana decriminalization should raise past-year use by about 5 – 8%, but have no effect on more frequent use (ie a few more people try it but do not become regular users). More impressively, Model 1993 (a source of some exasperation for me earlier) finds that after decriminalization, marijuana-related emergency room visits went up (trying to interpret their tables, I think they went up by a whopping 90%, but I’m not sure of this). This is sufficiently different from every other study that I don’t give it much weight, although we’ll return to it later.

Overall I think the evidence is pretty strong that decriminalization probably led to no increase in marijuana use among teens, and may at most have led to a small single-digit increase.

Proponents of stricter marijuana penalties say the experiment isn’t fair. In practice, decriminalization does not affect the average user very much – even in states without decriminalization, marijuana possession very rarely leads to jail time. The only hard number I have is from Australia, where in “non-decriminalized” Australian states only 0.3% of marijuana arrests lead to jail time, but a quick back-of-the-envelope calculation suggests US numbers are very similar. And even in supposedly decriminalized states, it’s not hard for a cop who wants to get a pot user in jail to find a way (possession of even small amounts can be “possession with intent to sell” if someone doesn’t like you). So the overall real difference between decriminalized and not decriminalized is small and it’s not surprising the results are small as well. I mostly agree with them; decriminalization is fine as far as it goes, but it’s a bigger psychological step than an actual one.

The next major milestone in cannabis history was the legalization of medical marijuana. Anderson, Hansen & Rees (2012) did the same kind of study we have seen above, and despite trying multiple different measures of youth marijuana use found pretty much no evidence that medical marijuana legalization caused it to increase. Other studies find pretty much the same.

This could potentially suffer from the same problems as decriminalization studies – the laws don’t always change the facts on the ground. Indeed, for about ten years after medical marijuana legalization, the federal government kept on prosecuting marijuana users even when their use accorded with state laws, and many states had so few dispensaries that in reality not a whole lot of medical marijuana was being given out. I haven’t found any great studies that purport to overcome these problems.

When we examined decriminalization, we found that the studies based on surveys of teens looked pretty good, but that the one study that examined outcomes – marijuana-related ER visits – was a lot less encouraging. We find the same pattern here, and the rain on our parade is Chu 2013, who finds that medical marijuana laws increased marijuana-related arrests by 15-20% and marijuana-related drug rehab admissions by 10-15%.

So what’s going on here? I have two theories. First, maybe medical marijuana use (and decriminalization) increase use among adults only. This could be because the system is working – giving adults access to medical marijuana while keeping it out of the hands of children – or because kids are dumb and don’t understand consequences but adults are more responsive to incentives and punishments. Second, we know that medical marijuana has twice as much THC as street marijuana. Maybe everyone keeps using the same amount of marijuana, but when medical marijuana inevitably gets diverted to the street, addicts can’t handle it and end up behaving much worse than they expected.

Or the studies are wrong. Studies being wrong is always a pretty good bet.

I can’t close this section without mentioning the Colorado expulsion controversy. Nearly everyone who teaches in Colorado says there has been an explosion of marijuana-related problems since medical marijuana was legalized. Meanwhile, the actual surveys of Colorado high school students say that marijuana use, if anything, is going down. A Colorado drug warrior has some strong objections to the survey results, but they center around not really being able to prove that there is a real downward trend (which is an entirely correct complaint) without denying that in fact they show no evidence at all of going up.

The consensus on medical marijuana seems to be that it does not increase teen marijuana use either, although there is some murky and suggestive evidence that it might increase illicit or dangerous marijuana use among adults.

There is less information on the effects of full legalization of marijuana, which has never been tried before in the United States. To make even wild guesses we will have to look at a few foreign countries plus some econometric simulations.

No one will be surprised to hear that the first foreign country involved is the Netherlands, which was famously permissive of cannabis up until a crackdown a few years ago. Despite popular belief they never fully legalized the drug and they were still pretty harsh on production and manufacture; distribution, on the other hand, could occur semi-openly in coffee shops. This is another case where we have to be careful to distinguish legal regimes from actual effects, but during the period when there were actually a lot of pot-serving coffee shops, the Netherlands did experience an otherwise-inexplicable 35% rise in marijuana consumption relative to the rest of Europe. This is true even among teenagers, and covers both heavy use as well as occasional experimentation. Some scientists studying the Netherlands’ example expect Colorado to see a similar rise; others think it will be even larger because the legalization is complete rather than partial.

The second foreign country involved is Portugal, which was maybe more of a decriminalization than a legalization case but which is forever linked with the idea of lax drug regimes in the minds of most Americans. They decriminalized all drugs (including heroin and cocaine) in 2001, choosing to replace punishment with increased treatment opportunities, and as we all have been told, no one in Portugal ever used drugs ever again, or even remembers that drugs exist. Except it turns out it’s more complicated; for example, the percent of Portuguese who admit to lifetime use of drugs has doubled since the law took effect. Two very patient scientists have sifted through all the conflicting claims and found that in reality, the number of people who briefly experiment with drugs has gone way up, but the number of addicts hasn’t, nor has the number of bad outcomes like overdose-related deaths. There are many more people receiving drug treatment, but that might just be because Portugal upped its drug treatment game in a separate law at the same time they decriminalized drugs. Overall they seem to have been a modest success – neither really raising nor decreasing the number of addicts – but they seem more related to decriminalization (which we’ve already determined doesn’t have much effect) than to legalization per se.

Returning to America, what if you just ask people whether they would use more marijuana if it’s legal? Coloradans were asked if they plan to smoke marijuana once it becomes legal; comparing survey results to current usage numbers suggests 40% more users above the age of 18; it is unclear what the effect will be on younger teens and children.

Finally, we let the economists have their say. They crunch all the data and predict an increase of 50 – 100% based solely on the likely price drop (even with taxes factored in). And if there’s one group we can trust to make infallible predictions about the future, it’s economists.

Overall I find the Dutch evidence most convincing, and predict a 25 – 50% increase in adult marijuana use with legalization. I would expect a lower increase – 15 – 30% – among youth, but the data are also perfectly consistent with no increase at all.

Conclusion for this section: that decriminalization and legalization of medical marijuana do not increase youth marijuana use rates, although there is some shaky and indirect evidence they do increase adult use and bad behavior. There is no good data yet on full legalization, but there’s good reason to think it would substantially increase adult use and it might also increase youth use somewhat.

II. Is Marijuana Bad For You?

About 9% of marijuana users eventually become addicted to the drug, exposing them to various potential side effects.

Marijuana smoke contains a lot of the same chemicals in tobacco smoke and so it would not be at all surprising if it had some of the same ill effects, like cardiovascular disease and lung cancer. But when people look for these effects, they can’t find any increase in mortality among marijuana smokers. I predict that larger studies will one day pick something up, but for now let’s take this at face value.

Much more concerning are the attempts to link marijuana to cognitive and psychiatric side effects. Meier et al (2012) analyzed a study of a thousand people in New Zealand and found that heavy marijuana use was linked to an IQ decline of 8 points. Rogeberg 2012 developed an alternative explanation – poor people saw their IQs drop in their 20s more than rich people because their IQs had been artificially inflated by schooling; what Meier et al had thought to be an effect of cannabis was really an effect of poor people having an apparent IQ drop and using cannabis more often. Meier et al pointed out that actually, poor people didn’t use cannabis any more often than anyone else and effects remained when controlled for class. Other studies, like Fried et al (2002) find the same effect, and there is a plausible biological mechanism (cannabinoids something something neurotransmitters something brain maturation). As far as I can tell the finding still seems legit, and marijuana use does decrease IQ. It is still unclear whether this only applies in teenagers (who are undergoing a “sensitive period of brain development”) or full stop.

More serious still is the link with psychosis. A number of studies have found that marijuana use is heavily correlated with development of schizophrenia and related psychotic disorders later in life. Some of them find relative risks as high as 2 – heavy marijuana use doubles your chance of getting schizophrenia, which is already a moderately high 1%. But of course correlation is not causation, and many people have come up with alternative theories. For example, maybe people who are already kind of psychotic use marijuana to self-medicate, or just make poor life choices like starting drugs. Maybe people of low socioeconomic status who come from broken homes are more likely to both use marijuana and get schizophrenia. Maybe some gene both makes marijuana really pleasant and increases schizophrenia risk.

I know of three good studies attempting to tease out causation. Arseneault et al (2004) checks to see which came first – the marijuana use or the psychotic symptoms – and finds it was the marijuana use, thus supporting an increase in risk from the drug. Griffith-Lendering et al (2012) try the same, and find bidirectional causation – previous marijuana use seems to predict future psychosis, but previous psychosis seems to predict future marijuana use. A very new study from last month boxes clever and checks whether your marijuana use can predict schizophrenia in your relatives, and find that it does – presumably suggesting that genetic tendencies towards schizophrenia cause marijuana use and not vice versa (although Ozy points out to meet that the relatives of marijuana users are more likely to use marijuana themselves; the plot thickens). When a meta-analysis tries to control for all of these factors, they get a relative risk of 1.4 (they call it an odds ratio, but from their discussion section I think they mean relative risk).

Is this true, or just the confounders they failed to pick up? One argument for the latter is that marijuana use has increased very much over the past 50 years. If marijuana use caused schizophrenia, we would expect to see much more schizophrenia, but in fact as far as anyone can tell (which is not very far) schizophrenia incidence is decreasing. The decrease might be due (maybe! if it even exists at all!) to obstetric advances which prevent fetal brain damage which could later lead to the disease. The effect of this variable is insufficiently known to pretend we can tease out some supposed contrary effect of increased marijuana use. Also, some people say that schizophrenia is increasing in young people, so who knows?

The exact nature of the marijuana-psychosis link is still very controversial. Some people say that marijuana causes psychosis. Other people say it “activates latent psychosis”, a term without a very good meaning but which might mean that it pushes people on the borderline of psychosis – eg those with a strong family history but who might otherwise have escaped – over the edge. Still others say all it does is get people who would have developed psychosis eventually to develop it a few years earlier. You can read a comparison of all the different hypotheses here.

I’ve saved the most annoying for last: is marijuana a “gateway drug”? Would legalizing it make it more or less of a “gateway drug”? This claim seems tailor-made to torture statisticians. We know that marijuana users are definitely more likely to use other drugs later – for example, marijuana users are 85x more likely than non-marijuana users to use cocaine. but that could be either because marijuana affects them in some way (implying that legalizing marijuana would increase other drug use), because they have factors like genetics or stressful life situation that makes them more likely to use all drugs (implying that legalizing marijuana would not affect other drug use), or because using illegal marijuana without ill effect connects them to the illegal drug market and convinces them illegal drugs are okay (implying that legalizing marijuana would decrease other drug use). RAND comes very close to investigating this properly by saying that when the Dutch pseudo-legalized marijuana, use of harder drugs stayed stable or went down, but all their study actually shows is that the ratio of marijuana users : hard drug users went down. This is to be expected when you make marijuana much easier to get, but it’s still consistent with the absolute number of hard drug users going way up. The best that can be said is that there is no direct causal evidence for the gateway theory and some good alternative explanations for the effect. Let us accept their word for it and never speak of this matter again.

Conclusion for this section: Marijuana does not have a detectable effect on mortality and there is surprisingly scarce evidence of tobacco-like side effects. It probably does decrease IQ if used early and often, possibly by as many as 8 IQ points. It may increase risk of psychosis by as much as 40%, but it’s not clear who is at risk or whether the risk is even real. The gateway drug hypothesis is too complicated to evaluate effectively but there is no clear casual evidence in its support.

III. What Are The Costs Of The Drug War?

There are not really that many people in jail for using marijuana.

I learned this from Who’s Really In Prison For Marijuana?, a publication of the National Office Of Drug Control Policy, which was clearly written by someone with the same ability to take personal offense at bad statistics that inspires my posts about Facebook. The whole thing seethes with indignation and makes me want to hug the drug czar and tell him everything will be okay.

Only 1.6% of state prisoners are serving time for marijuana, only 0.7% are serving for marijuana possession, and only 0.3% are first time offenders. Some of those are “possession” in the sense of “possessing a warehouse full of marijuana bales”, and others are people who committed much more dangerous crimes but were nailed for marijuana, in the same sense that Al Capone was nailed for tax evasion. The percent of normal law-abiding people who just had a gram or two of marijuana and were thrown in jail is a rounding error, and the stories of such you read in the news are extremely dishonest (read the document for examples).

Federal numbers are even lower; in the entire federal prison system, they could only find 63 people imprisoned with marijuana possession as the sole crime, and those people were possessing a median of one hundred fifteen pounds of marijuana (enough to make over 100,000 joints).

In total, federal + state prison and counting all the kingpins, dealers, manufacturers, et cetera, there are probably about 16,000 people in prison solely for marijuana-related offenses, serving average actual sentence lengths of three year. But it’s anybody’s guess whether those people would be free today if marijuana were legal, or whether their drug cartels would just switch to something else.

Looking at the other side’s statistics, I don’t see much difference. NORML claims that there are 40,000 people in prison for marijuana use, but they admit that half of those people were arrested for using harder drugs and marijuana was a tack-on charge, so they seem to agree with the Feds about around 20,000 pure marijuana prisoners. SAM agrees that only 0.5% of the prison population is in there for marijuana possession alone. I see no reason to doubt any of these numbers.

A much more serious problem is marijuana-related arrests, of which there are 700,000 a year. 90% of them are for simple possession, and the vast majority do not end in prison terms; they do however result in criminal records, community service, a couple days of jail time until a judge is available to hear the case, heavy fines, high cost of legal representation, and moderate costs to the state for funding the whole thing. Fines can be up to $1500, and legal representation can cost up to $5000 (though I am suspicious of this paper and think it may be exaggerating for effect). These costs are often borne by poor people who will have to give up all their savings for years to pay them back.

Costs paid by the government, which cover everything from police officers to trials to prison time, are estimated at about $2 billion by multiple sources. This is only 3% of the total law enforcement budget, so legalizing marijuana wouldn’t create some kind of sudden revolution in policing, but as the saying goes, a billion here, a billion there, and eventually it adds up to real money. And a Harvard economist claims that the total monetary benefits from legalization, including potential tax revenues, could reach $14 billion.

Some people worry that legalizing marijuana would cause an increase in car accidents by “stoned drivers”, who, like drunk drivers, have impaired reflexes and poor judgment, and indeed there is a small but real problem of marijuana-induced car accidents. But Chaloukpa and Laixuthai (1994) crunch the numbers and find that decreased price/increased availability of marijuana is actually associated with decreased car accidents, probably because marijuana is substituting for alcohol in the “have impairing substances and then go driving” population. This finding – that marijuana and alcohol substitute for each other – has been spotted again and again. Anderson & Rees (2013) find that states that legalize medical marijuana see a 5% drop in beer sales. There are however a few dissenting opinions: Cameron & Williams (2001), in complex econometric simulations that may or may not resemble the real world in any respect, find that increasing the price of alcohol increases marijuana use, but increasing the price of marijuana does not affect alcohol use, and the same researcher finds that banning alcohol on a college campus also decreases marijuana use. Also, possibly marijuana use increases smoking? This whole area is confusing, but I am most sympathetic to to the Andersen and Rees statistics which say that medical marijuana states are associated with 13% fewer traffic fatalities.

Overall conclusion for this section: full legalization of marijuana would free about 20,000 people from jail (although most of them would not be exactly fine upstanding citizens), prevent 700,000 arrests not resulting in jail time per year, save between 2 and 14 billion dollars, and possibly reduce traffic fatalities a few percent (or, for all we know, increase them).

IV. An Irresponsible Utilitarian Analysis

Decriminalization and legalization of medical marijuana seem, if we are to trust the statistics in (I) saying they do not increase use among youth, like almost unalloyed good things. Although there are some nagging hints of doubt, they are not especially quantifiable and therefore not amenable to analysis. Without a very strong predisposition to try as hard as possible to fit the evidence into a pessimistic picture, I don’t think there’s a great argument against either of these two propositions. Let’s concentrate on legalization, which would mean something like “People can grow and sell as much marijuana as they want and it’s totally legal for people over 21, with the same level of penalties as today for people under 21”.

Section (I) concludes that legalization could lead to an increase in adult marijuana use up to 50%. There’s not a lot of evidence on what it could do to teen marijuana use, but since it seems teen marijuana use is less responsive to legal changes, I made up a number and said 20%. Lest you think I am being unfair, note that this is well below the percent increase predicted by the survey that asked 18 year olds if they would start using marijuana if it were legal.

Right now about 1.5 million teenagers use marijuana “heavily”. Most of the detrimental effects of marijuana seem concentrated in teens and people in their early twenties; I’m going to artificially round that up to 2 million to catch the early 20 year olds. If this 2 million number increased 20%, 400,000 extra teens would start heavily using marijuana.

Those 400,000 teens would lose 8 IQ points each. IQ increases your yearly earnings by about $500 per point, so these people would lose about $4,000 a year. Making very strong assumptions about salary being a measure of value to society, society would lose about $1.6 billion a year directly, plus various intangibles from potential artists and scientists losing the ability to create masterpieces and inventions, plus various really intangibles like a slightly dumber electorate.

We need to use a different number to calculate psychosis risk, since the studies were done on “people who had used marijuana at least once”. The appropriate number turns out to be 8 million teenagers; of those, 1%, or 80,000, would naturally develop schizophrenia. If the 1.4 relative risk number is correct, marijuana use will increase that to 112,000, for a total increase of 32,000 people. Schizophrenia pretty much always presents in the 15 – 25 age window, so we’ll say we get 3,200 extra cases per year.

There were 35000 road traffic accident fatalities in the US last year. If greater availability of marijuana decreases those fatalities by 13% (note that I am using the number from medical marijuana legalization and not for marijuana legalization per se, solely because it is a number I actually have), that will cause 4500 fewer road traffic deaths per year. There may be additional positive effects of alcohol substitution from, for example, less liver disease. But there may also be additional negative effects from increasing use of tobacco, so let’s just pretend those cancel out.

So here is my guess at the yearly results of marijuana legalization:

– 20,000 fewer prisoners (but they might switch to other criminal enterprises)
– 700,000 fewer arrests
– $2 billion less in law enforcement costs
– Some amount of positive gain (let’s say $5 billion) in taxes
– 4500 fewer road traffic deaths (if you believe the preliminary alcohol substitution numbers)

– 400,000 people with lower IQ
– $2 billion in social costs from above dumber people
– 3,200 more cases of schizophrenia a year

We’ll proceed to calculate the nonmonetary burden of each of these in QALYs, then add the monetary burden in dollars, then convert.

The searchable public database of utility weights for all diseases (God I love the 21st century) tells me that schizophrenia has a QALY weight of 0.73. It generally starts around 20 and lasts a lifetime, so each case of schizophrenia costs us 0.27 * 50 or 13.5 QALYs. Therefore, the total burden of the 3,200 added schizophrenia cases is 43 kiloQALYs.

There’s no good way to calculate the QALY weight of having 4-8 fewer IQ points, and unfortunately this is going to end up being among the most important numbers in our results. If we say the lifetime cost of this problem is 3 QALYs, and divide the number by eight to represent eight years worth of teenagers in our sample population, we end up with 400,000/8 * 3 = 150 kiloQALYs.

My own survey tells me that being in prison has a QALY weight around 0.5. Marijuana sentences generally last an average of three years, which suggests that 1/3 of these marijuana prisoners are arrested every year, so the total burden of the ~6000ish marijuana imprisonments each year is 3 * ~6000 * 0.5 = 10 kiloQALYs.

Assume the average road traffic death occurs at age 30, costing 40 years of potential future life. The total cost of 4500 road traffic deaths is 40 * 4500 = 180 kiloQALYs.

The arrests are going to require even more fudging than normal. Average jail time for a marijuana arrest (when awaiting trial) is “one to five days” – let’s round that off to two and then use our prison number to say that the jail from each arrest is 2/365 * 0.5 = three-thousandths of a QALY. I am going to arbitrarily round this up to one one-hundredth of a QALY to account for emotional trauma and the burden of fines, then even more arbitrarily round this up to a tenth of a QALY to account for possibility of getting a criminal record. This sets the burden of 700,000 arrests at 70 kiloQALYs.

Now our accounting is:

Costs from legalization compared to current system: 200 kQALYs and $2 billion
Benefits from legalization compared to current system: 260 kQALYs and $7 billion

Although it’s not going to be necessary, we can interconvert QALYs and dollars at the going health-care rate of about $100,000/QALY ($100 million/kQALY):

Costs from legalization compared to current system: 220 kQALYs
Benefits from legalization compared to current system: 330 kQALYs

And get:

Net benefits from legalization: +110 kQALYs

Except that this is extremely speculative and irresponsible. By far the largest component of the benefits of legalization turned out to be the effect on road traffic accidents, which is based on only two studies and which may on further research turn out to be a cost. And by far the largest component of the costs of legalization turned out to be the effect on IQ, and we had to totally-wild-guess the QALY cost of an IQ point loss. The wiggle room in my ignorance and assumptions is more than large enough to cover the small gap between the two policies in the results.

So my actual conclusion is:

There is not a sufficiently obvious order-of-magnitude difference between the costs and benefits of marijuana legalization for a evidence-based utilitarian analysis of costs and benefits to inform the debate. You may return to your regularly scheduled wild speculation and shrill accusations.

But I wouldn’t say this exercise is useless. For example, it suggests that whether marijuana legalization is positive or negative on net depends almost entirely on small changes in the road traffic accident rate. This is something I’ve never heard anyone else mention, but which in retrospect should be obvious; the few debatable health effects and the couple of people given short jail sentences absolutely can’t compare to the potential for thousands more (or fewer) traffic accidents which leave people permanently dead.

So my actual actual conclusion is:

We should probably stop caring about health effects of marijuana and about imprisonment for marijuana-related offenses, and concentrate all of our research and political energy on how marijuana affects driving.

This cements my previous intuitions on irresponsible use of statistics – it’s unlikely to unilaterally solve the problem, but it can be very good at pointing out where you’re being irrational and suggesting new ways of looking at a question.

EDIT: People in the comments have pointed out several important factors left out, including:
– Some people enjoy smoking marijuana
– The opening of a permanent criminal record may mean arrests are worse than I estimate. I can’t find good statistics on how often this happens, but do note that decriminalization prevents a record from being opened.
– Loss of 8 IQ points may have wider social effects than I estimate, since IQ affects for example crime rate.
– Legalizing marijuana might remove a source of funding for organized crime

119 Responses to Marijuana: Much More Than You Wanted To Know

  1. David Gerard says:

    The tricky one would be comparing the costs of the drug war. Compare alcohol, which is a hard drug and hugely harmful, but so stupidly easy to make that banning it is handing buckets of free money to organised crime, blindness and brain damage from methanol in badly-distilled spirits, etc.

    I do concur that busting people’s asses for driving while stoned is a very important social bright line, of course.

  2. …700,000 ultimately inconsequential but extremely annoying arrests per year…

    I’m going to go out on a limb here and guess that getting arrested isn’t trivial. This isn’t something I have first-hand experience on (horray for being white). But I’m guessing the lifetime cost of having to answer “yes” on the “have you ever been arrested?” question on job applications is more than 0.1 QALYS.

    (For more on what the actual costs of getting arrested are, as well as how hard you have to try to get arrested as a white professional, I recommend this piece.)

    • Scott Alexander says:

      The racial arrest disparity is a big deal (and there’s some interesting research on it here and here), but let’s not exaggerate. Black people are proportionally more likely to be arrested (by about four times), but 460,808 white people were still arrested for possession of marijuana each year – and they still make up well above half of the total.

      I think many marijuana arrests don’t get added to the criminal record, and many others are by people who already have a criminal record. Nevertheless, I’d like to hear how many QALYs you think an arrest is worth (keep in mind 0.1 QALYs is the same as losing a month of your life).

      • David Gerard says:

        “and they still make up well above half of the total”

        … and are 90% of the population.


        That said, I wouldn’t be surprised if the negative effect of going from “no criminal record” to “criminal record” were 1 QALY or more. Say if someone first gains a criminal record at 25 and it has a lasting negative impact for the length of their career until age 65, it only needs to have a yearly impact of 0.025.

      • I should mention that I’m partly going on my overall impressions of the long-term effects a drug arrest/conviction can have on a person’s life. I suspect a drug conviction when you’re still in school can be especially bad, e.g. it disqualifies you from getting student aid in college.

      • Andy says:


        I’m wondering how many of these arrests are Hispanics who get shoved/pick the “White” box even though most outside observers would not call them white, and how many are “Non-Hispanic Whites” aka pasty-cheeked Anglos.
        The split multiple definitions of “Hispanic” are a huge headache for me when using US Census data in the last couple years.

        • Sarah says:

          By the census rules, “Hispanics” can be of any race. This is why white + black + Hispanic adds up to more than 100%. About 54% of American Hispanics also call themselves white. 2.5% call themselves black, 40% call themselves “some other race.” Hispanics are 16.7% of the US population, which means that white non-Hispanics are around 60-something percent of the population.

        • Julia says:

          I’m not sure where the records are from – at the jail where I work, I think inmates’ “race” for the purpose of records is usually picked by officers. So it does match up to what outside observers (or at least whichever officer checked the box) perceive.

    • Eric Rall says:

      In the US at least, the question should be “Have you ever been convicted of a felony?”

      With very few exceptions, it’s illegal under US employment discrimination law to ask a job applicant about arrests that did not lead to a conviction or about convictions below felony level. I’d expect most marijuana convictions to be infractions or misdemenors.


    Could it be that when marijuana is legal more people are happy to admit that their medical emergency is marijuana-related? Or, indeed, to go to hospital?

  4. Ivan says:

    The 8 IQ points statistic is very interesting. It’s new to me, but if it is true, it easily seems one of the more important parts in the equation.

    What would be really interesting to know is that same figure for alcohol. How much does drinking alcohol (at a young age) stunt your IQ? As much? More? Less? Each way it could turn out opens a new perspective to the question.

    • David Gerard says:

      I am much less convinced of the value of IQ scores (even as someone of stupendously high IQ and non-stupendous real-world achievements as I am) given the recent data on motivation as a factor in IQ scores. (Anyone questioning this will have seen the studies I mean.)

  5. Jai says:

    You skipped hedonistic effects! It seems wrong to do a utilitarian analysis of marijuana legalization and skip over “some people like getting high”.

    Maybe take a stab in the dark for QALY-adjustment-for-marijuana-being-difficult-and-risky-to-acquire? For a lower bound, guess the number of people who would give up one year of their life to decriminalize marijuana.

    • David Gerard says:

      How would you account for not “would” (which sounds like self-reporting), but observably “do”? (Per my alcohol example above. Which is where e.g. David Nutt on alcohol – where everything he says is clearly correct, versus the consideration that people really like alcohol and it’s stupidly easy to make. I know how easy it is to make because my house is basically a mead brewery.)

      • Jai says:

        True, it’s a tricky question – but that’s not a good reason to set the hedonistic effects to 0.

        • David Gerard says:

          Oh, I wouldn’t set hedonistic effects to zero – but then, I’m highly sceptical of utilitarianism as being a useful abstraction of human considerations of value in general.

    • Sarah says:

      how much people like a thing is usually related to how much they spend on it, nyet? also, good point!

  6. Macbi says:

    You’ve missed out the main benefit of legalisation: that people who want to use marijuana do so, which fulfils their preferences. Surely that’s the zeroth order purpose of legalisation.

    • Macbi says:

      (The comment above makes the same point. They must have replied whist I was reading the post.)

  7. Anonymous says:

    And I don’t think you said it explicitly, but the QALY weight from enjoyment of all-else-equal marijuana looks like rounding error, even in the limit of several million new users.

    Any thoughts on actual medical value, as compared with synthetic cannabinoids that followed more traditional routes for testing, approval, manufacturing, and regulation? My impression from talking to advocates of medical marijuana was that it was basically a play for increased acceptance of general use.

    • Jai says:

      This clashes with my intuition.

      For horrible-but-existent data on this, I googled marijuana

      Most of the questions are tradeoffs for marijuana legalization, and assuming that the answers bear even a slight semblance to reality suggest non-negligible QALYs.

      • Anonymous says:

        Wow, really? Smoking a couple nights a week (roughly corresponding to drinking habits) gives more than a percent increase in quality of life in less than a percent of a year? Do you expect the average person to find it more than twice as pleasant as normal experience, or that it would have positive flow-through effects on the rest of their life?

        • Jai says:

          I expect the median person to have a negligible difference in utility. I expect a small number of people to provide most of the utility here.

          The net gain in the OP napkin calculation was ~100k QALYs. If 0.1% of a population of 300 million is willing to trade 1 year of their lives for legalization, that’s 300k QALYs, dwarfing the original diff. I predict with high confidence that more than 1 out of 1000 people is willing to make this trade, and that this is a conservative estimate.

        • Jake says:

          OP’s numbers are annualized, while yours aren’t. The number of people willing to give up one year of life for one year of access to marijuana is much, much less than 0.1%.

        • Jai says:

          Jake: …you’re completely right. Not sure how I missed that.

          I do still want to get some estimate of hedonistic value, though.

        • Jai says:

          To further the collection of horrible-but-existent data, I’ve submitted a tradeoff to

    • houseboatonstyx says:

      What about the availability of the synthetic cannabinoids (prescription only?) vs the expected availability of legally backyard grown or casually purchased marijuana?

  8. Tarn Somervell says:

    Typos –
    expulsions controversy -> explosion controversy

    “although Ozy points out to meet that the relatives of marijuana users”
    Ozy points out to meet -> points out to me

  9. Alexander Stanislaw says:

    I’m surprised by how small the effect of legalization on usage is. However, I would have guessed that the main result of legalization would be that much of the stigma attached to marijuana use would gradually disappear over the course of a decade or two. Same deal for marijuana as a gateway drug: I would expect that a culture more tolerant of drug use would develop over the course of a decade or more. Of course this is speculative and hard to test, but long term social effects are important.

  10. Craig Gidney says:

    As cars become increasingly automated the effect of legalization on car accidents should drop. So your analysis suggests we expect legalizing to be a net benefit now, but become a net cost in ten-twenty years?

  11. I think the reason medical marijuana legalization doesn’t increase young people’s use is that major causes for medical marijuana seem to be cancer and severe chronic pain from injuries, and both are more common later in life.

    • anodognosic says:

      True, but statistics for medical marijuana users tend to skew much younger than you would suspect if you naively thought everyone who gets a script for medical marijuana is actually getting it for medical reasons. It’s hard to prove how much actual abuse of the system there is, but anecdotal evidence suggests that it’s rampant.

  12. ephion says:

    “plus various intangibles from potential artists and scientists losing the ability to create masterpieces and inventions, ”

    Given how many artists and scientists use marijuana and find it to be immensely helpful, I’m inclined to think that this wouldn’t be a negative.

    • anodognosic says:

      Seconded, adding that my own use of (legal) hallucinogens has always led to insight, artistic and not, that have held up well in sobriety. Any utilitarian analysis of drug use cannot leave out a consideration of the positive good that comes out of drug use.

  13. Paul Goodman says:

    I would very much recommend what Mark Kleiman and others have written on the subject (mostly at One major point he has made, which this touches on, is that the extent to which marijuana substitutes for or complements alcohol overwhelms pretty much all other effects. If weed decreases drinking significantly, than even if you make very pessimistic assumptions on every other axis legalization is still probably a very good idea, and vice versa.

    Secondly, something that isn’t really mentioned here: legalization is very different than decrim or medical because it allows legal advertising. As can be observed in the alcohol market, a large majority of sales comes from the small minority of consumers who abuse it, meaning that advertisers have a huge incentive to encourage overuse. For this reason Kleiman favors legalizing use and production by individuals and nonprofit coops, but not commercial sale.

    • Douglas Knight says:

      Legalization does not entail advertising. For example, tobacco advertising is illegal in Canada.

      • Fnord says:

        I’m not an expert, but might full legalization with advertising bans run into First Amendment problems in the US?

        • Andy says:

          Seriously doubt it. Tobacco advertising on billboards, TV etc, is pretty much illegal here in California, and I imagine the regulation is protected by the same rationales as not promoting cigarettes at children/teenagers.

        • Douglas Knight says:

          The ban on tobacco billboards might not be a law, but a voluntary restriction, agreed to in return for resolving the lawsuit over lying about cancer.

        • John H says:

          ‘Protecting’ children from the supposed evils of drug use is considered a state interest that overrides constitutional rights. The precedents have already been set; no *new* issues with cannabis re: advertising bans.

      • Paul Goodman says:

        An advertizing ban would make things better, but regardless of the specific mechanisms involved, full legalization will put a large amount of wealth and therefor influence in the hands of agents with a strong incentive to increase abuse. It seems very optimistic to assume this won’t have any consequences.

    • Dan says:

      I strongly second the recommendation to read Mark Kleiman. See his Wikipedia page, for starters.

      I was going to mention Kleiman’s thoughts on the two points that Paul Goodman highlighted, the relationship between marijuana & alcohol (complements or substitutes?) and the major question of how “legal” marijuana is regulated (will the marijuana industry be allowed to look like the beer industry?). I don’t have much to add on those topics, besides this link to a blog post by Kleiman.

      I’ll also add a brief bio: Kleiman is a professor of public policy at UCLA (and blogger at who has written a lot of sensible & well-informed things about drug policy & criminal justice policy. Last year he became the top marijuana policy advisor to the Washington state government.

    • Gunlord says:

      Respectfully, I would ponder how much difference the legalization of advertising would have on the use of pot. Look at all the TV shows and videogames which portray pot in a neutral or occasionally positive light (as contrasted to “harder” drugs like cocaine or bath salts, which are almost invariably portrayed as turning people into zombies). Heck, we even have entire genres like “stoner comedies.” Even if advertising for pot is legal, I’m not sure it would make that much of a difference, compared to the work our popular culture is already doing. ;o

  14. a person says:

    My guess is that the reason marajuana doesn’t cause cancer to the same extent as tobacco is that marajuana addicts generally inhale a lot less smoke than nicotine addicts. Two packs a day (40 cigarettes) is a pretty reasonable number for a heavy smoker, but I doubt even Wiz Khalifa regularly smokes 40 joints a day.

  15. Pingback: The whole sky

  16. Dave says:

    I hesitate to suggest that any analysis this long is missing something but it seems to me that there if a key element that you didn’t discuss. Crime. This whole thing reads to me like a discussion of prohibition that somehow fails to mention organized crime.

    The whole idea behind prohibition was that alcohol has all these bad effects and if you named they would go away. And you can argue about how much alcohol related problems increased or decreased during and after prohibition, but there was a huge secondary crime problem that seems crazy to ignore. Same here. There’s a lot out drug related gang violence.

    Now I have no idea how much is going to change with just marijuana legalization, but the effect seems like it could potentially be large. Especially if other drugs are displaced.

    • Sarah says:

      Agreed. Even if marijuana legalization’s effect on violent crime is uncertain and equivocal, it should be a major consideration outweighing others for the same reason that traffic accidents are. [Accidents are more common than homicides but, as I recall, within an order of magnitude.]

    • Sarah says:

      While we’re here, a mini-rant about medical marijuana:

      Cannabis is a plant. It contains over 80 neurologically active chemicals. The composition varies a lot based on varietal. Because it’s a plant.

      The chemicals that have painkilling properties are not the ones that make you high. THC is not a very good painkiller. If we took pain treatment seriously, we’d be isolating the chemicals that are the most effective at reducing pain, developing novel synthetic cannabinoid painkillers, or at least breeding varietals of cannabis for stronger pain relief (and weaker psychoactive properties, because if you have chronic pain you don’t necessarily want to be high all the time.) Chronic pain is one of the most common disabilities in the US, and a lot of it doesn’t respond to treatment at all. I’m pretty sure the utilitarian analysis works out dramatically in favor of researching novel chronic pain treatments.

      I have no idea if Charlotte’s Web ( is actually effective in treating epilepsy, as its producers claim; but it is a strain of marijuana specifically bred to have a therapeutic effect without making people high. That kind of experimentation is in its infancy. It could lead to drugs (cannabis-extracted or synthetic) which are safer, more effective, and more reliable than any medical marijuana is now.

      More broadly. The chronically ill often have to find kludges that work for them, by trial and error and informal networks of advice. There ought to be a tighter feedback loop between medical science and what people actually do to get relief. The cannabidiol thing is folk knowledge. When science is working right, folk knowledge will eventually get tested by experiments and either confirmed or disconfirmed. But sometimes I suspect that isn’t happening.

      • Doug S. says:

        I’ve read things that suggest some of the compounds in cannabis may be particularly effective at treating types of pain that opioid painkillers are bad at… I think my source is a friends-only LiveJournal post, though.

  17. Platypus says:

    In the introduction, you wrote: He says that “just because I’m a Democrat doesn’t mean I have to support stupid policies I know are wrong” and he’s able to back up his opinion with an impressive variety of studies.

    What studies does this other guy cite?

  18. Kaj Sotala says:

    I was previously in favor of full legalization. If I were to take these results at face value, then I would switch to only supporting decriminalization: in the long run, robotic cars are going to eliminate traffic fatalities anyway, but a drop of 8 IQ points is really quite bad.

    However, this analysis doesn’t seem to properly account for all the effects from marijuana substituting for alcohol usage. Reduced traffic fatalities is only part of it. A drop of 8 IQ points is really bad, but heavy alcohol use is associated with all kinds of bad outcomes as well, and it’s not clear that they’re much better. Intuitively, it seems like an alcoholic is much more likely to do active harm to the people around them than a marijuana addict is. Since the overall net effect of full legalization remains unclear, the heuristic that things should only be banned if their harms can be clearly demonstrated leads me to continue supporting full legalization, though with somewhat more reservations than before.

  19. Konkvistador says:


    The proper response would be to stop caring about marijuana in general and just concentrate on reducing driving accidents. I hope you meant to imply this?

    • Multiheaded says:

      It’s all a long running joke on the “Utilitarianism, eh?” theme, IMO.

    • Scott Alexander says:

      In the long run we should stop caring about everything but x-risk and maybe malaria. But given that marijuana is one of the Ten Things Everyone Must Have Tedious Debates About While Ignoring Everything Else, those debates should probably focus on the driving accident rate.

  20. BenSix says:

    Thank you, this was useful.

    I agree that the hedonic qualities of the drug are the most significant factor behind many people’s advocacy of legalisation, and, indeed, suspect that disgust towards such mindless bliss motivates its opponents. For at least some of them, it is not victims of psychosis that they bear in mind but the hippies that irritated them in decades past.

    In fairness, I do find that marijuana, like alcohol, makes people more tedious. Am I being unfair or was Hendrix an exception?

    • a person says:

      It kills your willpower and makes you lazy, but it also lets you interpret the world in a different light and (therefore) makes you more creative and a more original thinker. It’s a double edged sword.

  21. Cyan says:

    “odds ratio, but from their discussion section I think they mean relative risk”

    If the base rate is low enough, the odds ratio is approximately equal to the relative risk.

  22. houseboatonstyx says:

    Here’s a positive utility effect worth checking out. Our anecdotal neighbor, a disabled veteran, got a license for medical marijuana. This enabled him to cut out seventy of the pain pills he had been taking per day. (At least the pain pills he ordered from the VA pharmacy.)

    • Kaminiwa says:

      Wait, 70 pain pills PER DAY? How do you even swallow that many pills? o.o

      • houseboatonstyx says:

        Seven per hour for ten hours. Okay, I just checked with the person who managed his prescriptions. It was 70 total removed, but only about 25 of those removed were pain pills.

  23. Some aspects missing from your analysis (HT: people in my FB thread):

    1.) Presumably using marijuana is fun, at least in the short-term. That’s utility.
    2.) There’s some general inherent disutility in restricting freedom to things.
    3.) Legalizing takes away money from drug gangs.

  24. Katie Hartman says:

    My understanding is that it’s pretty well-accepted that marijuana (and some other recreational drugs) can trigger both the onset and individual episodes of depersonalization/derealization disorders, which are at least as prevalent as schizophrenia and possibly considerably more so. Not that this factors into the analysis very heavily, but it seems to be a risk many fewer people are aware of compared to schizophrenia.

  25. Sarah says:

    I’m a private citizen, not a policy wonk, so the question I really want to understand better is “is pot bad for you?” Should I, and the people I care about, be avoiding cannabis?

    For comparison, I did a bunch of my own legwork on “is social drinking bad for you?” and came to the conclusion that it probably is. Most studies of negative effects of alcohol focus on alcoholics, but there are enough that point to increased inflammation, poorer sleep, and higher risk of cardiovascular disease and diabetes among “heavy social drinkers” (7-14 drinks a week) and most of the claims that light drinking is good for you don’t account for the fact that teetotalers may be an unhealthy population (recovering alcoholics and sick people). I now try to keep my social drinking down — a few drinks a month, not a few drinks a week.

    To understand whether cannabis is actually a cause of psychosis, I’d like to have somebody do experiments, not just correlational studies. Isn’t there anybody looking at brains (even rat brains) before and after administering cannabis?

    Also, you’re a psychiatrist. Is there, or isn’t there, such a thing as a pre-psychosis phenomenon? Are people who grow up to be schizophrenic abnormal in any way as children and adolescents?

    • Anonymous says:

      there are enough [studies] that point to…higher risk of cardiovascular disease…among “heavy social drinkers” (7-14 drinks a week)

      I think you are misremembering the studies.

    • Julia says:

      Prodromal schizophrenia has symptoms like “preoccupation with new ideas often of an unusual nature, anxiety, social isolation, difficulty making choices, and problems with concentration and attention.” Try differentiating that from usual teenaged behavior.

  26. gwern says:


    Extract an income figure from a good-looking study in and convert to QALY at the previous rate you used.

    (Complications: it’s hard to estimate the positive externalities from higher IQ (hence, opportunity cost of lowered IQ), and a lot of the correlated gain seems to be zero-sum/positional, which are redistributional and largely irrelevant to this discussion.)

    I’m pretty skeptical it’s a real effect (correlations usually don’t turn out to be causal, or turn out to be overestimates since it’s the causal effect + all other systematic biase, selection effects, etc), but you can get a better estimate of the possible damage if you want.

  27. katlin says:

    This is one of the most dishonest blog posts I’ve seen posted here in quite a while.

    Where to begin? It dishonestly attempted to claim that the Netherlands saw a huge increase in use (post-liberalization) according to all other data on the topic they have lower use rates than surrounding countries, and the author’s source was non-existent. So let’s assume that was an outright lie.

    But the most laughable mistake came when the author actually used a study that found cannabinoids increase IQ in light users in attempt to say that cannabis use lowers IQ. The simple logic the author fails to grasp is that acute sedation of any type will lower IQ temporarily, but cannabinoids are neuroprotective and have been shown to grow new neurons. Sorry guy, weed is good for the brain.

    • Faul_Sname says:


          You mean Meier et al (2012)? With the title of Persistent cannabis users show neuropsychological decline from childhood to midlife?

          I can see where you’d think that study showed that cannabis raises IQ in light users, and I can probably point to the exact table you looked at (here). That table shows average IQ of non-users at 100.64, and light users at 101.25 at age 38. What you might not have noticed is that this study also included IQ measurements at ages 7 – 13, and the change from age 7-13 IQ and age 38 IQ was slightly positive (basically zero) for nonusers and slightly negative (basically zero) for light users. Which is to say that light cannabis use doesn’t have much effect one way or another. On the other hand, the study also showed that heavy use (defined as at least 4 days per week) does have a significant negative effect.

          So no, he did report the results of that paper accurately. And lets not assume that someone is maliciously lying if you don’t find the source they used. It was a formatting error — here is the unbroken link.

      • katlin says:

        You mean Meier et al (2012)?


        Fried et al (2002)

        Oh and thanks for helping source that broken link. But back to the author’s point about it, the increase he is talking about “post-liberalization” in the Netherlands was only 3 percentage points between 1984 and 1992. The Dutch started tolerating coffeeshops in 1975. Heck, Cheech and Chong filmed Still Smokin’ there before they started measuring this very insignificant change. My question to the author: Why bother citing it when this is nearly useless data?

        This is a troll blog at best, or this guy just believes that if he throws in enough blue links people will believe whatever he writes.


        Too bad the 2002 study found the opposite in light users eh?

        • Keratin says:

          You’re not allowed to act smug about your criticism being misinterpreted when you didn’t cite which study you were referring to in the first place.

          If you’d read the study more carefully, you would have noticed that the mild gains, when analyzed, were found to not be statistically significant. And if you’d even bothered to eyeball the sample sizes of the 2002 and 2012 study, you would have realized that the latter had ten times the sample size and likely used better statistical techniques, meaning that its results deserve much more weight in the first place.

          Personally, this blog post seems to be one of the most honest examples of research and belief-formation I’ve seen in a while. The author encounters an experienced countervailing viewpoint, goes and does a quick casing of the literature on marijuana use, and studiously records the whole thing in notes while clearly demarcating any points where the overall picture is unclear or he’s confused. Eventually, they come up with an inconclusive result, in a case that’s obviously not just the result of motivated cognition. Hell, if everyone was one-fifth as conscientious as Scott when it comes to this, the world would already be much better off.

  28. Calling Bullsnot says:

    I hate this one. We have evidence that it does adversly effect things like IQ, but very little.

    It’s more deserving a warning label like “May cause drowsiness” than the type of prohibition we have experienced. Because the study they are referring to showed as much as an 8 point drop in IQ. In my opinion that is very insignificant. Scientifically the Standard Deviation is 15 points, so even science does not consider 8 points a big deal.

    I get it though, it’s a play on peoples’ lack of scientific understanding, hoping they see a drop of IQ and have a knee-jerk reaction. In reality, they look very silly using an 8 point drop as a reason to prohibit Cannabis, if one realizes 8 points is a statistical drop in the ocean. Besides i’m willing to bet that a normal non-using adult’s IQ would fluctuate by twice that amount throughout a 24 hour period if you administered several randomized tests throughout the day.

    I smoke daily, multiple times a day, for 18 years. I am the Senior Network Engineer at my job. CCNA, CCNP, CCIE, VCP, and others… I don’t think it has caused a significant decline in cognitive abilities, if anything I have experience an increase in awareness and attention to detail which lend to improved cognition but that is purely anecdotal and subjective. As usual, I call bullshit on these sorts of exaggerated analysis, just that no one but the choir are listening.

    • gwern says:


      Bullsnot! Do you even know what you’re talking about? Half a standard deviation is huge. Most effects aspire to hit one day hit as high as a d of 0.5! For IQ, that’s nontrivial to say the least. For comparison, Ashkenazi Jews vs whites is a similar size, as is iodized vs iodine-deficient. And look at the cites I provide about IQ, and estimate the net lifetime total of half a standard deviation’s difference (hint: the financial impact should closer to six figures than zero figures).

      • Anonymous says:

        This is an exaggerated analysis. Inferring that I “don’t know what i’m talking about” does not make you correct. If you wish to contest the accepted standard deviation for IQ scoring, then you should publish a paper on it.

        While we are analyzing substances, there is another i’d like to talk to you about. It’s called hydroxyl acid and;

        contributes to the “greenhouse effect”.
        may cause severe burns.
        is fatal if inhaled.
        contributes to the erosion of our natural landscape.
        accelerates corrosion and rusting of many metals.
        may cause electrical failures and decreased effectiveness of automobile brakes.
        has been found in excised tumors of terminal cancer patients.

      • Anonymous says:

        Do you even understand science or are you pretending? Standard Deviation means; “In statistics and probability theory, the standard deviation shows how much variation or dispersion from the average exists” So with that now understood, if there is a standard deviation of 15 points from the average score under normal circumstances, then 8 points for a heavy cannabis user is indeed insignificant compared to the standard deviation. Anonymous is correct, you are giving an exaggerated analysis.

        • Randy M says:

          This doesn’t sound right, I think you are confusing a variation within a population with a variation in a single individual repeating the test.

        • Anonymous says:

          Randy M, i’m referring the accepted deviation in a populace of 15. So if 15 is typical in a population, why is 8 significant?

        • Anonymous says:

          Randy M don’t you people know how to use Google? Google “Standard Deviation”. Am I the only one here who understands this concept?

        • Randy M says:

          Well, you google what you don’t know, not what you know that isn’t so.

        • gwern says:


          That has nothing to do with whether _d_=0.5 is a large effect size! (And 15=1sd is a convention, so there’s nothing to ‘publish a paper on’ as you so condescendingly put it.)


          That has even less to do with anything.


          Your definition is wrong (it’s the square root of the variance, which only means what that summary does under specific circumstances/distributions), and you still are wrong: 15 points is normed as the entire population’s sd, and shifting someone up an entire sd corresponds to a huge within-population shift. Remember the 68–95–99.7 rule – just a standard deviation shifts you a huge amount within the population, and half a standard (8/15 points = 0.53) is likewise nothing to sneeze at. Look at the references I already supplied about the lifetime effect of just 1 IQ point (d=0.06). Look at Cohen’s classification of effect sizes in psychology, or if you want empirical proof that d=0.5 is an unusually large effect for psychology, check out “One Hundred Years of Social Psychology Quantitatively Described”,%202003.pdf , Bond et al 2003 while keeping in mind that effects which get replicated enough to meta-analyze will tend to be large or important or both.


          If it’s typical for people to differ in wealth by hundreds of thousands of dollars, why is blowing $50,000 on a Vegas spree significant? Your use of the term ‘significant’, by the way, suggests you are confusing ‘statistical significance’ with effect size.

          Am I the only one here who understands this concept?

          I’m not sure you understand it at all.

        • Andrew says:

          Er, a standard deviation is indeed a measure of variance from the average. That said, a standard deviation is a pretty huge number in absolute terms and is in no way connected to what is statistically significant.

          For example, the standard deviation of life expectancy from birth in developed countries is between 15 and 20 years depending on which country you’re looking at (p22). I’d kill a truckload of kittens for half a standard deviation more years of life, and I’d do the same for half a standard deviation of IQ.

    • Faul_Sname says:


      I’ll take that bet. How much are you willing to bet?

      • Matt S Trout says:

        Presumably that depends on what time of day you ask.

        • Doug S. says:

          Well, it’s not hard to imagine that sleep deprivation could cause a temporary and significant drop in IQ test scores…

      • Anonymous says:

        Are you telling me that you would score exactly the same on two seperate IQ testings, one given to you in the morning and a different one given to you at night? You would score EXACTLY the same score?

        • Randy M says:

          I think he’s saying his scores would have less than 16 point spread. Or possibily 30, depending on how one parses the parent comment. Make sure you nail that down before the bet is official.

  29. Scott Y says:

    So, I actually did some research on marijuana use and IQ a few years ago using rats as models. At the time, there was extremely inconclusive and predominantly anecdotal evidence in relation to humans (which I frankly don’t think has changed). Extremely heavy use was required to cause IQ damage in rats, and the rats had to be adolescent. There was some IQ lose associated with use for adults during and shortly after, but that disappeared within a reasonable amount of time. So in relation to IQ lose in teens, the question is what constitutes heavy use and do all studies use the same metric?

  30. gattsuru says:


    I’d honestly consider that a low-end estimate for “up to” — twenty hours of billable time at 250 USD/hour is not the sort of math that’d make a well-known lawyer spit-take. It’s still misleading, though, since the vast majority of minor marijuana charges go to plea bargains or involve a public defender (which costs the state money or public defenders money, but at a different rate). The maximum is seldom helpful.

    – Legalizing marijuana might remove a source of funding for organized crime

    Er, yeah. This one’s kinda important. The situation in just Mexico alone (~10,000 deaths per year, plus all the related costs of a society with 10,000 drug murders per year) would probably swamp every single other utilitarian metric you have on hand. I’d expect a major substitution effect, either toward harder drugs or toward more ransom, rather than the cartels shriveling up, but I don’t have nor can I find actual information to predict this, and the one test case we have (Prohibition) suggests my gut reaction is underinformed.

    • Scott Alexander says:

      Yeah, I was saying $5000 was a high estimate because it was given as the average for cases, across both those using lawyers and those that involve plea bargains/public defenders. I expect it’s a very normal/low amount for those where the defendant chooses to hire a private lawyer but that these represent a very small percent of arrests.

      Do people really need twenty hours of lawyer time just to defend against marijuana possession charges?

  31. Julia says:



    The look on the faces of a classroom of social work students, who are mostly 25-year-old women, when the professor mentions that schizophrenia usually manifests age 25-35 in women, is priceless.

  32. Paul Torek says:

    Has severe trauma, for example child abuse, significantly declined over the last 50 years? If so, that could easily explain the drop in schizophrenia. Any effects from marijuana use could easily be overwhelmed by such societal changes.

    But any historical data coming from before intercoder reliability was regularly and carefully evaluated should be firmly ignored. Never mind trying to compare medieval “mopishness” to modern “schizophrenia”.

    • Jake says:

      Surely mopishness is depression, not schizophrenia? The medieval concept mapping the strongest to schizophrenia is probably demonic possession.

  33. Alexander says:

    From my experience, being dutch, I agree with the people above that one of the biggest problems with prohibiting drugs is the power it gives to organized crime and the problems/nuisance it causes with drug dealers roaming the street.

    Two examples:

    Our (previous) government tried to change our coffee shops from stores to private clubs with a fixed limit to the amount of members, excluding tourists and casual users from the ability to legally purchase drugs. They started with a trail in the southern provinces of the Netherlands (bordering Belgium and Germany, two countries that don’t like our drug policies much). Right from the start the amount of drug dealers on the street skyrocketed (I have no numbers at hand, but this is acknowledged by both citizen accounts and statements by police officials and mayors). While the trail failed to reach its aims, and other local (city/province) governments fought and won a battle against the federal government, currently the laws are still set in most of the cities in the trail area, and continue to be a huge problem, both with crime and a general nuisance to inhabitants of those cities.

    Secondly, with our national drug policy as it is (Consumers can buy, and carry up to 5 grams of weed/hash legally, and can grow up to 2 or 3 plants in their home. Coffee shops can sell, but can not buy their product legally, and nobody can legally grow weed to sell to the shops). There is a lot of drug related crime in the growing and distributing of drugs. I can not remember a case of a drug raid by police where they did not find an arsenal of firearms (which are mostly prohibited here), and the amount of drugs they find suggest a huge income for organized crime.

    One last thing, to note about usage in a society where some drugs are “legal”:
    We might have more regular users of drugs (especially high-schoolers/students, where going to class smelling of weed might have you go to the principles office, but that’s about the extend of problems you will get) but society as a whole does not seem to suffer, a lot of people never use drugs, most who do (like me) only do so once every while and I have seen about as many “stoners” in my visits to the states as I do in my own country. And I have never seen any evidence that in general IQ’s are lower, that there is a higher rate of mental disorders or for that matter more traffic accidents then in countries that criminalize “soft” drugs.

  34. Jai says:

    Aside: What provoked the header change to PREVIOUSLY AN EXCEPTION TO “DON’T READ THE COMMENT SECTION”?

  35. Bob Unwin says:

    Landsburg has critiqued the point about tax revenues. See link .

    Another point: the Netherlands/Portugal comparisons are unlikely to give the fully story. Consider the large variation between countries in terms of alcohol, smoking, unhealthy food, dangerous driving (to give just a few examples). For instance, Japan has very little obesity and hard drug problems, but smoking is around 4 times more common than in Norway (though probably is declining). My sense is that binge drinking (and alcoholism) varies substantially between countries. Russia is the notorious example, but there’s variation between Nordic countries, UK, and Italy (esp. if you look at particular social classes). The cost of drinking may be reflected in how permissive the laws are, but only imperfectly.

    One important aspect is how legalization affects use of more dangerous drugs. A small impact on number of hard drug users may be a big QALY cost.

  36. JRM says:

    I just want to note that I’m a prosecutor and I think this is the most informative piece on marijuana legalization I have ever read. Most of the stuff published for public consumption is dishonest and hacktastic; your efforts are appreciated, at least by me.

  37. Pingback: What is the matter with Mary Jane? | mathew

  38. Stel says:

    The IQ study is far from conclusive. The results of the Meier et al. IQ study were already brought into question:

    -Ole Rogeberg. Correlations between cannabis use and IQ change in the Dunedin cohort are consistent with confounding from socioeconomic status.

    In addition:
    -It is likely regular teen use will drop with legalization as it did in Portugal.
    -The loss was only alleged in teens who were heavy users and continued heavy use into adulthood.
    -The heavy users only abstained for 1 week. It can take longer for effects to wear off in such users [Fried. 2005]
    -Abstention from using was not verified.
    -The study chart actually shows 5.23 point drop, not 8, in the heaviest using group.

  39. Stel says:

    If marijuana caused psychosis then the overall rates of psychosis should have risen with rising usage rates of marijuana in the U.S. but they have not. Also rates of psychosis should vary by country depending on their marijuana usage rate but they do not. They are roughly the same throughout the world.

    It does not increase the risk in adults without specific, uncommon, vulnerabilities.

    Parakh P & Basu D. Cannabis and psychosis: have we found the missing links? Asian J Psychiatr. 2013.

    It can even help people with schizophrenia.

  40. Pingback: The President’s Position On Pot | Konfeksiyon Tekstil

  41. Schmidty says:

    Anecdotally: I have been a “heavy” marijuana smoker who considers himself addicted for the last three years. During this time I taught myself software development and obtained a job at a university armed only with a highschool diploma. I have claimed to “self medicate” for depression and have experienced a wide assortment of alarming psychiatric symptoms throughout childhood.

    The theory that crazy people self-select as drug users cannot be stressed enough. A chronic behavior designed to alter one’s state of mind must be predicated on dissatisfaction with one’s natural state of mind – clinically, a chronic psychiatric disorder.

    A few points about the gateway drug effect: I would dismiss any study that included one-time-only users of lsd, mdma, basic pharmacy opiates, and anything else I forgot that’s well-understood as having basically zero injury risk for first time users. There is definitely a class of people who just try things and lsd is substantially safer than hang gliding. At minimum you need to include only participants who started with a chronic cannabis habit and developed an additional chronic habit. I would also seriously consider segregating users who moved on only to psychadelics such as lsd, mushrooms, peyote, ayahuasca and salvia divinorum. There is definitely a distinct class who smokes marijuana and uses these drugs, occasionally or heavily, but would never consider even trying cocaine, opiates, amphetamines, etc. Aside from mushrooms, the psychadelic drugs are relatively safe and are simply used for reasons separate than the reasons explaining heroine use. Exploring the gateway drug effect brings us instantly into contact with the question of which other drugs should be scrutinized for potential decriminalization or legalization – but we can improve our statistics by selecting a few of these drugs and seeing whether this class can emerge from clinically feasible samples. Anecdotally I could put you onto a full contiguous city block of them, all middle class.

    If you’d like me to focus my energy and attention on how marijuana affects driving, it’s my layman’s opinion that it’s an extremely small or even nonexistent problem. From the link you provided: “After alcohol, THC (delta-9-tetrahydrocannabinol), the active ingredient in marijuana, is the substance most commonly found in the blood of impaired drivers, fatally injured drivers, and motor vehicle crash victims. Studies in several localities have found that approximately 4 to 14 percent of drivers who sustained injury or died in traffic accidents tested positive for THC.” Blood tests for THC can be positive for months after the last consumption, and will light up like an oil rig fire long after the hour or so after smoking when a person might be markedly impaired. The percentage of fatally injured drivers with THC in their blood *should* be exactly the same as for all the people driving through the area – but in fact it seems to be substantially LOWER. Perhaps though this merely points to the blindingly obvious fact that people who smoke weed often drive slower and more carefully than average to avoid being pulled over – a fact which will disappear with legalization.

    • Stel says:

      No, this effect of greater caution has been demonstrated in the lab were there is no fear of being pulled over.

      H. Robbe. 1995. Marijuana’s effects on actual driving performance. HHMRC Road Research Unit, University of Adelaide. 1995.

      DOT HS 808 078, U.S. Department of Transportation, National Highway Traffic Safety Administration, Final Report, November 1993

  42. Ben says:

    If it is plausible that MJ has a substitution effect with alcohol to the point where it reduces road fatalities appreciably… Surely it is plausible that it also has some sort of mitigating substitution effect with the IQ-reduction effect of alcohol. From my admittedly limited interactions with drunks and stoners, I just can’t imagine that heavy MJ smokers are doing more damage to their brains than heavy drinkers.

  43. TallDave says:

    How much IQ should we trade for how much happiness? What ratio maximizes outcomes?

    We all face this choice every day, we know some activities will boost IQ and others won’t, we know some activities will boost lifetime earnings (e.g. studying, work) and others won’t.

  44. Stephanie says:

    I think if someone had told me I might lose 8 IQ points if I smoked weed before my frontal lobe was fully developed (I did a little but luckily not heavily) that would have been much more effective than the whole “drugs are evil” route. However, I probably have high IQ and I’m good at making smart decisions and delaying gratification. I wonder if the IQ differences are constant across IQ range? Also, how does it depend on when the weed was smoked? At what age is IQ no longer affected by smoking pot?

    The main problem with the legality is that it is so hard to answer these questions right now when there is so little data. I was trying to find studies about marijuana during pregnancy and couldn’t find much that is reliable. If we had accurate information, at least we could form public policy intelligently and inform the public of the risks.