user167: Magnus Carlsen's draw in yesterday's round of the 2012 London Chess Classic assured that his rating in the next published FIDE rating list will surpass Kasparov's previous record of 2851. I've seen/heard passionate chess fans debate the relative merits of Carlsen's rating achievement versus Kasparov's versus, say, Fischer's. To be clear, that's not what I'm after here.
One crucial element of such discussions is the notion of whether Elo ratings in general have undergone inflation over time: are there so many more 2700+ grandmasters today than there were 20 years ago because of a general rise in playing strength, or just because of some overall inflationary trend in the numbers? I'm also not trying to solicit bare opinions about whether or not that's so. What I am interested in knowing:

What serious research attempts have been made to answer the empirical question as to whether FIDE Elo ratings have naturally inflated over time because of something other than a rise in overall strength in the player pool?

The Wikipedia entry on the Elo rating system has a little bit to say about the matter, and also points to an article by Jeff Sonas of Chessmetrics. In addition to any pointers to work by others, I, for one, would also welcome an answer that gives a clear, concise summary of Sonas' main points.

user17330: The conclusions of Regan-Haworth paper should be taken with a grain of salt, as it seems to contradict other computer analysis of games, on better soft- and hardware and with more advanced mathematical methods. There they conclude (see Table 9), e. g., that Karpov in 1977 played at just slightly lower level than Kasparov in 2001 and Anand in 2008 (expected to score about 47% of points), and actually better than Topalov in 2005 and Ponomariov in 2011. Since Kasparov-2001 is 150 points higher rated than Karpov-1977, the rating would expect him to score 70% of points. I don't see how to reconcile this with the claim that there was no rating inflation.
Note that also, contrary to the implicit claim in the question, there is no mechanism by which rating would reflect a change in the overall strength in the player pool. It might be empirically the case that a typical strength of a 2600 player has not changed over certain time period, but this would be merely a coincidence rather than a reflection of fundamental properties of the ELO system, and certainly not generalizable.
If we rather define inflation naively and just measure the average rating of the top 100 players, then, as can be seen from this link, there was a steady inflation until 2012 and no inflation since then - the top-100 average rating oscillated betweeen 2700 and 2705 for the last 7 years.

user11874: I have a simple idea. Let's take a chess computer (hardware + software) that had it's rating measured 20 years ago, via play with other chess computers with known ratings that they had 20 years ago. Now let's measure it's rating now (exactly the same hardware plus exactly the same software), via play with modern chess computers, with known today's rating. The difference of two measurements would constitute rating inflation for the past 20 years. Simple enough?

user657: Elo's system had two components. One was independent of history, the other was not. His system for creating a "performance rating" over the course of an event or a period of time had no historical component to it; it was simply a measure of performance over the specified time. (Memory fails me on this point, but I think when he was calculating the ratings for FIDE this was the method he used.)
However the Elo system as used by federations around the globe does have a historical component, in that ratings are calculated by calculating a delta, a change from the previous rating.
The historically-based system has a natural tendency towards deflation. The system is a closed system, with no new points being created. So new players come in, take points from established players, and then exit (through death or retirement) before returning all those points back to the next batch of rising players.
Many ideas have been tried to compensate for this, some working better than others. Add to this the commercial pressure in the USCF of the early 70's to make ratings rise faster (the rather cynical view was that players would buy a book from the USCF and play in a tournament, their rating would go up, encouraging them to buy another book, etc.) and inflation was a real thing at some points in history.
Since Elo's system was based on a normal (bell) curve, it's nonsense to try and gauge inflation by measuring either extreme; the extremes are more likely to be affected by the total number of players being rated than by changes in actual strength or any sort of inflation.

user159: In absolute terms, Carlsen 2012 for sure is a stronger player than Kasparov 1985.
If Carlsen 2012 travelled in time played a match with Kasparov 1986, Carlsen would defeat Kasparov. This is simply because the technology-assisted preparation is a lot more efficient, and Carlsen has also an edge in opening theory, because he has the accumulated knowledge 1987-2012 that Kasparov does not have.
However, Kasparov is probably a stronger player than Carlsen.  If we take the FIDE Top 100 List for June 2000 (the oldest one that can be obtained), we see that Kasparov with 2849 Elo competes with an average of 2641 for the 99 followers (Elo distance 208 points) while Calsen in Fide Top 100 for December 2012 with a 2848 Elo competes with an average of of 2702 for his 99 followers (Elo distance of 146 points).
Elo is about the difference of points, not about absolute values (100 points of difference for Elo mean that player A is 2 times better than player B, 200 points means 4 times better, and so on. So with that list, it meant that Kasparov was on average more than 4 times better than all his 99 followers, while Carlsen is probably less only 3 times better than the average of his 99 followers.  
If we take the list were Kasparov has the maximum distance with his 99 followers and compare that distance with the best for Carlsen, we will be able to determine which player was actually the greatest, because with 99 data points, outliers (like another genius) get mitigate it.
I wonder however if Carlsen or Kasparov really care about who was better.

user70: I poked around some. You've probably seen these pages, but I'll post them anyway:
a. This page will interest you.  It includes a photocopy of a letter from Elo himself stating the possibility:  

Thus over time the rating scale could drift unless some measures are taken to stabilize it.

He further mentions that the ratings scale has no anchor, no fixed point. Compare to an athlete who runs a race in an hour; an hour now is the same as an hour 50 years ago.  Time is such a fixed point.
b. Also, hasn't the 'inflation' question already been answered by recent revelations of high ratings coming out of isolated areas?  See the "Pool of Players" section of this page for an allusion to the issue.   Additional support, though it is not scholarly nor particularly informative. Search for "isol".  Here's another anecdote showing what happens with isolated populations (and another candidate for the 'why are chess players crazy' thread!) I didn't fact-check it but should be easy enough to do.
c. The Elo wiki article talks about inflation as if it's an accepted fact.  
d. Here's a germane article about inflation, and the followup.  Look at that smoking gun in 1986!

user924: I am surprised that the paper "Intrinsic Chess Ratings" by Ken Regan and Guy Haworth hasn't been posted yet. It is exactly what's asked for, serious research into rating inflation. PDF
Basically they got games from three periods (1976-1979, 1991-1994, 2006-2009), in several rating ranges (e.g. both players within 10 points of 2200, within 10 points of 2300, etc), and excluded types of games that might be anomalous, like team matches. Read the paper, it looks quite thorough.
Then they compared the games systematically with Rybka 3.
Some sentences from the conclusion: 

We conclude that there is a smooth relationship between the actual
  players’ Elo ratings and the intrinsic quality of the move choices as
  measured by the chess program and the agent fitting. Moreover, the
  final sfit values obtained are nearly the same for the corresponding
  entries of all three time periods.

In my view, it's quite solid evidence against the existence of rating inflation.

167: <p>Magnus Carlsen's draw in yesterday's round of the 2012 London Chess Classic assured that his rating in the next published FIDE rating list will surpass Kasparov's previous record of 2851. I've seen/heard passionate chess fans debate the relative merits of Carlsen's rating achievement versus Kasparov's versus, say, Fischer's. <em>To be clear, that's not what I'm after here.</em></p>

<p>One crucial element of such discussions is the notion of whether Elo ratings in general have undergone inflation over time: are there so many more 2700+ grandmasters today than there were 20 years ago because of a general rise in playing strength, or just because of some overall inflationary trend in the numbers? <em>I'm also not trying to solicit bare opinions about whether or not that's so.</em> What I am interested in knowing:</p>

<blockquote>
  <p>What serious research attempts have been made to answer the empirical question as to whether FIDE Elo ratings have naturally inflated over time because of something other than a rise in overall strength in the player pool?</p>
</blockquote>

<p>The Wikipedia entry on the Elo rating system <a href="http://en.wikipedia.org/wiki/Elo_rating_system#Ratings_inflation_and_deflation">has a little bit to say</a> about the matter, and also points to <a href="http://www.chessbase.com/newsdetail.asp?newsid=5608">an article</a> by Jeff Sonas of <a href="http://www.chessmetrics.com/cm/">Chessmetrics</a>. In addition to any pointers to work by others, I, for one, would also welcome an answer that gives a clear, concise summary of Sonas' main points.</p>
 17330: <p>The conclusions of Regan-Haworth paper should be taken with a grain of salt, as it seems to contradict <a href="https://content.iospress.com/articles/icga-journal/icg0012" rel="nofollow noreferrer">other</a> computer analysis of games, on better soft- and hardware and with more advanced mathematical methods. There they conclude (see Table 9), e. g., that Karpov in 1977 played at just slightly lower level than Kasparov in 2001 and Anand in 2008 (expected to score about 47% of points), and actually better than Topalov in 2005 and Ponomariov in 2011. Since Kasparov-2001 is 150 points higher rated than Karpov-1977, the rating would expect him to score 70% of points. I don't see how to reconcile this with the claim that there was no rating inflation.</p>

<p>Note that also, contrary to the implicit claim in the question, <strong>there is no mechanism by which rating would reflect a change in the overall strength in the player pool</strong>. It might be empirically the case that a typical strength of a 2600 player has not changed over certain time period, but this would be merely a coincidence rather than a reflection of fundamental properties of the ELO system, and certainly not generalizable.</p>

<p>If we rather define inflation naively and just measure the average rating of the top 100 players, then, as can be seen from <a href="https://ratings.fide.com/toplist.phtml?list=men" rel="nofollow noreferrer">this link</a>, there was a steady inflation until 2012 and <strong>no inflation since then - the top-100 average rating oscillated betweeen 2700 and 2705 for the last 7 years</strong>.</p>
 11874: <p>I have a simple idea. Let's take a chess computer (hardware + software) that had it's rating measured 20 years ago, via play with other chess computers with known ratings that they had 20 years ago. Now let's measure it's rating now (exactly the same hardware plus exactly the same software), via play with modern chess computers, with known today's rating. The difference of two measurements would constitute rating inflation for the past 20 years. Simple enough?</p>
 657: <p>Elo's system had two components. One was independent of history, the other was not. His system for creating a "performance rating" over the course of an event or a period of time had no historical component to it; it was simply a measure of performance over the specified time. (Memory fails me on this point, but I <em>think</em> when he was calculating the ratings for FIDE this was the method he used.)</p>

<p>However the Elo system as used by federations around the globe does have a historical component, in that ratings are calculated by calculating a delta, a change from the previous rating.</p>

<p>The historically-based system has a natural tendency towards deflation. The system is a closed system, with no new points being created. So new players come in, take points from established players, and then exit (through death or retirement) before returning all those points back to the next batch of rising players.</p>

<p>Many ideas have been tried to compensate for this, some working better than others. Add to this the commercial pressure in the USCF of the early 70's to make ratings rise faster (the rather cynical view was that players would buy a book from the USCF and play in a tournament, their rating would go up, encouraging them to buy another book, etc.) and inflation was a real thing at some points in history.</p>

<p>Since Elo's system was based on a normal (bell) curve, it's nonsense to try and gauge inflation by measuring either extreme; the extremes are more likely to be affected by the total number of players being rated than by changes in actual strength or any sort of inflation.</p>
 159: <p>In absolute terms, Carlsen 2012 for sure is a stronger player than Kasparov 1985.</p>

<p>If Carlsen 2012 travelled in time played a match with Kasparov 1986, Carlsen would defeat Kasparov. This is simply because the technology-assisted preparation is a lot more efficient, and Carlsen has also an edge in opening theory, because he has the accumulated knowledge 1987-2012 that Kasparov does not have.</p>

<p>However, Kasparov is probably a stronger player than Carlsen.  If we take the FIDE Top 100 List for June 2000 (the oldest one that can be obtained), we see that Kasparov with 2849 Elo competes with an average of 2641 for the 99 followers (Elo distance 208 points) while Calsen in Fide Top 100 for December 2012 with a 2848 Elo competes with an average of of 2702 for his 99 followers (Elo distance of 146 points).</p>

<p>Elo is about the difference of points, not about absolute values (100 points of difference for Elo mean that player A is 2 times better than player B, 200 points means 4 times better, and so on. So with that list, it meant that Kasparov was on average more than 4 times better than all his 99 followers, while Carlsen is probably less only 3 times better than the average of his 99 followers.  </p>

<p>If we take the list were Kasparov has the maximum distance with his 99 followers and compare that distance with the best for Carlsen, we will be able to determine which player was actually the greatest, because with 99 data points, outliers (like another genius) get mitigate it.</p>

<p>I wonder however if Carlsen or Kasparov really care about who was better.</p>
 70: <p>I poked around some. You've probably seen these pages, but I'll post them anyway:</p>

<p>a. <a href="http://www.chesshistory.com/winter/extra/ratingstitles.html">This page will interest you</a>.  It includes a photocopy of a letter from Elo himself stating the possibility:  </p>

<blockquote>
  <p>Thus over time the rating scale could drift unless some measures are taken to stabilize it.</p>
</blockquote>

<p>He further mentions that the ratings scale has no anchor, no fixed point. Compare to an athlete who runs a race in an hour; an hour now is the same as an hour 50 years ago.  <em>Time</em> is such a fixed point.</p>

<p>b. Also, hasn't the 'inflation' question already been answered by recent revelations of high ratings coming out of isolated areas?  <a href="http://www.kaggle.com/c/ChessRatings2/data">See the "Pool of Players" section of this page</a> for an allusion to the issue.   <a href="http://www.chessforums.org/general-chess-discussion/11013-evile-ratings.html">Additional support</a>, though it is not scholarly nor particularly informative. Search for "isol".  <a href="http://www.chess.com/chessopedia/view/bloodgood-claude-frizzel">Here's another anecdote</a> showing what happens with isolated populations (and another candidate for the 'why are chess players crazy' thread!) I didn't fact-check it but should be easy enough to do.</p>

<p>c. The <a href="http://en.wikipedia.org/wiki/Elo_rating_system#Ratings_inflation_and_deflation">Elo wiki article</a> talks about inflation as if it's an accepted fact.  </p>

<p>d. Here's a <a href="http://members.shaw.ca/redwards1/">germane article about inflation</a>, and <a href="http://members.shaw.ca/redwards1/update.html">the followup</a>.  Look at that smoking gun in 1986!</p>
 924: <p>I am surprised that the paper "Intrinsic Chess Ratings" by Ken Regan and Guy Haworth hasn't been posted yet. It is exactly what's asked for, serious research into rating inflation. <a href="http://www.cse.buffalo.edu/~regan/papers/pdf/ReHa11c.pdf">PDF</a></p>

<p>Basically they got games from three periods (1976-1979, 1991-1994, 2006-2009), in several rating ranges (e.g. both players within 10 points of 2200, within 10 points of 2300, etc), and excluded types of games that might be anomalous, like team matches. Read the paper, it looks quite thorough.</p>

<p>Then they compared the games systematically with Rybka 3.</p>

<p>Some sentences from the conclusion: </p>

<blockquote>
  <p>We conclude that there is a smooth relationship between the actual
  players’ Elo ratings and the intrinsic quality of the move choices as
  measured by the chess program and the agent fitting. Moreover, the
  final sfit values obtained are nearly the same for the corresponding
  entries of all three time periods.</p>
</blockquote>

<p>In my view, it's quite solid evidence against the existence of rating inflation.</p>
