Monday, April 21, 2014

Shared environment not a factor

A 2000 paper summarized the evidence:
When genetic similarity is controlled, siblings often appear no more alike than individuals selected at random from the population. ...

Why Are Children in the Same Family So Different?


An important indicator of the influence of Plomin and Daniels' (1987) article is that an entire field of empirical research was generated in an attempt to answer the question posed in its title: Why are children in the same family so different?
This was explained to the general public is a book titled, The Nurture Assumption. It argued against the assumption that parental nurturing is important.

The terminology is a little confusing. The twin studies divide influences into these factors:
genetic effects (heritability);
unshared, or unique, environment - events that occur to one twin but not another, or events that affect each twin in a different way.
So the "shared environment" is the family environment shared by siblings. Many people assume that this is the most important thing in a child's life, but the studies do not show any significance at all.

A recent blogger JayMan elaborates:
In my earlier post on Gregory Clark’s work, The Son Becomes The Father, I laid bare the case for the known high heritability of human behavioral traits (including values and attitudes) and life outcomes. As well, equally important, I illustrated the complete absence of shared environment influences on these – that is, the effect common environmental forces that children growing up together share. This includes parents and upbringing, making it abundantly clear that parents don’t leave a lasting impact on who we grow up to be. These are towards what I’m calling the “75-0-25 or something” rule,
These results are hard to accept, as nearly all parents believe that they are doing something special for their kids with their care.

My theory is that parenting does not seem to make much difference because most moms just do what all the other moms in the neighborhood do. Then kids seem to be influenced more by schools and peers, than by parents. Of course this means parents still have influence by choosing where to live, what schools for the kids to attend, and what friends are encouraged. Parents can have even more influence if they are willing to have policies different from the crowd.

Whether I am right or not, there is very little scientific evidence that any parenting style is any better or worse than any other. If someone tries to tell you that you should be doing something different, ask for the research. It will usually not exist.

Here is another article about exaggerated parental influences:

Dr Jan Macvarish, a Research Fellow at Kent's Centre for Parenting Culture Studies, analyzed the policy literature for the study.




Anonymous said...

I think the idea that siblings are really sharing the same environment is silly. I'm the oldest of five. When I was 14 and my youngest sister was 5, we were living in the same environment, but experiencing it very differently. The household was very different by the time she was 14, and very different when I was 5. So even though we grew up in the same household, there were way to many factors to make that kind of statement.

George said...

Child-rearing is a lot of work, and it is reasonable to assume that some parents are a lot better at it than others. Your parents put the same skills, attitudes, practice, time, energy, money, etc. into both you and your sister. I would expect some correlation in the benefits.

Anonymous said...

Sure, and my sister and I have a lot in common, but to say it was the same environment is crazy. I had young parents, she had older parents, I had a SAHM until KG, she didn't, I have 4 younger siblings, she has 4 older ones, my parents had a lot more money when she was a kid, my dad wasn't in school, my parents knew what they were doing, nut had less energy. The list is endless, and I think it makes studies like this ridiculous.