Nature vs Nurture: A Natural Experiment
But recently someone pointed out an interesting paper which provides evidence on a related question of considerable interest: To what extent are outcomes for a child determined by the environment he is raised in, to what extent by his genes? Children of well off and well educated parents tend to end up richer and better educated than children of parents with the opposite characteristics. Is that because of a superior environment or superior genes?
From 1970-1980, a large number of Korean-American children were placed for adoption by an agency which assigned them at random to adoptive families. That meant that any correlation between characteristics of the families, such as maternal education, socio-economic status, or income, and characteristics of the children as adults, would be due to environment not genetics. By comparing the strength of the relation between characteristics of parents and adoptive children with the corresponding figure for parents and biological children raised by those parents, one can get at least some estimate of how much of the relation comes from which cause.
The conclusion is striking. An increase of one year in maternal education produces an increase of only .07 years in the education of an adoptive child but an effect four times as large on the education of a biological child. Similar results apply to a variety of other characteristics. It looks as though being brought up by well off or well education parents is indeed an advantage, but a considerably smaller advantage than being the biological child of such parents.
One important qualification to that result is to note that all of the adoptive families had to be certified by the adoption agency as suitable to adopt. That would presumably cut off some of the lower tail of the distribution—an alcoholic unmarried mother would be unlikely to qualify. And, they had to be families that wanted to adopt, which again would eliminate some. But at least over the range of environments in the sample, nurture seems to be a good deal less important than nature.
The study, as so far described, is limited to particular, readily measured, characteristics of the adoptive parents. Having a well educated adoptive mother doesn't have much effect on how much education you get. But having an adoptive mother who cares a lot about her children and pays them a lot of attention might.
To test that possibility the author of the paper looked at the relation between characteristics of siblings. Adoptive siblings, like biological siblings, are brought up in the same household, but, unlike biological siblings, are not genetically related. So if some households are much better places to be brought up in than others, one would expect the result to show up in the relation between (say) years of education of adoptive siblings.
There is such a relation, but it is only a little stronger, relative to the corresponding relation between biological siblings, than in the parent/child case. An extra year of education by an adoptive sibling predicts, on average, an extra .09 years of education, for a biological sibling an extra .29. The effect is stronger for income: .16 vs .29.
A further limitation in the study is that it does not distinguish a relation between biological parent and child or biological siblings due to shared genetics from one due to pre-natal environment. Arguably, better educated and higher income mothers are in better condition during pregnancy, which could result in better children for (pre-natal) environmental reasons rather than genetic reasons. That does not affect the absolute level of the effect for adoptive children but might make the genetic contribution to the difference between adoptive and biological look larger than it really is.