From the blog “Three-Toed Sloth.” This is just a couple of excerpts, it’s worth it to go and read the whole thing (even though it’s a slog if you don’t have a science background):
Many twin data sets show that the correlations in twins’ IQs actually change with the environment, in a pretty crude way which nonetheless goes beyond what people generally include in their models. I will quote from an old paper by Bronfenbrenner [3, pp. 159--160], because it’s handy and it makes the point:
The importance of degree of environmental variation in influencing the correlation between identical twins reared apart, and hence the estimate of heritability based on this statistic, is revealed by the following examples.
a. Among 35 pairs of separated twins for whom information was available about the community in which they lived, the correlation in Binet IQ for those raised in the same town was .83; for those brought up in different towns, the figure was .67.
b. In another sample of 38 separated twins, tested with a combination of verbal and non-verbal intelligence scales, the correlation for those attending the same school in the same town was .87; for those attending schools in different towns, the coefficient was .66. In the same sample, separated twins raised by relatives showed a correlation of .82; for those brought up by unrelated persons, the coefficient was .63.
c. When the communities in the preceding sample were classified as similar vs. dissimilar on the basis of size and economic base (e.g. mining vs. agricultural), the correlation for separated twins living in similar communities was .86; for those residing in dissimilar localities the coefficient was .26.
d. In the Newman, Holzinger, and Freeman study, ratings are reported of the degree of similarity between the environments into which the twins were separated. When these ratings were divided at the median, the twins reared in the more similar environments showed a correlation of .91 between their IQ’s; for those brought up in less similar environments, the coefficient was .42.
(Let us pause a moment to contemplate the sense in which identical twins, growing up in the same town and attending the same school, are “raised apart”.)
By the time you’re done partitioning the twin pairs into classes in this way, n is pretty small, and the sampling errors in the correlations are going to be large, so I wouldn’t give the 0.86 vs 0.26 contrast a lot of credence, but the fact that the differences are all in the same direction, and get pretty big, ought to be hard to ignore. (And it’s worth remembering that n is never very large for identical-twins-raised-apart studies.) The obvious explanation for such results is that the developmentally-relevant environment of twins raised apart, but in similar towns, is much more highly correlated than that of twins raised apart in dissimilar towns. This means that a substantial chunk of the correlation you thought was genetic is actually due to shared environment, and pushes your heritability estimate down. Alternately, you could abandon the lack of correlation between genetic and environmental contributions, or the strictly additive nature of the model by including a very substantial interaction between genes and environment, so that identical genotypes respond very differently to those differences in environment. However you slice it, your estimate of heritability was too high.
And from later in the essay….
There are also randomized studies of interventions with “at risk” families, e.g. ones with unusually low birth-weight children. Depending on the study, the treated groups had IQs 10 to 15 points above the controls. Because of the random assignment, not only is there no problem of endogeneity, it’s also idle to worry about placebo effects — it would be fantastic if a placebo raised IQ by 10 points.
Another nice example (not from Wahlsteen) comes from Heber’s work on Rehabilitation of Families at Risk for Mental Retardation. Rather than summarizing it my own words, I’ll quote someone else’s summary (though no doubt I’ll be told he understood neither genetics nor experimental methods):
It describes an experiment on ghetto children whose mothers had IQs of below 70. Some of these children received special care and training, while others were a control group. Four years after the training period the IQs of the former averaged 127 and those of the latter 90, a spectacular difference of 37 points. The fact that the control children had a 20-point advantage over their mothers is not unexpected [because of regression toward the mean]. [4, pp. 14--15]
At this point, the ritual is for people to begin saying things like “there’s nothing you can do if the environment is already decent”, “the changes didn’t last long after the program” (which would equally show that exercise can’t really change physical fitness; see below), or to raise irrelevancies. (My favorite, among the last, is to point to adoption studies showing that adoptees’ IQs are more correlated with their biological than their adoptive parents’ IQs, conveniently side-stepping which set of parents had scores closer to the adoptees’.)