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”.)
How do you know this?
This is a standard criticism of the twin studies. For example, Shalizi http://vserver1.cscs.lsa.umich.edu/~crshalizi/weblog/520.html
Thanks! That’s yet another problem—but few solutions!