Since identical twins share twice the genetic covariance of fraternal twins, the impact of family environment can be backed out from the respective intra-pair correlation coefficients. In the limit case if genetics played no factor, then fraternal and identical twins should have identical pairwise correlations.
For example suppose pairs of identical twins have 45% pairwise correlation for adult IQ. And say fraternal twins have 25% correlation for the same measure. That would tell us that the population level variance of adult IQ is 40% attributable to genetic heritability, 5% to environmental heritability , and 55% to non-heritable factors (i.e. not genetics and not family environment).
"Twin studies of adult individuals have found a heritability of IQ between 57% and 73%[6] with the most recent studies showing heritability for IQ as high as 80%[7] and 86%.[8]. IQ goes from being weakly correlated with genetics, for children, to being strongly correlated with genetics for late teens and adults. "
Can you think of a reason selection bias could be an issue, thus invalidating twin study cohorts?
A twin study should normalize everything genetically. Ergo epigenetic expression and environment are the only two causal levers. It sounds like you're arguing that parental involvement has a causal impact, which is the point of the studies to a degree.
Perhaps my original question was unclear: is there something the twins do or have done to them that cause a latent selection bias before adoption, thus invalidating the twin studies approach as a whole?
