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).