What Can We Learn From the Study of Twins?: An Evaluation of the Equal Environments Assumption

This is the most comprehensive analysis of the equal environments assumption to date. Most evidence about the effects of genes on behavior comes from twin studies. The classic twin study method compares the similarity between monozygotic (MZ) twins on some trait with the similarity of dizygotic (DZ) twins on that same trait. If the rearing environments of MZ and DZ twins are analogous, then the magnitude of genetic influence on variation in a trait can be estimated. This is based on the extent to which the correlation between MZ twins exceeds the correlation for DZ twins. If the rearing environments for MZ and DZ twins are not analogous, then the estimates of heritability generated from this method may be confounded.