Apart from genetics: What makes monozygotic twins similar?

Identical (monozygotic) twins have attracted special attention for the study of behavior genetics. Some of the assumptions and results of these studies are reviewed with special attention given to the natural experiment of identical twins adopted by different families. However, the correlation for any behavior between the adopted twins of a monozygotic pair is affected by their common prenatal environment as well as by the pervasive similarity of the two adoptive environments. The genetic contribution to complex social phenomena, but also to physical characteristics such as height, is usually overestimated. Both for adopted and unadopted twins differential effects of cultural, family, and prenatal environments, and the correlated experiences of twins and their physical appearance affect their development. Taxonomies and samples of environments need be examined in order to be able to estimate the genetic contributions to behavioral traits