Twins: a test of the equal environments assumption.

We asked a sample of 343 adult same-sex twin pairs a number of questions about the similarity of their social environment during childhood and early adolescence. A factor analysis of their responses indicated that their common environment was derived from two sources, one being similar treatment "imposed" upon them by their parents, the other being "elicited" by the twins' similar interests and behavior. Monozygotic (MZ) twins reported experiencing more similar "imposed" and "elicited" environments than dizygotic (DZ) twins. The extent of imposed similar treatment received during childhood and early adolescence was unrelated to either MZ or DZ twins' current behavioral similarity, as indicated by absolute intrapair differences in their Neuroticism, Anxiety, and Depression scores. Similar treatment imposed upon MZ twins on the basis of their zygosity alone is therefore not a threat to the validity of the twin method.