Biometric analysis of twin and sibling studies use similarities and differences between twins and siblings with varying degrees of genetic relatedness to decompose the variance in a phenotype into additive genetic (A), shared (C) and nonshared (E) environmental components. Genetic influences are indicated if the correlation of sibling/cousin 1’s and sibling/cousin 2’s externalizing are more similar among sibling/cousin types sharing more genes on average than sibling/cousin types that share fewer genes on average (i.e., MZ twin correlation is twice that of DZ/full sibling correlations, and a cascade such that DZ/full siblings > HS > US, and cousin pairs whose parents are MZ twins > whose parents are DZ twins). Shared environmental influences are nongenetic influences that contribute to similarity among family members (indicated by sibling correlations of similar magnitude across the different sibling types and by US correlations > 0), whereas nonshared environmental influences are nongenetic influences that contribute to differences among family members (indicated to the extent that MZ twins’ externalizing are not perfectly correlated).
