Twin data were analyzed with Mx (Neale, 2004) using structural equation modeling. This method compares the similarities between twins of different genetic relatedness to partition phenotypic variances (VP) into genetic (VA), shared environment (environment that makes members of a family more similar to one another, VC), and non-shared environment (environment that is uncorrelated among family members, VE) variance components. Identical, or monozygotic (MZ), twin pairs share 100% of their alleles identical by descent and the shared environment whereas fraternal, or dizygotic (DZ), twin pairs share on average 50% of their alleles identical by descent and the shared environment. Assuming equal shared environmental influences for MZ and DZ twins, if MZ twin pairs are more similar to one another for a phenotype than DZ twin pairs, there is evidence of a genetic contribution to the variance of that trait. If the MZ twin pairs are less than twice as similar as the DZ twin pairs, shared environment influence is implied; however, if the MZ twin pairs were at least twice as similar as DZ twin pairs it would indicate that the
