Univariate variance component analysis characterized the relative importance of genetic and environmental effects on pack-years. Univariate analyses for all brain measures have been reported extensively in prior studies and are detailed (Kremen et al. 2010; Eyler et al. 2011; Eyler et al. 2012). The degree of relatedness between members of MZ and DZ twin pairs is used to estimate the contribution of genetic and environmental effects to the phenotypic variation of nicotine use and brain structure. Phenotypic similarities between MZ twins arise from the sharing of all their additive genetic factors and all shared environmental influences. Similarities between DZ twins result from sharing on average 50% of their additive genetic factors in addition to shared environmental influences. Influences from unique environmental effects diminish the similarity between members of a twin pair, regardless of their zygosity. Models using twin data thus utilize MZ and DZ twin pair variances and covariances to estimate the proportion of total phenotypic variance due to additive genetic, shared environmental and unique environmental influences.
