Having established evidence of genetic influences on regular smoking, we then formally quantify the contribution of genetic factors to regular smoking by using maximum likelihood variance components models (Purcell 2008). This process estimates the best fitting model by comparing the covariance of MZ and DZ pairs, and it provides estimates for two components of genetic influence (additive [A] and dominant [D]) and two components of environmental influence (shared [C] and unshared [E]). We assess the best fitting model by comparing model fit indices (e.g., likelihood ratio tests), and we present the most parsimonious model. The purpose of this model is to establish an average heritability measure for this trait.4 These estimates are presented in Table 2.
