Classic twin methodologies have been useful in quantifying genetic and environmental factors associated with cigarette use phenotypes. Generally, twin study methods have been used to compare the agreement in the behavior of monozygotic or identical twins that share the same genetic make-up and dizygotic or fraternal twins who share, on average, 50% of their genetic make-up. Statistical models estimate the percentage of variance in the trait explained by genes (i.e. heritability) and by common environment (i.e. experiences that render family members more alike) and unique environment (i.e. experiences that cause dissimilarity between family members) (15). Heritability estimates differ according to phenotype and age. For the initiation of cigarette use, shared environmental factors account for a small proportion of the liability (16), relative to additive genetic factors, which account for ~60% of the variance (17). Data from one meta-analysis showed differences in the heritability of initiation by sex, suggesting that genetic and environmental factors may contribute differently to individual differences in initiation in male and female smokers. Whereas the weighed mean heritability for females reached ~50%, the weighted mean heritability for
