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 males was ~40% (18). Meanwhile, heritability estimates for smoking persistence range from 50% to 70%, for smoking quantity from 40% to 60%, for nicotine dependence from 60% to 80%, and for smoking cessation ~50% (19–21). It has also been suggested that the liability to smoking initiation, regular tobacco use, and nicotine dependence are correlated. Specifically, more than 80% of the variance in liability to initiation and regular use is shared, while a smaller proportion is shared between regular use and nicotine dependence (17). Added to this, age-dependent genetic effects have been identified, whereby the genetic liability influencing later cigarette use behaviors is more influential when cigarette use is initiated during adolescence (22), implying a gene-environment interaction with E being operationalized as age.
