Over the last several decades, a number of twin studies throughout the world have yielded results consistent with the overall conclusion that both genetic and environmental factors contribute to the risk of becoming a long-term smoker (for reviews, see [1,2]). After performing a meta-analysis of most of the reported twin studies on smoking-related behaviors in the literature, we found that genetic factors contribute approximately 50% to smoking initiation and 59% to smoking persistence [3].
