Using a correlated liability dimensions model, we examined the extent to wh
ich the same genetic and environmental factors influence both initiation of
regular cigarette smoking and maintenance of the smoking habit in men and
women. We analyzed questionnaire survey data obtained from large samples of
male and female like-sexed twins from three countries, Australia (N = 1535
pairs), Sweden (N = 5916 pairs), and Finland (N = 4438 pairs), subdivided
into three age bands (18-25, 26-35, and 36-46 years of age). We found that
familial influences on risk for persistence in smoking cannot be entirely e
xplained by the same factors responsible for risk of smoking initiation. To
tal genetic variance for smoking persistence varied little by age band and
sex (range, 39-49% in women and 42-45% in men); however, even among twins i
n the youngest group (18-25 years of age), who on average have the fewest y
ears of cigarette use, less than 40% of the total genetic variance in smoki
ng persistence was accounted for by the same genetic factors that increased
risk of smoking initiation, and this percentage decreased to less than 10%
in the 36-46 year olds.