When twins were asked to record their average daily cigarette usage over their lifetime11, analyses of their responses yielded the following results (Fig. 2). In the early years of adolescence, twin resemblance for smoking is entirely the result of environmental experiences shared between the twins; for example, from the home, school and/or from peer groups. As twins age and smoke more, genetic factors begin to emerge (probably because of genetic differences in their responses to nicotine and vulnerability to the development of dependence). From age 16 to 30, genetic factors become progressively more important in their influence on cigarette use, while shared environmental experiences become progressively less important. Similar patterns are seen for other substances of abuse11.
