We also analyzed data from the 1989 Australian Adult Twin Cohort (Heath et al., 2001; Knopik et al., 2004; Lessov et al., 2004)- male and female twins born 1964–1971 (aged 24–36 at the time of interview; mean age = 29.9) from the population-based Australian twin panel maintained by the Australian National Health and Medical Research Council. These twins were volunteered by their parents in response to media appeals and appeals through the Australian school system in 1980–1982. From 1996–2000, participants were interviewed by telephone using astructured diagnostic assessment similar to that described above for the Missouri adolescent twins. Telephone interview data were available from a total of 6257 individual twins (3454 women and 2803 men). Of relevance to this study, twins who reported smoking 100 or more cigarettes lifetime were asked detailed questions about nicotine withdrawal after they quit or cut down on their smoking. Approximately 47.8% (N=1651; mean age = 30.0, SD = 2.5) of women and 52.1% (N=1461; mean age = 30.1, SD = 2.5) of men reporting smoking and a 24-hour period of quitting or cutting down
