Descriptive statistics are provided in Table 1 for both the full sample and the subsample of current smokers, including sample sizes, number of twin pairs, and mean (SD) of CPD. In the full sample there are around 1000 or more participants assessed at each of the ages. In the subsample the number of participants is reduced considerably, especially at younger ages. At older ages the samples of current smokers are much larger, comprising 1/2 to 3/4 of the full sample. Our criteria for current smoking was intended to be inclusive, and to capture all individuals who were smoking cigarettes even to only a small extent (e.g., smoking on average one cigarette per month) during the assessment period. Individual trends are reported graphically in Figure 1, along with a running mean and standard deviation (based on overlapping windows of 500 subjects). Very few 11-year-olds smoke to any extent. Some individuals dramatically increase their intake at age 14, but again the vast majority uses no cigarettes (the median at all young ages is zero). By the age 17 assessment the mean CPD
