Proportions of participants who reported ever using each substance are reported in Table 1, along with mean counts for males and females at 11-, 14-, and 17-years-old. (While the 17-year-old assessment in fact included individuals over the age of 18, and therefore able to legally use tobacco products, there was not a significant difference between those who were 17-years-old and those who were over the age of 17 on tobacco use endorsement rates, 62% and 67% respectively, z=1.65, p=0.10.) Whether or not males differed from females in the diversity of substances used was investigated by regressing substance use counts at each age on sex (coded dichotomously as 0/1 for females and males, respectively, as well as taking into account the clustered nature of twin data). Males reported higher total substance use counts than females at all three ages (11 years: Mmale=0.52, Mfemale=0.21, r=−0.31, z=−7.88, p<0.0001; 14 years: Mmale=1.39, Mfemale=1.21, r=−0.19, z=−1.84, p=0.066; 17 years: Mmale=1.98, Mfemale=1.61, r=−0.47, z=−3.29, p=0.001), although the difference was not quite significant at 14 years. Variance in diversity of substances used remained similar between males and females across ages.
