In our study, men were three-times more likely to smoke heavily than women at age 31. In contrast, sex differences were not found in adolescence. This is in line with findings showing overlapping smoking rates in boys and girls younger than 16 years(59) and increased rates among boys than girls aged >16 years(59) and among adult men than women(60). A plausible explanation is that sex differences in smoking habits might become evident only after puberty because they are mediated by differences in sex-hormone levels(61, 62).
