Substance use and sexual intercourse onsets in adolescence: a genetically informative discordant twin design.

PURPOSE Using a genetically informed, discordant twin analysis, the objective of this study was to examine whether earlier onset of drinking and smoking behaviors predicted early sexual intercourse onset. METHODS Over 3,400 adult same-sex twins from the Australian Twin Registry completed a structured interview that included retrospective reports on onsets of smoking, drinking, intoxication, and sexual intercourse and conduct disorder symptoms. A two-level frailty model estimated within-twin-pair and between-twin-pair comparisons. Onsets of smoking, drinking, drunkenness, and conduct disorder symptoms were estimated as sexual intercourse onset predictors. RESULTS After controlling for conduct disorder, smoking and drinking onset did not predict sexual intercourse onset for either within-twin-pair or between-twin-pair comparisons. Drunkenness onset had a significant effect on sexual intercourse onset, such that twins who first experienced alcohol intoxication at a younger age than their co-twins were also more likely to have sex earlier than their co-twins. CONCLUSIONS Relationships between substance use and sexual intercourse onsets may be due mostly to shared underlying factors; there was only a small relation between intoxication onset and sexual intercourse onset, and no direct relation between smoking and drinking onset and sexual intercourse onset.