Testing Putative Causal Associations of Risk Factors for Early Intercourse in the Study of Twin Adults: Genes and Environment (STAGE)

Adverse childhood experiences and substance use have been identified as potential causal risk factors for early-onset sexual
intercourse. While it is possible that exposure to these risk factors directly increases the likelihood of engaging in early
intercourse, an alternative explanation is that observed associations between these variables are due to shared familial confounds.
These unmeasured confounds may increase the likelihood of being exposed to these risk factors and of engaging in early intercourse.
Participants drawn from a population-based study of Swedish adult twins (ages 19–47 years; N = 12,126) reported on their history of exposure to early physical and sexual trauma, cigarette use, and cannabis use. We
investigated the nature of the association between these risk factors and young age at first intercourse, using a comparison
of twins differentially exposed to each risk factor. When compared to non-exposed, unrelated individuals, participants who
reported adverse childhood experiences or who engaged in early cigarette use or cannabis use were more likely to engage in
early intercourse. However, co-twin comparisons indicated that observed associations between these risk factors and early
intercourse may be due to familial factors shared within twin pairs, and risk factor exposure may not lead directly to early
intercourse. Our results suggest that preventing trauma exposure or preventing or delaying adolescents’ cigarette smoking
or cannabis use may not effectively delay intercourse onset; instead, other aspects of the adolescent’s environment should
be addressed.