Hazard ratios from unadjusted and adjusted Cox regression models predicting timing of first sex are shown in Tables 1 and 2 for AA and EA twins, respectively. Among AA twins, effects of parental separation and parental alcoholism were nonsignificant with a single exception: in covariate-adjusted models, AA twins from alcoholic intact families were at 8.61 times increased likelihood of sexual onset (thus, earlier first sex) from age 21 onwards, compared to AA twins from nonalcoholic intact families. Here a violation in the PH assumption was observed, such that modeling of an age interaction (i.e., separate risk periods of birth through age 20 and from age 21 onwards) was necessary.
