Cox proportional hazards (PH) regression was conducted predicting age at first sex from dummy variables coding for parental separation and parental alcoholism, without and with adjustment for control variables. Parental separation was modeled as a time-varying predictor to ensure onset before or at the same time as first sex. Intact families were right-censored at twins’ age at last interview if younger than age 18, and in the case of parental death during childhood, right-censored at twins’ age when their parent(s) died. Parental alcoholism was modeled as time-invariant given age of AD onset is available by parent self-report only. Control variables with ages of onset (offspring CD, SAD, MDD, suicidality, and physical and sexual abuse) were included as time-varying predictors in adjusted models. The Efron approximation [31] was applied for survival ties. To examine potential violation of the PH assumption, such as might be the case if the effects of parental separation or parental alcoholism differ for earlier versus later age periods, the Grambsch and Therneau test of Schoenfeld residuals [32] was employed, with age-interactions modeled to correct observed violations [33].
