Associations between timing of first sex and both parental separation and parental alcoholism were examined using individual-versus pair-level data, with standard errors and confidence intervals adjusted for non-independence in the statistical package STATA[29]. In descriptive analyses, cumulative failure curves were estimated using the Kaplan-Meier survivor function [30]. Kaplan-Meier log-rank tests were conducted to identify differences in equality of survivor functions by race/ethnicity and zygosity, the latter to identify potential limitations to the generalizability of twin data.
