The study found that twins differed significantly in their age at first sexual intercourse, indicating that environmental factors that were not shared were the strongest influence on the timing of first sexual intercourse. Thirty-one percent of the variance in age at first sexual intercourse was due to additive genetic influences and 10 percent to shared environmental influences. The remaining 59 percent of variance in age at first sexual intercourse was due to environmental influences that were not shared. This indicates that even identical twins differ significantly in the timing of their first sexual intercourse. The twin pairs who, on average, had earlier ages at first sexual intercourse also had higher levels of delinquency in early adulthood, on average; however, once the research controlled for between-family differences, the twins who had an earlier age at first sexual intercourse had lower levels of delinquency. Analyses were restricted to same-sex twins in order to prevent bias in estimates of genetic influence due to monozygotic twins necessarily being identical for sex. At each assessment wave, participants reported whether they had ever had vaginal intercourse and their age at first intercourse. At Wave I, participants were asked how often in the last 12 months they had engaged in each of 15 delinquent behaviors. At Wave III, the frequency of delinquent acts was assessed with a 19-item scale. The analytical procedures used are described. 1 table, 2 figures, and 71 references