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Whereas the majority of research on adolescent sexual initiation has focused solely on environmental factors, the present study used behavioral genetic analyses to investigate the relative contributions of genetic and environmental influences. Structural equation models were fitted to data from adoptive and non-adoptive sibling pairs (231 biologically related pairs and 169 unrelated pairs) from the Colorado Adoption Project. Information from censored individuals who had not yet experienced sexual initiation was maximized by adapting the twin survival analysis method of Pickles et al. (Behav Genet 24(5):457-468, 1994) to accommodate adoptive and non-adoptive siblings. Point estimates of variance components from an ACE model, including additive genetic (A), shared environmental (C), and non-shared environmental (E) influences were 28%, 24%, and 48%, respectively. Despite the lower point estimate for shared environmental effects than additive genetic effects, a CE model provided the best fit to the data. However, because adoptive siblings provide a direct estimate of shared environmental influences there is greater power to detect shared environmental effects in adoption designs. Evidence for genetic influences from our data were somewhat lower than those obtained in previous twin studies, possibly reflecting a return to more socially conservative sexual attitudes, changing sexual behaviors, or ambiguities in the wording of questions commonly used in research on adolescent sexuality.

Heart rate was recorded on 210 MZ and 174 DZ same sex twin pairs participating in the MacArthur Longitudinal Twin Study (MALTS) at age 14, 20, 24, 36 months and 7 years. Heart rate was monitored in the laboratory at all ages. At ages 14 to 36 months, heart rate was monitored prior to a set of cognitive tasks. At age 7 years heart rate was recorded during a mood-eliciting videotaped presentation. At this age only heart rate monitored during neutral portions of the presentation were used. Mean heart rate declines substantially across this age range, but is similar in boys and girls and for MZ and DZ twins at each age. Heart rate is moderately correlated across all time points suggesting that individual differences in heart rate are relatively stable over this age range. Multivariate genetic and environmental models were fitted to the raw data. In general, genetic factors contribute to the stability of individual differences over time. Shared and non-shared environment factors tended to be occasion specific, with non-shared environment contributing substantially to the individual variation at each age. Shared environment and non-shared environment also contributed a modicum to the stability across time. Thus, individual differences in resting heart rate is a relatively stable, heritable trait from infancy to early childhood.

The adolescent substance abuse (ASA) study collected information concerning drug use and psychopathology on male adolescent probands in treatment for substance abuse and also on matched control adolescents, as well as all available family members of both groups. Information was obtained through direct interview and the family history method of assessment. Both methods revealed greater alcohol and drug use, conduct disorder (CD) and antisocial personality disorder (ASP) in the relatives of treatment probands as compared with control relatives. These results suggest familial transmission, not only for alcohol abuse, but also for non-alcohol substance abuse. Familial transmission for CD and ASP is also evident for both male and female relatives, although the prevalence of these disorders is significantly greater in males than females.