Genetic and environmental contributions to variance in age at first sexual intercourse

Little is known about the relative importance of genetic and environmental factors as determinants of age at first sexual intercourse. In this study, subjects were 5,080 individuals from the Australian Twin Registry (3,310 females, 1,770 males; age range: 27-70 years, median: 40 years) who completed a semistructured interview by telephone in 1992-1993. Self-reported age at first intercourse correlated higher for identical (monozygotic) twins than for nonidentical (dizygotic) twins. Structural equation model fitting found that the genetic contribution to variance was considerably greater among twins aged 40 years or less (72% for males and 49% for females) than for those aged from 41 to 70 years (0% for males and 32% for females). Among the older cohort, there was evidence that somewhat different aspects of the shared social environment influenced age at onset in males and females. In a more laissez-faire social climate in recent decades, it is likely that biological and psychological characteristics that are partly under genetic control significantly influence the age at which a person commences sexual activity.