Age at first intercourse in twins reared apart: Genetic influence and life history events

The first analysis of age at first intercourse in monozygotic (MZ) and dizygotic (DZ) reared apart twins was conducted. Data were gathered for 34 MZA twin pairs, 25 DZA twin pairs and 23 individual twins, via a comprehensive Sexual Life History Assessment. Genetic influence on age at first intercourse was indicated by higher MZA (ri = .34, p < .05) than DZA (ri = −.22, ns) intraclass correlations. Associations among age at first intercourse, selected life history events and rearing family relations for female twins were also examined. Life history events included age at menarche, the interval between menarche and first intercourse, as well as age at first kiss, marriage and childbirth. These associations were assessed with reference to parental investment theory and child development theory. The ages at first kiss, marriage and childbirth, as well as the menarche–intercourse interval, for female twins were significantly associated with age at first intercourse. Twins engaging in earlier intercourse felt less happy than their peers and less fulfilled in their rearing home than those engaging in later intercourse. Age at menarche and most rearing family variables showed no association with age at first intercourse.