Genetical, environmental and personality factors influencing the age of first sexual intercourse in twins.

A trend towards younger sexual initiation has been reported in recent years sug~ gesting that the trait is currently influenced by cultural factors. It is reasonable to SUspect, however, that in the past the age of first sexual intercourse could have bOOn partly under genetica.J.Coritrol and subject to natural selection. Although it would be premature to speculate about any relationship between age of first intercourse and reproductive fitness, there is little point in attempting to do so before genetical variation has been demonstrated. In this paper we report the findings of a recent twin study of this trait.