Timing of embryo split may explain sexual orientation in twins

By John Hayman

Same-gender sexual attraction is a feature of human sexuality. Estimates of same-gender proclivity vary from the original Kinsey reports of 10 per cent of adults having predominant or exclusive homosexuality to more recent estimates of 20 per cent having preferred same-sex relationship.

The factors resulting in same-sex sexual orientation remain controversial, but there is growing evidence that sexual orientation is heavily influenced by prenatal biological mechanisms rather than by unidentified factors in postnatal socialisation.

Although there is some kinship association, sexual orientation appears as an intrinsic human characteristic and the direction of orientation to be essentially random in occurrence.

A paradox in our understanding of an inheritable basis of same-sex orientation is sexual orientation in twins.

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