Genetic analysis of orgasmic function in twins and siblings does not support the by-product theory of female orgasm

The evolutionary basis of human female orgasm has been subject to furious scientific debate, which has recently intensified. Many adaptive explanations have been proposed, invoking functions from pair bonding and mate selection to sucking up sperm, but these have been attacked as being based on flawed logic and/or evidence. The popular alternative theory is that female orgasm is not adaptive and is only evolutionarily maintained as a by-product of ongoing selection on the male orgasm–ejaculation system. This theory has not been adequately tested. We tested one of its central tenets: that selection pressure on the male orgasm is partially transmitted to the female via a positive cross-sex correlation in orgasmic function (susceptibility to orgasm in response to sexual stimulation). Using questionnaire data from over 10 000 Finnish twins and siblings, we found significant genetic variation in both male and female orgasmic function, but no significant correlation between opposite-sex twins and siblings. This suggests that different genetic factors underlie male and female orgasmic function and that selection pressures on male orgasmic function do not act substantively on female orgasmic function. These results challenge the by-product theory of female orgasm.