Dizygotic twinning, the simultaneous birth of siblings when multiple ova are released, is an evolutionary paradox. Twin bearing mothers often have elevated fitness1-5; but despite twinning being heritable6, twin births only occur at low frequencies in human populations7. We resolve this paradox by showing that twinning and non-twinning are not competing strategies, instead dizygotic twinning is the outcome of an adaptive conditional ovulatory strategy of switching from single to double ovulation with increasing age. This conditional strategy when coupled with the well-known decline in fertility as women age, maximizes reproductive success and explains the increase and subsequent decrease in twinning rate with maternal age that is observed across human populations8-10. We show that the most successful ovulatory strategy would be to always double ovulate as an insurance against early fetal loss, but to never bear twins. This finding supports the hypothesis that twinning is a byproduct of selection for double ovulation rather than twinning.

Twins, Conditional strategy, Maternal age