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Does polyovulation counterbalance a high abortion rate in humans?

We tested the fertilization insurance hypothesis of human twinning, which suggests that twin births should be seen as an unadaptive consequence of polyovulation compensating for high spontaneous abortion rates in humans. Females with high incidence of polyovulation were predicted to benefit by becoming pregnant more rapidly and by having a higher likelihood of successful pregnancy than other females. As a disadvantage, polyovulating mothers should be more prone to presumably maladaptive twin deliveries. We studied birth intervals of twin and singleton mothers living in pre‐industrial Finland by using demographic records of local churches. The analysis revealed that, contrary to the prediction of the fertilization insurance hypothesis, polyovulating females did not display shorter intervals between deliveries. Therefore, if the tendency towards polyovulation and therefore occasional twin births in humans is adaptive and maintained by natural selection, other benefits should be evident.