Why is lifetime fertility higher in twinning women?

In a recent paper published in Proceedings of the Royal Society B, Robson & Smith (RS) presented an analysis of a database comprising the life histories of more than 50 000 women born in the nineteenth century UT, USA. They addressed the question of whether a woman's likelihood of producing twins is related to overall phenotypic 'quality' [1]. Controlling for a number of confounding factors, they show that, compared with women who do not produce twins in their lifetime, twinners have lower post-reproductive mortality, shorter inter-birth intervals, later ages at last reproduction, longer reproductive lifespan and higher lifetime fertility. They conclude: 'our results strongly support the hypothesis that twinning is an index of phenotypic quality associated with other dimensions of maternal heterogeneity'. We focus here on the question of why women in RS's sample have higher lifetime fertility if they have also produced twins. In this respect, the results of RS's analysis are consistent with those carried out on smaller databases from natural fertility populations [2– 5]. Thus, at first glance it would indeed appear that there is a strong basis to infer that there is some specific aspect of the propensity to produce twins that is biologically linked to lifetime fertility. Indeed, rates of dizygotic twinning (the kind that arises from polyovulation and accounts for most of the inter-societal variation in twinning rates [6]), have been found to be higher in women who conceive more readily [7,8], suggesting that there may indeed be some heterogeneity in fecundability that cov-aries with twinning propensity. However, while we agree that this hypothesis is both plausible and interesting, comparison of the life-history traits of women who have and have not ever produced twins does not allow one to draw any conclusions about the covariation between individual variation in twinning propensity and those traits. This is because the fact that a woman has produced twins during her lifetime is not only a product of her propensity to produce twins, but also of the number of times she has given birth. Even if one assumes that a twin birth is a random event that can happen to all women with equal probability per delivery, one should nonetheless expect that twinning women would have higher lifetime fertility than non-twinning women. Focusing on the number of deliveries rather than children, a simple way to realize this is to reverse the implication of causation by altering the statement …