Twins and motherhood Thrice blessed
Throughout history, twins have provoked mixed feelings. Sometimes they were seen as a curse—an unwanted burden on a family’s resources. Sometimes they were viewed as a blessing, or even as a sign of their father’s superior virility. But if Shannen Robson and Ken Smith, of the University of Utah, are right, twins have more to do with their mother’s sturdy constitution than their father’s sexual power.
At first blush, this sounds an odd idea. After all, bearing and raising twins is taxing, both for the mother and for the children. Any gains from having more than one offspring at a time might be expected to be outweighed by costs like higher infant and maternal mortality rates. On this view, twins are probably an accidental by-product of a natural insurance policy against the risk of losing an embryo early in gestation. That would explain why many more twins are conceived than born, and why those born are so rare.
Dr Robson and Dr Smith, however, think that this account has got things the wrong way round. Although all women face a trade-off between the resources their bodies allocate to reproduction and those reserved for the maintenance of health, robust women can afford more of both than frail ones. And what surer way to signal robustness than by bearing more than one child at a time? In other words, the two researchers conjectured, the mothers of twins will not only display greater overall reproductive success, they will also be healthier than those who give birth only to singletons.
Alas, pinning down evolved relationships between fertility and health is tricky. Modern medicine and the pampering effects of economic growth mean that, these days, women everywhere give birth to fewer children than they did in the distant evolutionary past, when human bodies and physiology were forged—even as more of the offspring they do bear survive into adulthood. In Europe and North America this demographic transition began in earnest around 1870.
To test their theory that the bearers of twins are supermums, Dr Robson and Dr Smith scoured the Utah’s records (contains 1.6m genealogical records for people in the state from the early 1800s to the 1970s) for women born between 1807 and 1899 who survived past the age of 50, when the menopause typically strikes. They excluded those married more than once, those widowed before they were 50. This left them with some 59,000 women, around 4,600 of whom had given birth to twins at least once.
To assess the effects of the demographic transition, the two researchers split the sample into women born before 1870 and those born in or after it. The results confirm their hypothesis. Mothers of twins in the pre-1870 batch gave birth, on average, to 1.9 more children than the mothers of singletons. Those in the post-1870 batch had 2.3 more. More importantly, from an evolutionary point of view, more of their children made it to maturity. The mothers of twins in the pre-1870 batch had 1.24 more children who survived past the age of 18 than did other mothers. After that date the figure was 1.56.
Of course, having twins automatically increases a mother’s lifetime fertility by one, relative to her number of pregnancies. But, since few women have twins more than once, the data imply that mothers of twins are more productive, even ignoring that effect. Moreover, they also outperformed the mothers of singletons on a range of other reproductive measures, including intervals between births (shorter) and the age at which they gave birth to their last child (older).
Most strikingly, the pre-1870 group of mothers of twins actually lived longer than those who were not thus blessed. They were, in other words, simply healthier. This suggests that, in evolutionary terms at least, twins are far from a curse. On the contrary, they are indeed the mark of a supermum. (645 words)