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.
One way round this is to look at records gathered before the demographic transition began. The problem is that few authorities then kept tabs diligently enough to provide adequate fodder for statistical analysis. One exception was Utah, a state largely settled in its early days by Mormons, who have always taken such matters seriously. As a result, Utah’s Pedigree and Population Resource, a database of which Dr Smith is the director, contains 1.6m genealogical records for people in the state from the early 1800s to the 1970s.
To test their theory that the bearers of twins are supermums, Dr Robson and Dr Smith scoured the records 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 and also (this being Utah) those wives who were part of one of the polygamous marriages once legal among the state’s Mormon settlers. (In a study published earlier this year, Dr Smith and his colleagues found that the more wives a Mormon woman had to compete with for her husband’s attentions, the fewer children she was likely to produce.) This left them with some 59,000 women, around 4,600 of whom had given birth to twins at least once.