Male preference for younger women explains today’s menopausal age.

Abstract

Among several evolutionary explanations for the menopause, the most frequently heard is that female longevity has overtaken the point at which women cease to become fertile. The fixed stock of ovarian follicles has simply depleted long before death, and chronological ageing has progressed more slowly than ovarian ageing. So today, most women in developed countries can expect around 30 years of active life after the menopause, even though that activity excludes the ability to conceive and deliver a baby. And one anthropological reason why that cut-off age has for so long been fixed at around 50 is that women giving birth at an older age would not have the ability to help their children grow and become adults. In terms of evolutionary theory, according to a recent report, this has prompted two explanations for the menopause: trade-offs between prolonged life span and reproduction; and fitness benefits for older, nonreproductive women through increasing the reproductive success of their offspring (the ‘‘grandmother effect’’ whereby older women must not look after their own children but are fit enough to help their grandchildren). This same report, however, has now suggested a third evolutionary explanation for the menopause – that human male ‘‘mating preference’’ for younger women has led to the accumulation of gene mutations which are incompatible with female fertility, and thus to the menopause. As ever, it is the man who gets the blame. The theory was tested in a ‘‘two-sex computational model’’ which showed that ‘‘neither an assumption of pre-existing diminished fertility in older women nor a requirement of benefits derived from older, nonreproducing women assisting younger women in rearing children’’ is necessary to explain the origin of menopause. Instead, this complicated model was based on an evolving population with constant size, without preexisting diminished fertility in females, and incorporating mutations that affected fertility as well as mortality. However, only when a matrix involving male preference for younger females was added to the model did femalespecific mutations with a late age of onset begin to accumulate in the population – otherwise infertilitycausing mutations did not accumulate, fertility and survival remained high, and there was no menopause. Indeed, the model suggested that, if the matrix were to encode female preference for younger males (rather than male preference for younger females), the role of the sexes would be reversed. ‘‘Male menopause never arose because male-specific infertility-causing mutations were subjected to purifying selection and did not accumulate,’’ the investigators propose. Only in women did the fertility mutations arise and accumulate. Bringing this futuristic model back down to earth, one of the investigators, evolutionary geneticist Professor Rama Singh from McMaster University in Canada, told reporters that men choosing younger partners were ‘‘stacking the odds’’ against continued fertility. Conversely, however, he also suggested that the most recent trend for later motherhood might eventually extend the age of menopause, as fertilityassociated gene mutations in older age were absorbed into the gene pool.