Menopause is the cessation of a woman's menstrual periods, demarcating the end of her childbearing years. In the U.S., women experience menopause at the average age of 51, according the the Mayo Clinic.

While menopause is a natural process, evolutionary biologists have tried to explain alternative social and evolutionary reasons for its occurrence, especially given that men's reproductive abilities continue as they get older, but women's don't.

This ties into the theory of natural selection, the mechanism by which biological traits are kept in the gene pool because certain individuals who are more fit for reproducing than others. Those without favorable traits are often infertile and unable to reproduce.

Or, menopause could be socially constructed. Rama Singh, Ph.D. and evolutionary geneticist, has found that over time, competition among men of all ages for younger mates has left older females with a reduced chance of reproducing. The forces of natural selection, Singh said, are concerned only with the survival of the species through individual fitness, so they protect fertility in women while they are most likely to reproduce.

This does not mean that infertility is necessarily selected for by evolution, but rather, women who are more fertile are likely to be sought by men of all ages, instead of women unlikely to reproduce.

"How do you evolve infertility? It is contrary to the whole notion of natural selection. Natural selection selects for fertility, for reproduction — not for stopping it," he said.

There is some debate over whether people choose their mates based on the other person's perceived ability to reproduce. This ties into natural selection — those who are likely to reproduce and have viable offspring are selected for by others in the gene pool.

This new theory, which states that men of all ages show preference for younger women, suggests that men select against fertility in older women. If men are choosing younger women, an older woman's lack of reproduction in her later years could lead to menopause.

"This theory says that natural selection doesn't have to do anything," Singh said. "If women were reproducing all along, and there were no preference against older women, women would be reproducing like men are for their whole lives."

However, this is not the case; it is likely that keeping reproductive machinery viable in older women simply deteriorates their health or is otherwise unfavorable. The March of Dimes, an organization dedicated to the birth of healthy children, states that women over the age of 35 who try to carry children to term have higher risks of premature delivery, stillbirth, and miscarriage.

Singh points out that if women had historically been the ones to select younger mates, the situation would have been reversed, with men losing fertility. A 2010 study, cited by the Mayo Clinic, suggests an increased likelihood of childhood development of autism born to a couple in which the father is older than 40 and the mother is younger than 30. Similarly, in a 2009 study, children born to older men scored lower on tests measuring memory, reading, and reasoning skills through age seven. Each situation comes with its own drawbacks and risks.

Singh's evolutionary evidence is clear: mate choice is a serious factor in the cessation of fertility and the evolution of menopause.

Source: Morton RA, Stone JR, Singh RS. Mate Choice and The Origin of Menopause. PLoS Comput Biol. 2013.