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In-Law Competition Hypothesis

The hypothesis suggests that the reproductive overlap between a mother and her mother-in-law drove selection for decreased age of fertility. Mothers-in-law will be more related to their offspring than their daughter-in-law’s offspring, and daughter-in-laws will be completely unrelated to their mother-in-law's offspring. Therefore, if they are in conflict for reproductive resources, the daughter-in-law’s offspring has a stronger chance of survival due to his or her higher degree of relatedness. On the other hand, reproductive overlap between a mother and a daughter is not seen as a selective factor for the evolution of menopause. The daughter is equally related to her mother’s offspring, and the mother is half-related to her daughter's offspring. Thus the higher degree of relatedness reduces selection for one offspring over the other. For example, one study determined that when a woman and her mother-in-law have children within two years of each other, infant survivor-ship reduced by 50%, while there was no decline in survivor-ship when women and their own mothers had children simultaneously. Therefore, the In-Law Hypothesis proposes that the degree of relatedness during reproductive overlap between generations of women influenced the evolution of menopause.

Reproductive Conflict Hypothesis

While post-reproductive survival can be selected for to help a woman's own children or grandchildren survive as in the mother and grandmother hypotheses, many evolutionary biologists argue that kin-selected fitness gains do not outweigh the potential gains of continued reproduction. Kin-selected benefits explain post-reproductive survival but not early reproductive senescence. Reproductive senescence coincides with the age at which reproductive competition from younger females would begin. Therefore, the reproductive conflict hypothesis suggests that menopause is the outcome of reproductive competition in generations within social units and could complement the other adaptive hypotheses.

Mother Hypothesis

In an article from 1957 that inspects the effects of pleiotropy and natural selection on senescence, G.C Williams first introduced the idea that menopause may not be the result of the aging syndrome but rather the subject of positive selection. He suggests the “Mother Hypothesis” to explain the mismatch between reproductive and somatic senescence. As women age, their risk of death from childbirth elevates. Williams suggests that menopause decreases the risks associated with childbirth and allows mothers to allocate remaining resources to care for extant offspring. According to his theory, the benefits dedicated towards immediate offspring outweigh the fitness of extended reproduction since a child’s survival relied heavily on his or her mother’s survival. This could help explain the discrepancy with many of our primate relatives. Our pelvic anatomy, which allows for our bipedalism compared to our primate counterparts, increases our risk of mortality during childbirth and represents a unique selective factor for human menopause. One problem with the Mother hypothesis is the age of menopause onset, 45-55 years, is generally past the age of critical parental care.

Additions to Non-Adaptive Hypothesis

One study shows that reproductive timing is conserved with our closest primate relatives. They suggest that menopause results from the evolution of survival as opposed to the evolution of reproduction.

Combination

These hypotheses are not mutually exclusive though. One research group was not satisfied with individual factors alone accounting for this unique aspect of human life history. They provide evidence to support the mother and grandmother hypotheses acting together. The “mother” and “grandmother” hypotheses could act in tandem as they both suggest that selection has favored post-reproductive survival because older women, even if they do not bear more children themselves, can nevertheless gain inclusive fitness by helping their existing offspring or grandchildren to survive. Another hypothesis, the Self-Determination Hypothesis, suggests that the non-adaptive reasons initiated menopause, then adaptive reasons maintained it. Our rapid lifespan extended past our reproductive capacity. Then adaptive pressures such as the grandmother and mother hypotheses stabilized the mismatch. Overall, menopause presents a curious evolutionary phenomenon and is an area of active evolutionary research.