The grandmother hypothesis is meant to explain why menopause, rare in mammal species, arose in human evolution, and how late life infertility could actually confer an evolutionary advantage. The hypothesis suggests that this is because of risks associated with pregnancy and childbirth and the relative importance of parental investment to the human species. Grandmotherly investment may also be important in the few other animals which experience menopause, such as whales. Kristen Hawkes originated the hypothesis, and C.G. Williams was the first to posit that menopause may be protective.
Both pregnancy and childbirth are extremely detrimental to the health and longevity of women. Pregnancy increases a woman’s caloric intake requirements and childbirth exposes women, especially older women, to deadly infections. For these reasons physical anthropologists think that older women in primitive times were less productive child bearers than younger women.
It is conceivable that older mothers that lost their fertility were able to spend more of their time helping, protecting and teaching their children and grandchildren. Such an investment of time is referred to by behaviorists as parental investment. Experiments and observation have shown that those animals that have had time invested in them by family members, in the form of protection and education, are much more likely to live to the age at which they are able to reproduce.
On average, prehistoric women who experienced menopause may have lived longer lives and were better able to spend time supporting their children and their grandchildren. The progeny of these menopausal women benefited from additional parental investment and were therefore more likely to live to procreate. These progeny also benefited from inheriting their mother’s genes because these genes caused them to experience menopause and have more prosperous progeny of their own. This rationale is used by anthropologists to provide an evolutionary theory of menopause which holds that menopause in modern women is a remnant of a protective adaptation that allowed older females to better focus their maternal resources.
Alternatively, the debilitating symptoms that usually accompany menopause in Western cultures could be seen as a natural cull of non-reproductive members of a species. Hot flashes, loss of short term memory, decreased ability to concentrate and difficulty in the learning of new tasks would, in the wild, leave the sufferer at greater risk from predators and topographical dangers such as falls from a height. This natural cull would leave more food, usually in relatively short supply, for the reproductive members of a species whose youth could mean that they are less experienced at finding it. However, cross-cultural studies of menopause have found that menopausal symptoms are quite variable among different populations, and that some populations of females do not recognize, and may not even experience, these "symptoms." This high level of variability in menopausal symptoms across populations brings into question the plausibility of menopause as a sort of "culling agent" to eliminate non-reproductive females from competition with younger, fertile members of the species.
The grandmother theory appears to work because species continue to recognize, value, and therefore protect, closely related members of their species who would otherwise fall to predation. Only with this protection is the grandmother available to take a mentoring and supportive role.
Alvarez HP. Grandmother hypothesis and primate life histories. American Journal of Physical Anthropology. 2000 Nov;113(3):435-50. PMID: 11042542
Melby, Melissa K. "Vasomotor symptom prevalence and language of menopause in Japan." Menopause. 12.3 (2005): 250-257.