Are all grandmothers equal? A review and a preliminary test of the “grandmother hypothesis” in Tokugawa Japan

Abstract

An unresolved question arising from human evolutionary research relates to the function of the postreproductive period in human females. If menopause is not merely an artifact resulting from the benefits of civilization, there must be an adaptive mechanism favoring the offspring of women who continue to thrive well past the time of their last ovulation. The "grandmother hypothesis" was developed on the basis of the original suggestion by Williams (1957 Evolution 11:32-39) that "stopping early" would benefit already-born children. This idea, combined with the concepts of kin selection (Hamilton 1964 J Theor Biol 7:1-52) and parental investment (Trivers 1972 Sexual Selection and the Descent of Man, Chicago: Aldine, p. 136-179), was expanded to suggest that postreproductive women (in contrast to males) contribute to their inclusive fitness by extending support to their grandchildren. We used discrete time event history analysis (Allison [1984] Event History Analysis, Newbury Park: Sage; Allison [1995] Survival Analysis, Cary, NC: SAS Institute) and logistic regression on data provided in population registers (Shūmon Aratame Chō, or SAC) from a village in central Japan, covering the period from 1671-1871, in a preliminary investigation of the effects of household grandparental presence on the probability of a child's death. We found that after accounting for the presence of other household members, the only grandparent whose presence exerted a consistent negative effect on the likelihood of a child's death was the mother's mother. Due to the small sample size of households that contained maternal grandmothers, these results failed to achieve statistical significance. Their importance, however, is in what they suggest about future research, i.e., census data from preindustrial societies can provide a basis for testing evolutionary proposals, including the "grandmother hypothesis."

Grandma plays favourites: X-chromosome relatedness and sex-specific childhood mortality

“…To date, only two previous studies known to the authors have distinguished between boys and girls when investigating the effect of both grandmothers. A study of a Japanese population found that the presence of a PGM had a negative effect on boys and a positive effect on girls, while the presence of an MGM had a positive effect on children of both sexes, especially on boys (Jamison et al 2002). They point to cultural features of the society as a possible mechanism for these findings.…”

Intergenerational relations and child development in England

“…In pre-modern and traditional populations the presence of grandmothers are found to improve child survival rates (e.g. Jamison et al 2002;Ragsdale 2004;Lahdenperä et al 2004;Sear et al 2000;2003;Voland and Beise 2002), while grandfathers are found to have much less beneficial or even unbeneficial impact on child survival (e.g. Dong et al 2016;Lahdenperä et al 2007).…”

Does grandparental help mediate the relationship between kin presence and fertility?

“…It may, conversely, underestimate kin effects if after a death, other kin step in to provide necessary help. A popular alternative measure of kin availability is co-residence with kin, sometimes measured by postnuptial residence (see examples: Snopkowski and Sear 2013;Thornton et al 1986;Skinner 2004;Jamison et al 2002;Tsay and Chu 2005). This measure may be biased if individuals who live with kin after marriage are systematically different from those who do not.…”