A model explaining the matrilateral bias in alloparental investment

Abstract

Maternal grandmothers invest more in childcare than paternal grandmothers. This bias is large where the expression of preferences is unconstrained by residential and lineage norms, and is detectable even where marriage removes women from their natal families. We maintain that the standard evolutionary explanation, paternity uncertainty, is incomplete, and present an expanded model incorporating effects of alloparents on the mother as well as on her children. Alloparenting lightens a mother's load and increases her residual nepotistic value: her expected fitness from later investments in personal reproduction and in her natal relatives. The mother's mother derives fitness from all such investments, whereas her mother-in-law gains only from further investment in children sired by her son, and thus has less incentive to assist the mother even if paternity is certain. This logic extends to kin other than grandmothers. We generate several hypotheses for future research.

Matrilateral Bias in Human Grandmothering

“…If paternity uncertainty and the differential fitness returns from boosting the NV of relatives versus in-laws are indeed the evolutionary engines of the matrilateral bias, as Perry and Daly (2017) have proposed, their relevance is not limited to grandmothers. The logic extends to matrilateral versus patrilateral kin in general, and there is already considerable evidence for matrilateral investment biases by grandfathers (many of the studies cited in Section "Grandmothering in Modern Democracies"), aunts and uncles (e.g., Gaulin et al, 1997;Perry, 2017a), and cousins (e.g., Jeon and Buss, 2007).…”

“…However, analyses of "load lightening" and its effects in birds have not incorporated the possibility that the assisted breeders might then provide help to kin of the original helpers, perhaps because helping is a pre-reproductive life stage in most cooperatively breeding birds and collateral nepotism on the part of those who have already attained the status of breeders is rare. The human life course, however, is very different from that of birds, with plenty of opportunity for former reproductives to act as helpers later, and this eventual nepotistic payback probably played a significant role in the evolution of the matrilateral bias (Perry and Daly, 2017), a possibility reinforced by recent evidence that misattributed paternity is much rarer than previously supposed (Anderson, 2006;Larmuseau et al, 2016;Sear, 2016).…”

“…Recently, Perry and Daly (2017) have identified a further selective basis for a matrilateral bias in alloparenting. Because the costs of parenting are paid primarily by mothers, not fathers, in all societies and alloparents alleviate that burden (Hrdy, 2009), their help constitutes a de facto investment in the mother as well as her children.…”

Interdisciplinary perspectives on grandparental investment: a journey towards causality

“…This assumption can explain why maternal grandmothers, certain of their relationships with their daughters and their daughters' relationship with their grandchildren, invest more than paternal grandfathers (see Smith, 1987). The fact that these patterns of grandparental investment may be confined to industrialised societies and are not always present in rural (Kaptijn, Thomese, Liefbroer, & Silverstein, 2013;Pashos, 2000) and more traditional populations (Snopkowski & Sear, 2015) means there is some question about the actual impact of paternity uncertainty alone (Perry & Daly, 2017), perhaps because paternity uncertainty varies between populations (Anderson, 2006;Sear, 2016). The recognition that paternity uncertainty is typically low in human populations (Anderson, 2006) has led to the recent development of additional evolutionary arguments to explain grandmaternal bias: it may be that care allocated to the mother will translate into greater fitness benefits for the maternal kin because it enhances the reproductive success of both mother and child, whereas paternal kin only gain fitness benefits through the child (Perry & Daly, 2017).…”

Do mothers also “manipulate” grandparental care?

“…Paternity uncertainty has recently been included in a larger model emphasizing parent's and alloparent's benefits from investing in the mother as well as in her children [48]. That is, as mothers most often are prime parental caregivers, parenting targeted towards mothers rather than towards fathers gives the highest payoff in residual nepotistic value and, in turn, fitness.…”