Coall & Hertwig (C&H) offer a comprehensive survey of literature pertaining to grandparental “altruism” and call for an integration of disparate perspectives. Although the scope and coherence of a unified theoretical approach are not clearly defined, the authors are to be commended for raising important issues. We argue that evolutionary and rational actor perspectives could be expanded to provide a framework that encompasses both ultimate and proximate-level explanations.

Highly encephalized brains, slow growth, and long lives are derived features of human life history, with juvenile dependency, complex skill development, and grandparenting as key components. If the function of post-reproductive lifespan is to improve fitness of descendant kin, a wide range of cognitive and behavioral traits that focus attention on perceiving and responding to needs of particular kin is expected. Emotions, like motivations, could further modulate behaviors that either benefit or burden particular kin. Psychological studies of wisdom among older adults in modern societies (Baltes et al. Reference Baltes, Smith, Staudinger and Sonderegger1992) and of kin-favoring dispositions despite age-related physical decline (Carstensen & Lockenhoff Reference Carstensen and Lockenhoff2003) are consistent with an evolutionary perspective. Norms and institutions might help facilitate delivery of benefits, even when co-residence is unlikely, as codified in inheritance rules. Norms and institutions are considered features of the sociological domain and emotions as part of psychology, yet evolutionary theory and economics are required to make sense of why norms, institutions, and emotions occur in particular forms and expressions. The evolutionary study of emotions and norms is a rich industry.

Evolution has led to a long human lifespan with a substantial post-reproductive phase, yet, despite the adaptive value that grandparenthood must have provided our ancestors, the authors point out a conundrum: Grandparents in the past overlapped with grandchildren for a brief period but with large fitness impact, whereas longer-living grandparents today have more overlap and thereby greater potential to help, but few grandchildren. As a consequence, grandparents in the past increased fitness by reducing infant mortality, but today mostly have only “soft” impacts on well-being and cognition. We feel that (1) the contrast made between past and present opportunity is overstated, and (2) differences in investment patterns depend on marginal benefits of grandparental help, which varies among societies based on differences in fertility, production patterns, co-residence, and inheritance.

Contrary to the statement that grandparental opportunity is strongest today, evidence suggests that the opportunity to help grandchildren was higher among our hunter-gatherer ancestors. First, while mortality and fertility are lower today, age at first marriage is also much later, and so Westerners become grandparents about 12 years later on average than do hunter-gatherers. Thus, the average number of years lived as a grandparent may not be very different between groups (Table 1). Second, hunter-gatherers are more likely to be co-resident with grandchildren and the total number of grandchildren to potentially impact is higher (fertility of hunter-gatherers is 4–8 births).

Table 1. Demographic parameters for hunter-gatherers and modern populations

Data Sources: Hunter-gatherers: Gurven and Kaplan (Reference Gurven and Kaplan2007), Hewlett (Reference Hewlett1991); Spain: Instituto Valenciano de Investigaciones Económicas (Goehrlich, http://www.ivie.es); United States: National Center for Health Statistics (http://www.cdc.gov/nchs/data/nvsr/nvsr57/nvsr57_14.pdf).

Third, support for the idea that grandparents in traditional societies increase fitness has relied on historical demographic datasets to measure the impact of their presence on early life mortality. Anthropological studies of grandparental contributions focus primarily on food production of older adults. To our knowledge, caloric production (or any other grandparental behavior) has yet to be causally linked to child welfare in any of these studies. Despite the popularity and importance of the Grandmother Hypothesis and alternate explanations of post-menopausal lifespan, all studies of grandparental impacts on kin fitness are indirect, based on whether a grandparent was alive or dead, or in rare cases, co-resident, in a given year. To what extent is the early weaning of infants, higher infant and child survivorship, and earlier reproduction, influenced by grandparents? Until these pathways are studied, phenotypic correlations may confound any observed positive relationship between living grandparents and kin survivorship or fertility. Without an understanding of the proximate mechanisms by which grandparents likely improved kin welfare, detailed predictions about what grandparents should be doing today (and whether their behavior is maladaptive) are difficult to make.

Fitness is impacted by accumulating and transferring material, embodied, and relational wealth, and societies vary in the extent to which each of these is inherited and needed for cultural and biological “success” (Borgerhoff Mulder et al. Reference Borgerhoff Mulder, Bowles, Hertz, Bell, Beise, Clark, Fazzio, Gurven, Hill, Hooper, Irons, Kaplan, Leonetti, Low, Marlowe, McElreath, Naidu, Nolin, Piraino, Quinlan, Schniter, Sear, Shenk, Smith, von Rueden and Wiessner2009). Grandparents should facultatively adjust their aid behavior where they can have the highest marginal benefit at lowest personal cost. Whether in small-scale societies or modern post-industrial ones, we suspect that the greatest impact of grandparents may be realized during rare, but fitness-relevant, periods. The authors describe postpartum depression and teenage pregnancy in modern societies as examples. We mention a few others here based on ten years of fieldwork among Tsimane forager-horticulturalists of Bolivia. Tsimane grandparents are often primary caretakers when parents die: 17% of adult Tsimane interviewees had a parent die before age 18, and 19% of these went to live with a grandparent (9.8% with maternal, 9.0% with paternal). Even when not holding leadership positions, older adults mediate conflicts between different kin factions, which helps to promote coordination in activities so as to more efficiently reap gains from divisions of labor and economies of scale. While the “soft” impact of grandparents in traditional societies has not been described, we suspect that further inquiry may reveal that the marginal impact of grandparents is not primarily in the form of calories. Grandparents are named as important transmitters of Tsimane skills and knowledge (Gurven & Kaplan Reference Gurven, Kaplan and Sokolovsky2008; Schniter Reference Schniter2009); they account for 8% of identified contributors to early-life skill acquisition. They are twice as likely to be named for rare but important skills, such as making pottery, punishing bad behavior, singing traditional songs, and telling old stories and myths.

Finally, an evolutionary perspective emphasizes not only grandparents' cumulative fitness impacts, but also the increasing costs on descendants with age. Few hunter-gatherers and horticulturalists live beyond the seventh decade of life. Among Tsimane, we observed that grandparents in their 70's no longer make net-transfers of food to grandchildren. Whether the complementary contributions listed above are sufficient to slow the decline in utility is an open question, but we suspect that net utility is negative by the late 70's. In pre-industrial societies where production is costly and resource competition is high, geronticide and neglect are commonly practiced (Maxwell et al. Reference Maxwell, Silverman, Maxwell and Sokolovsky1984). Elderly populations today, whose knowledge and traditions may be devalued, given rapid cultural change, show increasing evidence of neglect and abuse (Lachs & Pillemer Reference Lachs and Pillemer2004). Intervention programs that focus on the marginal benefits grandparents can offer may be helpful for strengthening familial care networks and building communities (Denham & Smith Reference Denham and Smith1989). The total value of grandparents as fallback caretakers, educators, mediators, storytellers, and as sources of wisdom is too important as social insurance to risk losing, even in modern societies.