Grandparental assistance in helping to raise their children's young is common in many industrialized and traditional societies today, and is hypothesized to be important in the evolution of modern life histories (Hawkes & Paine Reference Hawkes and Paine2006; Hawkes et al. Reference Hawkes, O'Connell and Blurton Jones1997). Coall & Hertwig (C&H) make a compelling case, both for integrating evolutionary perspectives on grandparenting with economic and sociological views, and for moving evolutionary models forward to explain grandparenting under post-demographic transition conditions. In doing so, they raise a number of salient points about grandparenting specifically and allocare in general.

Humans have been characterized as cooperative breeders, a reproductive strategy in which mothers routinely rely on the help of nonparental individuals to raise young. Because allomothers divert energy and resources to another's reproductive success, it presents an evolutionary puzzle. In addressing this, much of the cooperative breeding literature has centered on the question of why helpers help. C&H evaluate grandparenting and its altruistic basis in industrialized populations. My comment focuses on this question in traditional human societies.

Hamilton's Rule articulates the conditions under which altruism and cooperation might evolve and provides a heuristic means to view allocare as an adaptive behavior. While Hamilton's Rule has broad appeal as a framework to understand cooperative breeding, the challenge has been to operationalize measures of costs and benefits. Recent research has questioned whether the cost of helping may be overstated (Clutton-Brock Reference Clutton-Brock2002), and whether kin-biased behaviors may be motivated by factors other than, or in addition to, kin selection (de Waal Reference de Waal2008; Silk Reference Silk, Kappeler and van Schaik2004). These reevaluations are particularly prescient for humans living in subsistence economies where the provisioning of young occurs in the context of intergenerational transfers.

Because fitness payoffs are often time-delayed, C&H point out that kin selection per se may be insufficient to motivate helping behaviors (de Waal Reference de Waal2008). In contrast to the psychological mechanisms overviewed in the target article, an alternative mechanism may lie in intergenerational transfers of food and labor that characterizes subsistence economics. The human diet consists of a wide variety of foods, most of which require processing and specialized technology. In all traditional societies, resources and labor are pooled and transferred intergenerationally, but also bidirectionally (Kramer Reference Kramer2005b; Lee & Kramer Reference Lee and Kramer2002). These transfers flow downward from older to younger generations, but also upward from children to adults. Rarely does any individual of any age do all of the tasks necessary to grow, survive, and reproduce. This economic interdependence affects helping behaviors in two ways. Children growing up in transitional economies are not as costly as they are often characterized, and bidirectional transfers link older and younger generations in an economic relationship that minimizes the opportunity cost to help.

Because dependency of human young extends into juvenility, the energetic burden of supporting children often is assumed to increase in step. While children growing up in industrialized societies are “gas guzzlers,” they are not so in many traditional societies (Bliege Bird & Bird Reference Bliege Bird and Bird2002; Blurton Jones et al. Reference Blurton Jones, Hawkes, O'Connell, Segal, Weisfeld and Weisfeld1997; Cain Reference Cain1977; Kramer Reference Kramer2005b; Nag et al. Reference Nag, White and Peet1978). Juveniles have a complex relationship with their caretakers. They may not have the skills and body strength to perform all of the tasks necessary to survive, but in most traditional societies children provide some portion of what they need. For example, Hadza children provide 50% of their caloric requirements by the age of five during some seasons (Blurton Jones et al. Reference Blurton Jones, Hawkes, O'Connell, Standen and Foley1989). When subsistence work is defined to include processing and household tasks, Maya girls produce 50% of what they consume by the age of six (Robinson Sullivan et al. Reference Robinson Sullivan, Lee and Kramer2008). But juveniles are also able to overproduce at some tasks – fetching water, harvesting, fishing, collecting shellfish, and foraging for fruit and nuts are good examples. At these tasks, children may meet not only their own consumption, but their surplus production contributes to the “common pot” (sensu Stecklov Reference Stecklov1999) and is redistributed among members of the sharing group. Although a greater cross-cultural sample is needed, where bidirectional transfers have been quantified, children's subsistence effort and upward transfers allow mothers to raise more children than they otherwise could provision on their own (Kramer Reference Kramer2005a; Lee & Kramer Reference Lee and Kramer2002). In societies where children make economic contributions, not only do children underwrite part of their cost, but others, including older individuals, benefit from their transfers as well.

Older adults in many traditional forager and agricultural societies produce surpluses that exceed the level of their own consumption. Although they are net producers (Hawkes et al. Reference Hawkes, O'Connell, Blurton Jones, Standen and Foley1989; Kaplan Reference Kaplan1994), they too rely on exchanges from others, including younger generations. In subsistence forager and agricultural economies where the time and effort spent in allo-provisioning is embedded in self-provisioning tasks (e.g., the time a grandmother spends collecting roots or weaving a hammock that she shares with a grandchild), the cost to subsidize grandchildren may be relatively low. Although the intergenerational transfer literature has focused on unidirectional flows and net costs, subsistence economies function in a more nuanced and bidirectional manner. Grandparental helping behaviors such as childcare are clear examples of altruistic allocare – activities that benefit another but not the helper – and may be explained by kin selection. However, in humans much of what helpers provide is food and other resources to juveniles. Because provisioning juveniles occurs in the context of general food sharing and labor cooperation and helpers also receive resources from others including those they help, it introduces potential mutualistic benefits and other motivations..

In natural fertility populations, the help mothers receive has been associated with younger ages at first birth, shorter birth intervals, and higher offspring survival (Flinn Reference Flinn, Betzig, Borgerhoff Mulder and Turke1988; Hawkes et al. Reference Hawkes, O'Connell and Blurton Jones1997; Kramer Reference Kramer and Alvard2004; Turke Reference Turke, Betzig, Borgerhoff Mulder and Turke1988). Assistance, which traditionally came from grandparents and others, is today further augmented by governments and institutions. In many natural fertility populations today, through external sources of nutritional and medical assistance child survival has increased concomitant with a rise in birth rates (Dyson & Murphy Reference Dyson and Murphy1985; Kramer & Greaves Reference Kramer and Greaves2007; Kramer & McMillan Reference Kramer and McMillan2006; Romaniuk Reference Romaniuk1980). In post-demographic transition populations childbearing occurs much less often in a kin-based context. As generational time lengthens and families are more prone to geographic dispersion, childrearing support networks often diminish or evaporate. Even though families are small, help from others often is still essential, but support shifts to nonkin-based assistance, paid childcare or institutional assistance. An interesting outcome is that, although mothers make allocation decisions and rely on help, the central evolutionary question about why helpers help is no longer pertinent if caretakers are paid or otherwise compensated. In post-demographic transition populations, while traditional grandparental roles may be supplanted, C&H point out that grandparents continue to have an important effect on the fertility decisions for working mothers and on their grandchildren's well-being.