Who keeps children alive? A review of the effects of kin on child survival
Introduction
Human life history poses a problem for women: that of raising several dependent children simultaneously. The human birth interval, of about 3 years in natural fertility populations, is out of line with that of other great apes of similar body size. The orangutan, for example, has an interbirth interval of about 8 years, and the chimpanzee 4–5 years (see Galdikas & Wood, 1990, for a review). If human females are capable of such rapid reproduction, most anthropologists now agree that this is due to the support they receive from other family members. Whilst demographers used to argue that children were an economic asset rather than a cost (Cain, 1977, Caldwell, 1978), Kaplan (1994) showed that wealth in families tends to flow down the generations, not up, helping to establish the view that children are especially costly in humans requiring much more parental investment than, for example, do chimpanzees (Kaplan & Lancaster, 2003). The ‘traditional view’ of the family has been that help for the mother comes from the father—hence, the human pair-bond is based on mutual interdependence of husband and wife to raise their children (e.g., Lovejoy, 1981). In hunter–gatherer societies, the division of labour is nearly always such that men bring back meat to the band, whereas women gather. However, the observation that the number of calories brought back from gathered foods often exceeds that from hunting, combined with the fact that meat is often shared widely throughout the band rather than strictly within the nuclear family (Hawkes, 1990, Hawkes et al., 2001, Kaplan & Hill, 1985), has led to the suggestion that women are not as dependent on men to raise their family as once thought (Hawkes, O'Connell, & Blurton Jones, 1997).
If human life history poses a problem for women, then it may also provide the solution. Unusually, human females spend a relatively high proportion of their lives in a nonreproductive state. Both pre- and postreproductive individuals may be available to help mothers in raising offspring as they can do so at relatively little cost to their own reproduction. Grandmothers, in particular, are often proposed as an alternative to male care. If grandmothers are helping to support their daughters' children, then two unusual features of human female life history—menopause and high birthrates—can potentially be explained at once. Both may arise because menopause is an adaptation to enable grandmaternal support, which in turn enables a high human birth rate (Hawkes, O'Connell, Blurton Jones, Alvarez, & Charnov, 1998). Mothers may also use the labour of their older children, particularly daughters, to spread the costs of raising offspring. The extended juvenile period of human young is another unusual characteristic of our species, and the economic contributions of older children may also help to ameliorate the costs of large family size, although this effect has only been shown in agricultural societies (Kramer, 2005, Lee & Kramer, 2002).
How might empirical studies help us to distinguish between the two views of the human family: that the pair-bond with the father is key or that other kin, especially grandmothers, are more important as allocarers? Empirical studies on hunter–gatherer communities are data-limited, due to both the very small number of such societies that survive and the very small number of individuals living in something approaching a hunter–gatherer lifestyle within those societies. This may have contributed to the fact that a consensus view on the relative importance of fathers as compared to grandmothers has not emerged.
The main line of evidence in this debate came from nutritional studies. Hawkes et al. (1997) point out that in the Hadza of Tanzania, children with older female relatives in their band are better nourished, and their data suggest that the hunting season is not actually a particularly good time of year for children (see also Hadley, 2004). Some studies on foraging strategies in the Ache of Paraguay and in the Hadza highlight the fact that total calories and energy return rates from gathering often equal or even exceed those from hunting (Blurton Jones et al., 2000, Hill et al., 1987, Marlowe, 2003). Isotope studies on prehistorical Californians suggest that male and female diets may have been very different (Walker & Deniro, 1986), with males having been living predominantly off marine resources, whereas the females must have been eating food mostly terrestrial in origin—though small sample sizes have not enabled this to be properly tested. But Hill, Kaplan, and others (see, e.g., Gurven & Hill, 1997, Gurven & Kaplan, 2006, Hill, 1993, Kaplan & Lancaster, 2003) have argued that the nature of the food brought back by males is superior and very important, leading them to conclude that the contribution of males to group nutrition is very significant, perhaps around 80% of the calories (though note that an important contribution by males to the diet does not necessarily imply that fathers are directly provisioning their families). As an extreme example, Arctic hunters like the Inuit are almost entirely dependent on hunted food brought in by men. In the coldest areas, babies and young children could barely survive outside for much of the year, and thus, women are dependent on men for almost everything. Marlowe (2003) shows that male provisioning occurs at very important times in the Hadza, such as when a woman's foraging is handicapped because she recently gave birth.
These findings suggest that the ecology of the system influences the relative importance of fathers, grandmothers, and potentially other kin such as siblings or older offspring in the rearing of human children. This should come as no surprise to evolutionary ecologists. The variability in hunter–gatherer ecology further highlights the fact that data from just one type of population cannot answer the question of whether humans are cooperative breeders. We will argue here that it is not necessary or sufficient to restrict our studies to extant hunting and gathering communities, none of which are necessarily cases of special importance in human history. Furthermore, very few hunter–gatherer studies can generate large-enough sample sizes to estimate important determinants of rare events like mortality or low variance measures like fertility. There are a small number of natural fertility and natural mortality populations for which large sets of demographic data are available, some of which are historical populations. These are now being analysed to enhance our understanding of which kin have an influence on the fitness of their descendants. Most of these populations are farmers but with high workloads, high disease burdens, and high reproductive rates. Whilst most of these populations are/were growing rather than stable, the same can be said of contemporary hunter–gatherers populations too. We need to use as much data as is available to us to untangle the full story of the evolutionary ecology of human family life.
Section snippets
Kin effects on child survival in a range of natural fertility/natural mortality populations
There are many studies on the contributions of various relatives to child care, nutrition, and other aspects of development (Hewlett et al., 2000, Hurtado & Hill, 1992, Ivey, 2000) that contribute greatly to our understanding of social networks and child-rearing, but it is not always easy to determine from these studies the extent to which such help enhances the fitness of the beneficiary. In this review, we shall concentrate solely on studies that have examined the effects of kin on one
The importance of mothers
It comes as no surprise that in all 28 populations in which the association between mother's death and child death has been investigated, the death of the mother is clearly associated with higher child mortality (Table 1A, Table 1B). That this effect exists is expected. What we wanted to determine from this analysis was, firstly, how long this association lasted (i.e., is it seen throughout the whole period of childhood, or do mothers only matter to young children?), and secondly, can even
Evolution and the human family
What does this review tell us about the evolution of the human family? Whilst there clearly is a problem using data on current populations to infer anything about evolutionary history, it is all we have. Certainly the study of a single society tells us little about evolution of a particular trait. In the Gambia, we found positive effects of maternal grandmothers and no effect of fathers on child survival, but this does not constitute strong evidence in favour of the importance of older women
Conclusion
We have presented evidence that human children benefit from an extended family and that kin support can enhance female reproductive success. There are several studies focussing on components of reproductive success that further support this view, but we narrowed our discussion here to those that could identify a kin effect on child survival, an unambiguous determinant of reproductive success, so that we could unpick differing influences within the family. This analysis reveals some
Acknowledgments
Thanks to Monique Borgerhoff Mulder, Mhairi Gibson, and two anonymous reviewers for helpful comments on the text. Thanks also to Mhairi Gibson, Krzysztof Tymicki, and Jeff Winking for permission to use unpublished data.
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