It has been reported that humans are a cooperatively breeding species that depends on individuals other than the mother and father for the successful rearing of offspring (Hrdy, 2005a, 2009). These individuals are known as alloparents and may be siblings, aunts, uncles, grandparents, and other kin or nonkin. One of the most thorough studies took place among the Maya of the Yucatan, Mexico. Hardworking girl-farmers paid back to their parents 94% of their cumulative consumption costs before leaving home to marry, and boys repaid 80%, enabling mothers to have more closely spaced births (Kramer, 2005a,b). In other species, alloparents seem to enhance their inclusive fitness by helping (Emlen, 1997a), but it is not yet clear whether this is also the case for Mayan children. In a study on the fishing atoll of Ifaluk in Micronesia, women (n = 7) whose two firstborn children were daughters had a mean of nine surviving offspring, and women (n = 11) who bore two sons first had a mean of five surviving offspring (Turke, 1988). This difference of four offspring was attributed to the role of elder daughters as helpers at the nest and was an enormous effect size for such a small sample (n = 18). Given that the analysis was bivariate, there is a strong possibility that the results reflect confounding variables.
In 19th century Finland, the survival of the maternal grandmother was correlated with improved grandoffspring survival (Lahdenperä et al., 2004). This study controlled for occupation (e.g., farmer, priest, or landless laborer) but did not distinguish among farmers by the size of their holdings; hence, the phenotypic correlation between grandmater-nal survival and grandoffspring survival may be caused by variation in wealth. A recent review (Sear and Mace, 2008) concluded that cooperative breeding was prevalent in traditional farming populations that had high fertility and high mortality, but only 6 of 17 studies controlled for wealth. A meta-analysis showed that the positive association between grandparental and grandchild survival was found only for the maternal and not the paternal side, although in these farming populations, the children were more likely to live with their paternal grandparents (Strassmann and Garrard, 2011).
Even in forager populations, the data on grandparental investment are mixed. In the Hadza of Tanzania, foraging grandmothers worked longer hours gathering tubers than they did in their prime, and children who received food from a grandmother or great aunt had higher nutritional status than children who were without alloparents (Hawkes et al., 1997, 1998). In the !Kung, having four surviving grandparents was not associated with improved grandchild survival or nutritional status (Draper and Howell, 2005).
Here, I use evolutionary social theory to explore the family dynamics of the Dogon of Mali, West Africa. My underlying premise is that