Jones, 1993, and Rogers and Blurton Jones, 1992). One result is that the value of investments in income-related capital depends on the probability of surviving to future ages (Kaplan et al., unpublished work; see also Ben-Porath, 1967).5 If the expected future life span is short, it pays little to invest in future earnings, favoring allocation of resources to current reproduction instead. The corollary is also true. The value of investments in survival depends on expected future income. If income is increasing through time, higher investments in survival are favored (for a related result, see Hamilton, 1966, and Charlesworth, 1994: chapter 5). Another result is that the value of allocations to each form of mortality reduction depends on the probability of dying from other causes. For example, if one is likely to die from predation, it pays less to invest in cell repair and immune function, which would affect future condition and the likelihood of dying from disease. Low probabilities of predation are probably an important determinant of why birds, bats, and primates allocate more resources to maintaining physical condition and senesce at late ages for their body size (Austad and Fischer, 1991, 1992; Kirkwood, 1981; Kirkwood and Rose, 1991), as do some animals living on islands with few predators (Austad, 1993). Lastly, the model also suggests that optimal investments in offspring survival and offspring income are complementary and positively associated (Kaplan, 1996).
These general results can be applied to the specific model of human life-history presented above. The nature of our feeding niche increases the value of both one's own and parental investment in income-related capital. Because many investments in income-related capital do not mature fully until after the age of 25. the value of investments in survival increases in order to realize the dividends from investments in income. The low productivity of hunter-gatherer children probably reflects both investment in skills and investments in survival. For example, boys in many hunting and gathering societies spend a great deal of time hunting very small animals. They achieve very low returns from those activities and could probably acquire more food by collecting. This is evidently an investment in future income. However, it also appears to be the case that adults restrict children to safe places (Draper, 1976; Blurton Jones et al., 1994b). When hunter-gatherers make camps in which they will reside for more than a few days, a great deal of effort is spent clearing the area of all underbrush. This is the safe zone, especially for very young children. Moreover, informants among both the !Kung (ibid.) and the Aché (Kim Hill, personal communication) report that they prefer to leave children in camp rather than to take them foraging. The food the children could collect on those foraging trips is apparently less valuable than the risks and costs associated with taking them along.6 Such effects appear to be ecologically variable, however. The comparative analysis of children's behavior and parental