Such considerations demonstrate a central principle of life history organization, namely, to establish optimal trade-offs among competing demands under consistently unpredictable circumstances. Modeling trade-offs is essential for understanding the design or the biological organization of the functions and capacities of the human organism. Life history strategies can be usefully viewed, on the level of the organismic design of a species, as a set of algorithms that negotiate the trade-offs to optimize spatiotemporal allocation of limited resources.
One cardinal rule underlies the ubiquity of trade-offs: the allocation rule. Related to thermodynamic notions of energy conservation, the allocation rule states that consumable resources used for one purpose cannot be used for another. This rule will be critically examined below.
In schematic terms, maintenance may be subtracted from resource intake to determine the net resources available for growth or reproduction, which is labeled productivity. In comparison to productivity estimated from juvenile growth rates in other determinate growers, the productivity of primates appears remarkably low, only 40 percent of that of mammals in general—and the productivity of humans is only 20 percent of that of mammals, because human children grow very slowly between the ages of 5 and 10 (Kaplan et al., 2000). Slow growth affects life history strategy in several ways. It reduces the burden of energetic demands for growth (given the species has relatively low juvenile mortality). This effect may be particularly important for easing the demands on human parents, who provision their young throughout juvenility (Bogin, 1997, 1999). Slow growth may also permit greater investments in the maintenance required to reduce juvenile mortality or to meet the high metabolic costs of large brain size. Finally, low productivity may be the product of increased developmental costs not reflected in growth (e.g., play) but required for attaining adult competence.
Humans are large, long lived, and obligately social primates that have altricial singleton births, spaced at 2 to 5 years, which are first breastfed and then provisioned well into the second decade. Young are carried, tended, defended, instructed, and systematically exposed to experiences or adult-driven ecologies aimed not only at teaching attitudes and techniques for survival but also developing social competence and coherence as well as adult productivity. Reproductive timing is set through distinctive physiological switches that inhibit gonadal function in childhood and activate it in puberty.