In Part I, I reported Hadza population parameters, the central tendencies of fertility, mortality, age structure, and population growth. My focus was on doing this to a knowable level of accuracy, and independently from model populations such as those of Coale and Demeny (1983) and Weiss (1973). I wanted to allow the Hadza to be as different as they might be from other known populations, to be able to assess the degree of accuracy of my estimates, and to look for evidence about change during the second half of the twentieth century. I answered my main question: have we got the main parameters right? With “Yes. Measures collected independently of each other, yet linked in a stable population, fit well enough with predictions derived from our estimates of fertility and mortality that we are unlikely to be wildly wrong.” Details of Hadza demography support Marlowe's (2010, chapter 10) characterization of them as “the median foragers,” and their demography is close to that of many historical and third-world populations. A normal human demography can be supported by hunting and gathering in sub-Saharan savanna. I finished by discussing the puzzles of hunter-gatherer population dynamics. Simulations suggested that, in a population where helpers were important, the effects of sporadic population crashes could be magnified.

In the second part of this book, I leave the anthropological version of classical demography and try to use the variation in our data, looking at individual differences in the whole population, to address issues our team has long pondered. Issues such as: Are grandmothers effective helpers? How big are the effects of helpers? Do inter-birth intervals influence child mortality? Does the reproductive system successfully manage the allocation of resources between fertility and childcare? Is big game hunting a paternal investment, the way men help their children survive, or a display, a way to gather good neighbors, or what? What are the origins of marriage?

This takes me into another group of disciplines, evolutionary ecology, behavioral ecology, and sociobiology, which have successfully been applied to studies of people in a variety of contexts. Excellent introductory accounts of behavioral ecology in biology are by Davies et al. (2012) and Krebs and Davies in Behavioural Ecology: An Evolutionary Approach (1978, and subsequent editions), which first included a chapter on humans in its 1991 edition (Borgerhoff Mulder, 1991).