"is the study of . The primary foci of demography are rates and levels of , , and and how these all interact to produce population growth (or decline), density, and age- and sex-structures; how these rates or levels vary across time and space and what produces such variation; and what consequences these have on other aspects of human (or nonhuman) existence. These demographic phenomena lie at the very heart of . occurs as a result of differential fertility and mortality within a population; gene flow occurs because of migration between populations; and the effects of genetic drift are dependent upon population size, which is an outcome of the interactions among mortality, fertility, and migration (Gage, DeWitte, & Wood, 2012). These demographic forces also affect, are affected by, and reflect many of the things that anthropologists find most interesting. For example, the age–sex structure of a population influences the population’s ratio of consumers to producers and numbers of potential marriage partners, and thus places limits on such things as subsistence strategies and household structure."