The scientific study of human populations

Demography is commonly defined as “the scientific study of human populations, primarily with respect to their size, their structure and their development.” This definition brings out three essential aspects of the discipline. First, its primary concern is with aggregate populations, large groups of people historically bounded by geography and time; a “population” is the basic unit of study. Second, the mode of study is “scientific” in that demographers seek to understand a population objectively, above all through statistics that describe it. Third, a demographic description of a population focuses especially on the numerical size of a population, its composition, and its change over time. What demographers isolate are objective explanations for gross population trends.

There are two main demographic approaches. First, we may look at a given population, like that of the Roman Egypt in the first to third centuries AD, as an entirety, and ask, for instance, how many persons this population had, whether it was growing, how it was geographically distributed, and so on. Second, we may instead study the more basic demographic functions that bring about population change: mortality, the rate at which persons leave the population by dying; fertility, the rate at which persons enter the population through birth; and migration, the rate at which persons enter or leave the population by physically relocating.

The second approach is more interesting because it incidentally raises issues of great social importance. These issues include the level of overall welfare in a given society, its structures of family life, the emphasis it places on bearing and rearing children, its treatment of women and the elderly, and so on.