Human Population

Population structure refers to the relative abundance of males and females, and of individuals in various age classes. The latter type of structure is significantly different for growing versus stable populations and has important implications for future changes in population size.

Populations that have not been increasing or decreasing for some time have similar proportions in various age classes. In other words, there are comparable numbers of people aged five to 15 years old as those 35–45 years old. The distribution of people is even among age classes except for the very young and the very old, for whom there are, in many societies, disproportionately high risks of mortality. (In industrialized societies, the death rate for infants and young children may be very low; for the elderly, it remains high.)

In contrast, populations that are growing rapidly have relatively more young people than older people. Therefore, the age-class structure of growing populations is triangular, that is, much wider at the bottom than at the top. For example, more than one half of the people in a rapidly growing human population might be less than 20 years old. This type of population structure implies inertia for further growth because of the increasing numbers of people that are continually reaching reproductive age.

Human populations that are growing rapidly for intrinsic reasons (i.e., birth rather than immigration) have a much higher birth rate than death rate and a markedly triangular age-class structure. The so-called demographic transition refers to the intermediate stage during which birth rates decrease to match death rates. Once this occurs, the age-class structure eventually becomes more equitable in distribution until zero population growth is achieved.