Human Population Dynamics

For nearly all of human history, crude birth rate and crude death rate tracked each other closely. Both were high — disease, famine, and poor sanitation killed people about as fast as they were born — so total population barely grew. Starting in the 1700s and accelerating through the 1900s, sanitation systems, vaccines, antibiotics, and better food distribution cut death rates sharply and quickly. Birth rates stayed high, because the things that lower them — female education, access to contraception, urbanization, lower infant mortality convincing parents they do not need extra children as insurance — spread much more slowly through a society. That gap between a fast-falling death rate and a slow-falling birth rate is what bends the flat historical line into a steep upward curve.

This gap is the engine of the demographic transition. Birth and death rates eventually converge again as a country finishes the transition, but while they are split apart, population grows fast. A country's position in that transition, not its current size, sets its growth rate. A small nation early in the transition can post a high percentage growth rate while a billion-person country further along the transition grows slowly in percentage terms even though it still adds large numbers of people.

Projecting the future means tracking total fertility rate, the average number of children per woman, against replacement level, about 2.1 in most countries. Because projections compound over decades, a shift of even 0.5 in assumed fertility changes the projected global population by billions by the year 2100, which is why demographers present a range of scenarios rather than one number.