Birth Rate vs. Death Rate: What Actually Drives Population Growth

A demographics primer

Every country's population is the result of one simple subtraction, repeated continuously: births minus deaths, plus or minus migration. The two rates that matter most — birth rate and death rate — sound self-explanatory, but the way demographers actually measure them, and how they combine into a growth rate, is worth understanding if you want to read population data correctly.

Crude birth rate and crude death rate

The crude birth rate is the number of live births per 1,000 people in a population per year. The crude death rate is the number of deaths per 1,000 people per year. Both are "crude" in the technical sense — simple to calculate, but sensitive to a country's age structure. A country with a young population will have a naturally low crude death rate even if its healthcare system is mediocre, simply because young people are less likely to die in any given year. That's part of why the same crude rates can mean different things in different countries.

How growth rate is calculated

Subtract the crude death rate from the crude birth rate and divide by 10, and you get the annual population growth rate as a percentage. A country with a birth rate of 30 per 1,000 and a death rate of 8 per 1,000 has a growth rate of (30 − 8) ÷ 10 = 2.2% per year — a rapidly growing population. A country with a birth rate of 8 and a death rate of 13 has a growth rate of −0.5% — a shrinking one.

Birth rate isn't the same as fertility rate

These two get conflated constantly. Crude birth rate measures births per 1,000 people in the total population — men, women, children, everyone. The total fertility rate measures something more specific: the average number of children a woman would have over her lifetime, given current age-specific birth rates.

Fertility rate is the number demographers actually use to talk about long-term population trajectory, because it strips out the effect of age structure. A population needs a fertility rate of roughly 2.1 births per woman — "replacement level" — to hold steady in the long run without immigration. Below that, and a population eventually shrinks, even if it's still growing today because of population momentum from past generations.

Why some countries grow fast and others are already shrinking

The gap between birth rate and death rate — and therefore growth rate — varies enormously by country, mostly tracking what demographers call the demographic transition: as a country develops economically, death rates fall first (better healthcare, sanitation, nutrition), then birth rates fall a generation or two later (urbanization, education, especially for women, and access to family planning). Countries early in that transition — much of sub-Saharan Africa — still have high birth rates and fast growth. Countries further along — Japan, South Korea, much of Europe — have birth rates below death rates and are already shrinking.

You can see this directly: switch the region on World Pulse's home page between, say, Niger and Japan, and watch the growth rate flip from strongly positive to negative.