Demography

By Richard Grossman MD | 2 June 2026
Population Matters-USA

(Credit: Sean X Liu / Flickr / CC BY-NC-SA 2.0)

One of the responses to my request for ideas of topics for Population Matters! suggested that I write about demographic terms. I have chosen a few of the terms that I think most useful, and tried to explain them simply.

The word population has several meanings. It is the number of individuals living in an area, but it can also denote a subgroup of people. Finally, it can mean the study of population—similar to “demography”.

Two measures of fertility are commonly used. I favor the Total Fertility Rate (TFR)—it is the average number of children women bear in their lifetime.

Replacement fertility is the TFR that results in a stable population. It is 2.1 for a country with good healthcare—one child to replace the mother and one to replace the father. The 0.1 child is to compensate for children who die before they can reproduce. ZPG (Zero Population Growth) is another term for this. The Birth Rate (the other measure of fertility) for ZPG is around 12 or 15 births per 1000 people.

Unfortunately, people die, and the rate at which we die helps determine the size of a population. Thus, the death rate is an important demographic measure—but it is not something over which we have much control. I much prefer to focus of voluntary slowing of the birth rate.

Lowering the birth rate to replacement fertility doesn’t immediately stop growth of a population because of population momentum. This time lag is due to the continued population growth due to a young population structure, even after ZPG is reached.

High rates of child mortality were universal before the industrial revolution. Populations were fairly steady then, with the number of births more-or-less equalling the number of deaths. With better food supply, better health care and better living standards, more children survived and populations increased rapidly. Parents realized that they didn’t need to have so many kids, so the TFR gradually declined until a new steady state was reached, and births equaled deaths. Access to family planning services is an important catalyst for this to happen. This decrease in deaths, then births, rates is called the demographic transition. Developed countries have gone through this transition, but some developing countries have not.

The age dependency ratio is the ratio of people who are too young or too old to work versus the workers (aged 15 to 64). Some people are worried that our dependency ratio is becoming unfavorable with fewer babies being born and at the same time people experiencing greater longevity.

Better health care and living conditions have caused some countries’ economies to blossom by increasing the number of healthy young workers. For a brief time the dependency ratio may decrease, and help the country progress. This is called the demographic dividend; examples are four East Asian countries—the Asian Tigers (Singapore, South Korea, Hong Kong and Taiwan).

When I started writing Population Matters! I realized that I needed to learn a bit about demography. I began reading a brief treatise on the subject, “Demography: A lively Introduction,” but fell asleep. There is a new, better version available free from the Population Reference Bureau (www.prb.org).

If you really like studying population, or are interested in global foods, I suggest Apoorva Jadhav’s blog “Demography Matters”. She was the senior demographer for USAID until that organization was disbanded last year. While unemployed, she started writing weekly essays, each focused on a country and its food. The demographic information was focused on an outstanding attribute of that country. The latest essay was on the USA and our high maternal mortality rate.

Richard Grossman became concerned about overpopulation in high school and chose a medical path to work in the field. He practiced gynecology and obstetrics, including abortion care, for 40 years in Southwest Colorado. Now retired, he writes a monthly essay on aspects of human population. If you like his blog, www.population-matters.org, you can subscribe using the form at the bottom of the page.

Doctor Richard Grossman

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