The Joy of Population Pyramids

One of the first tools you learn in public health school is the population pyramid. It is a simple an effective way to see the distribution of a population by age and sex.

Population by Age and Sex, Washington DC

These frequently are shown broken down by race and ethnic group, too. For example, look at the following two pyramids that show only whites and only blacks in Washington, DC.

White Population by Age and Sex, Washington DC

Black Population by Age and Sex, Washington DC

There’s a pretty remarkable difference between the two that are broken down by race. You can see at a quick glance that there are few young or old whites (US Census designation) in the District, but there is a far more even distribution of blacks. Other population pyramids overlay the same data for different dates to give an indication of how the population changed over time:

Swiss Population Pyramid

Swiss Population Pyramid

There’s another interesting story here, too, if you stop to consider both the Swiss graphic and the first Washington DC graphic. Look at the population distribution in 1900 in Switzerland. While it shows that the Swiss people lived significantly shorter lives, on average, it also shows that there are far more young people than old people. This is somewhat typical of historical population pyramids, merely revealing that people died at a younger age.

The consequence of having a baby boom — more babies than adults — is that the pool of people that are economically productive will grow in size as the babies enter their adult years. The strain on adults for providing for a cohort of babies larger than themselves dissipates when the economically productive adults start having fewer babies again. The 2008 numbers for Switzerland show a large cohort of tax-paying, economically beneficial 40-50 year olds. They’re saving a lot of money because they have far fewer children in relation to their size as a group. Their tax base supports ongoing social programs in Switzerland. The top pyramid for DC shows a similar story. The positive economic period that follows a baby boom is called The Demographic Dividend and is described in a Rand monograph by David Bloom, David Canning and Jaypee Sevilla (public health professors of several Perihelion members).

The next chapter in the story is an interesting one, though. What happens when the economically productive adults retire and begin to rely on the youth to support their social security and tax-based public services? This potentially puts a huge burden on the country. Many of the discussions about the collapse of Social Security in the United States revolved around exactly this issue: as with Switzerland, there was a post-WWII baby boom. Those baby boomers soon will retire and start to collect Social Security. Those of us who remain in the work force — a far fewer number than will be collecting social security — will struggle to support the tax-based social systems.

Public health programs that reduce morality can lead to baby booms and demographic transition. When fewer people on average die in a population, the population grows. A decline in the birth rate usually follows a decline in the death rate by a number of years. This wikipedia demographic transition chart explains the change. The space between the the death rate and the birth rate curves represents the baby boom.

The Demographic Transition

The Demographic Transition

The point at which the birth rate declines to meet the death rate is what sparks the Demographic Dividend. This can be artificially initiated. For example, China’s economy began to boom when China instituted its (draconian) one-child policy. It set the state for a decline in the size of China’s population and also for economic growth. The economic growth lags behind the decline in birth rate by the number of years that it takes the final large cohort of babies to enter their economically productive years. Ireland’s recent economic boom and decline is an oft-cited example of the demographic transition, the Demographic Dividend, and the consequences of an aging baby boom population.

And to think — one quick glance at a population pyramid can reveal all of that information!

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