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Demography is the study of the structure of human populations, including factors like births, deaths, aging, as well as health and economic factors. Some demographic changes happen slowly, over decades, but in a predicable way. For example, if you want to look at projections for the year 2050 of the ratio of the US working-age population–say, ages 25 to 64–to the ratio of the US over-65 population, you need to start by recognizing that all of the the native-born US population that will be 25-and-over in 2050 has already been born! So your demographic projections can tinker around the edges with possible future immigration rates, or how how health and mortality rates may change for the elderly, or how labor force participation rates may evolve for different groups in the next 25 years. But the basic age-group ratio for working-age vs. elderly in 2050 is already baked into the cake.

The Congressional Budget Office offers a look at these kinds of projections in “The Demographic Outlook: 2026 to 2056” (January 7, 2026). As you think about the future shape of the US economy–number of people, number of workers, overall population aging, the sustainability of government programs in which working-age people support the elderly, and more–these projections shape what’s possible.

For example, this figure shows how, in the past, US population growth has been a mix of immigration (gray bars) and births-exceeding-deaths (blue bars). But the blue bars are shrinking, and in a few years the number of deaths will exceed births. As result, all of US population growth will be tradeable to immigration–and the US population growth rates is headed for zero in the next 30 years or so. Overall, the US population rises from about 349 million people this year to 364 million in 2056–and then declines after that.

As the US gets older the ratio of (traditionally) working-age population to over-65 population will shrink. This is a long-term-trend, going back 70 years and more. The “baby boom” generation born in the 15 years or so after World War II helps support the working-age population for a time, but the US is in the middle of that population age group entering retirement age. The ratio of working-age to elderly does level off around 2036 (given the assumptions about immigration above).

US fertility rates have been dropping, and have fallen below the “replacement rate” of roughly 2.0. One big shift here is that the fertility rate for 30-and-older women now exceeds the fertility rate for 29-and-younger women. Between these forces, the overall US fertility rate seems to be levelling out.

Meanwhile, the overall mortality rate is falling as life expectancy rises.

The immigration rate spiked during the last few years of the Biden administration, then plummeted. There are rises and falls related to economic factors and the pandemic. The projections expect total immigration in the next few decades closer to average of levels prevailing in the first two decades of the 21st century.

Many of these underlying factors are susceptible to policy and happenstance. But such policies and events are likely to be around the edges of the big-picture evolution taking place.

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