When economic times are bad and people worry about the future, they become cautious about undertaking long-term projects. There are few initiatives that have longer-term implications than having babies. So the recent news that the number of births in our country dropped last year for the first time in a decade is not surprising.

This dip in childbearing is a classic human response to adverse economic conditions, one that often occurred even before the widespread use of modern contraceptive technology. It was particularly marked during the Great Depression, but the upturn in fertility rates in subsequent years set the stage for the population we live in and many of the social and economic challenges we face.

During the Roaring ’20s, the U.S. population increased by 1.46 percent per year, rising from 106 million to 123 million. But in the 1930s, growth fell by half, to 0.74 percent per year, with the total population rising to 132 million. A bit of this decline in overall population growth was due to higher mortality, as some people were malnourished and many did not have funds for medical care. But most of the decline was due to lower birth rates.

Early in the 1930s, the Census Bureau issued a report projecting that, at then-prevailing birth and death rates and average age of the population, the U.S. population would peak at 135 million in the 1940s and thereafter decline.

But after the United States joined World War II, growth rates rose above 1 percent even though some 14 million people were in uniform, most away from their homes and many overseas.

Once the war ended, and the nation experienced peacetime prosperity for the first time in 16 years, birth rates soared. The population sailed right past the Census projection of 135 million and hit 150 million in 1950, the year I was born. It hit 200 million by 1968 and 300 million a few years ago.

Eighty-one million people were born from Jan. 1, 1946, through Dec. 31, 1964, the span usually defined as the Baby Boom. The first official boomer qualified for Social Security retirement benefits on New Year’s Day 2008, and boomer retirements soon will become a torrent.

The challenges of large increases in the number of potential retirees relative to the working population will play a dominant role in economic policy for the next half-century, regardless of how we fund Social Security.

The difficult aspect of unraveling intertwined relationships between economic conditions and population growth is that of separating shorter-term cyclical fluctuations like depressed birth rates in the 1930s or 2008 from longer-term structural changes in fertility.

That long-term decline in the number of children per woman has been especially steep in wealthy industrialized countries but is also occurring in nearly all developing countries. But given the much younger average age in poor countries and a higher proportion of women in or yet to reach child-bearing age, the population of poorer countries will grow markedly relative to the richer ones over the next half-century. That will pose economic challenges in itself.

St. Paul economist and writer Edward Lotterman can be reached at elotterman@pioneerpress.com.