For decades, women have told those who keep track of such things that they would like to have two children. And many of them say they want to have those kids when they are around the ages of 20 to 24.

But the fertility rate in the United States was 1.71 in 2019, the lowest it’s been since the 1970s.

To put it simply: Young families aren’t having the number of children they say they want to have, at least not when they say they want to have them.

A new study from researchers at the Ohio State University and the University of North Carolina examined the phenomenon as part of a deep dive into data gathered by the National Survey of Family Growth, which has been asking people about their childbearing goals and behaviors for several decades.

The study focuses on data gathered by the survey at several points between 1988 and 2019.

One of the big takeaways: The country’s fertility rate, defined as the total number of births in a year per 1,000 women of reproductive age, went down 11% over the Great Recession, the four years from 2007 to 2011.

The rate has not recovered since. Following the recession, there came a global pandemic, which, for many reasons, has made some young adults wary of starting families.

The researchers say they don’t yet know what COVID-19’s impact on the fertility rate will be, whether it will be just a temporary ­shock or a more permanent change, but it bears more study.

Why does it matter if birth rates rise or fall?

Not accounting for migration or mortality trends, a fertility rate of 2.1 children per woman ensures a stable population, which can affect society and the economy, among other things.

One need only look to Japan, where an aging population and decades of low fertility rates have left the country with the world’s oldest population and young adults reluctant to start families. Only 40% of the workforce is fully employed. And those who do have full-time jobs often report that fatigue from being overworked leaves them too exhausted to even think about dating.