The crude birth rate in the U.S. has dropped by more than half since the 1960s. Per the St. Louis Fed, sixty years ago approximately 24 babies were born per 1,000 people, in 2022 that figure stood at 11.

This drop—combined with the fact that the nation’s population is living longer—is a serious concern for economists who question how economies will function with fewer people available to do the work.

Melinda Mills is a professor of demography and population health at Oxford University’s Nuffield Department of Population Health. Mills explains: “Sustained low fertility combined with longer life expectancy results in aging populations.

“This causes strains in the labor market such as health care for older populations, the closing of schools, rethinking housing and infrastructure, and rethinking pension systems and age of retirement.”

The resulting drop in GDP from this aging population could be as much as 4%, James Pomeroy, HSBC’s global economist, previously told Business Insider.

Previously experts believed that economies would see a post-COVID “baby bump,” spurred by a brief uptick in births in 2021.

But data from 2022 and 2023 made it clear births were reverting back to their pre-pandemic trend with couples increasingly choosing a dual-income-no-kids (DINK) lifestyle, as the CDC reported last year that U.S. fertility rates fell to a historic low of about 55 births for every 1,000 females ages 15 to 44.

“In a low-fertility scenario, the number of people of the traditional working age could start falling within 20 years,” Pomeroy wrote in his latest note on the subject, though Mills warned the tension between fewer births and an older population is already being felt.

She explained many countries are already struggling to fill health care positions, which previously had relied on migrant workers to fill.

“This has happened in the U.K., for instance where in 2022 around 33% of migrants were to work in the health care system,” Mills, director of the Leverhulme Centre for Demographic Science, tells Fortune.

“This has also caused political tensions, with countries increasingly facing choices related to sustaining the labor force and pension systems while also thinking about reskilling or urging existing inactive populations into the labor market.”

“You’ll find it more difficult to find somebody to cut your hair, do your nails, set up the X-ray machines at the hospital. The sheer decrease in the number of people…becomes a problem,” he added.

What’s the motivation to have children?

Young people have plenty of reasons not to want kids right now: expensive childcare, an unaffordable housing market, high costs of groceries and household essentials, career disruption, and concerns for the future of the planet.

A Pew Research study from July of this year spoke to more than 3,000 people who either haven’t had children or don’t plan to.

Of those aged between 18 and 49—who fall predominantly into the Gen Z and millennial generations—who said they didn’t plan on having children, the top reason is simply that they didn’t want to or wanted to focus on other things.

Additionally, 38% said they didn’t want to have children because they were concerned about the state of the world, and 36% said they couldn’t afford to raise a child.

A further 26% said they didn’t want to have children because of environmental concerns and 24% said they wouldn’t have children because they hadn’t found the right partner.

One factor impacting birth rates is also women’s increasing power and influence within the economy.

Mills explains: “The main reasons are manifold, including shifts such women obtaining higher education and remaining in the labor market, work-family reconciliation, but also housing problems, gender equality, and uncertainty for the future.

“The age at first birth is also above 30 in many countries for women and even higher for men at 32 and older. This also causes increasingly biological limits of fertility.”

Another consideration for many DINK couples is the freedom they can enjoy in their careers if they don’t have the pressure of children to provide for.

Heather Maclean and her husband Scott Kyrish told Fortune last year that the choice not to have children has allowed them to have a “rose and gardener” approach to their careers—the idea that while one person can grow and take risks, the other remains the stable supporter.

“I never thought I’d quit my job to try and write a book. It was never something I saw as an option,” Maclean said.

“But then I took the time to think about what I really wanted to do if I could do anything, and it took a lot of convincing and months of assurances that I could take the time off and afford it, to decide to do it.”