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Economist Caitlin Myers has a striking explanation for why women are having fewer babies: It's the smartphones.

Myers and other researchers have been searching for what's behind the sharp drop in fertility over the last two decades. Birth rates in the U.S. have fallen by 22% since 2007.

At first, economists assumed that the Great Recession was to blame but that births would soon rebound, as they'd done after previous downturns.

But then the economy recovered — and birth rates just kept falling.

If the recession wasn't responsible for the baby bust, what was?

"Whatever it is, it must be big, and it needs to coincide with about 2007 because that's when we see all the births go down," says Myers, a professor of economics at Middlebury College in Vermont.

That happens to be the year that Apple CEO Steve Jobs introduced the iPhone, declaring, "Every once in a while, a revolutionary product comes along that changes everything."

Maybe even birth rates.

In a provocative new working paper titled "Is the iPhone Birth Control?" Myers argues that the spread of smartphones could explain between a third and a half of the decline in birth rates during that period.

Births fell more in places where you could get an iPhone in the early years

To test that theory, she makes clever use of an accident of history that creates a kind of natural experiment. When iPhones first came out, they worked only with AT&T.

"In some areas of the country, AT&T had broadband coverage and you could get an iPhone, and in other areas, including where I live in Vermont, that coverage was much more limited," Myers recalls. "And what you can see in this simplest of comparisons, births start to fall in the places where you can get one, and they're not falling nearly as much in the places where you can't."

One might argue the results are skewed because smartphones spread faster in urban areas or wealthier communities. But the results hold up even when Myers controlled for variables like population density and local economics.

"You're probably not going to get pregnant if you're not interacting with people in person"

The drop in birth rates has affected women of all ages, but it's most pronounced among teenagers. That sounds plausible to Jean Twenge, a professor of psychology at San Diego State University.

In books like Generations: The Real Differences Between Gen Z, Millennials, Gen X, Boomers, and Silents, Twenge has documented the profound behavioral changes that accompanied smartphones, especially among young people.

"The smartphone fundamentally changed the way adolescents spent their time outside of school," Twenge told NPR. "They started spending a lot more time online and on their phones and a lot less time hanging out with their friends in person and driving around in a car or going to the mall or just hanging out."

Myers says it's not a stretch to think that this would result in fewer babies.

"If there's one thing I learned in abstinence-only sex ed in the '90s in Georgia growing up, it's that you're probably not going to get pregnant if you're not interacting with people in person — if you're not having sex," Myers says.

In the paper, co-authored with her 24-year-old stepson, Ezekiel Hooper, Myers suggests smartphones also placed access to information about contraceptives and abortion in the palm of users' hands.

The devices also might have depressed birth rates by making it easier for people to find pornography.

"When I talk to my students at Middlebury College, this is the first one they actually bring up," Myers says. "Pornography was proving to be a substitute for in-person relationships."

Apple didn't respond to an inquiry about Myers' paper.

Eventually, copycat phones came along that could be used on other networks, and today smartphones are ubiquitous. Myers says that this raises the question of whether birth rates will level off now or continue to fall.

"I think it's possible that we'll continue to see effects of phones on behavior and outcomes like fertility for years to come," she says. "But we'll just have to keep watching."

Apple is a financial supporter of NPR.