Two new studies ask: Did the iPhone cause the birth rate to drop?

The enduring mystery of fertility decline has a new culprit: the smartphone.

Experts have long wondered whether phones played a role in the decline in the birth rate — which began in 2007, the same year Apple introduced the iPhone — but until now there was no hard evidence to prove it.

Two new papers, one posted on Monday and the second in Mayare the first academic efforts to test whether the smartphone was the cause.

It is the latest attempt to explain the steep decline in birth rates in the United States and other countries over the past 20 years. Researchers have already looked at contraceptive use, abortion rates, rising levels of female education, and even the popular TV show “16 and Pregnant.”

Proving that phones caused the decline is a tricky endeavor. There were a number of major events in those years, including the Great Recession, and isolating smartphone use is difficult.

The gold standard for scientific evidence is known as random assignment. It compares outcomes for people randomly selected for a treatment (such as getting a smartphone) with people who are not.

But this is not possible when it comes to uncovering the reasons for declining fertility.

So the researchers looked for smartphone data that introduced randomness.

Caitlin Myers, an economist at Middlebury College, and Ezekiel Hooper, her student, used the spotty early introduction of the iPhone as a way to isolate the phone’s effects on fertility. The first iPhone was launched in June 2007, they wrote, and was only available on the AT&T network until February 2011. The study compared birth rates in US counties that had near-universal AT&T coverage with counties that had little or none.

Their work, published in the National Bureau of Economic Research, found that the iPhone caused up to half of the decline in fertility between 2007 and 2011. The effects were most pronounced among young people aged 15 to 24.

What happened in the districts with iPhones? One theory, Professor Myers said, is that young people started socializing more on their phones and less in person, and as a result were less likely to have sex and get pregnant.

Professor Myers said iPhones could also have made pornography more accessible, leading young people to substitute it for sex, or young people could have used it to get better information about how to avoid pregnancy, including contraception and abortion.

Scientists not involved in the study said the results were compelling.

Phillip B. Levine, an economist at Wellesley College, said he’s “a little jealous” of the Middlebury data, which he says provides real insight into a potential driver of major social change.

He said some variance in AT&T’s data could throw off the final finding. For example, the company may have settled in counties that were wealthier or more densely populated, establishing a pattern “that is unlikely to be random anymore,” he said.

He said Professor Myers had tried to explain these variations and that her findings made sense.

But he cautioned, “You shouldn’t take the result so literally and say, ‘Oh, it’s the iPhone.’

Declining birth rates, once a feature of wealthy societies, are now an almost global phenomenon. The sweep of the decline leads researchers to look for common drivers. Authors from the second study also decided to look at smartphones.

“Countries with very different health care systems, welfare regimes, abortion laws, religious traditions, recessions and demographic trends all experienced similar turning points in the same window,” wrote the authors, Hernan Moscoso Boedo, professor of economics at the University of Cincinnati, and Nathan Hudson, Ph.D. student.

“Whatever caused it was something global — something that arrived in roughly the same form in all these places at roughly the same time,” they wrote.

They analyzed World Bank data measuring smartphone penetration and teen fertility rates in 128 countries. In countries as diverse as Iran, Costa Rica, Guatemala, Chile, Mexico and Turkey, they found that the decline in teenage fertility accelerated as smartphones became a mass phenomenon.

They tested their technological theory in the United States using data from cable broadband and high-speed 4G mobile networks. They looked at where access was better and where it was worse, and found a substantial effect: that teenage birth rates fell fastest in counties with faster access.

Theodore Joyce, an economist at Baruch College, said he was skeptical of both studies. The teenage birth rate has been declining since the 1990s, he said, long before technology came on the scene. Professor Myers’ paper, he said, examined the short period before smartphones had fully penetrated.

The hypothesis, he said, could be correct but “remains speculative.”