As governments around the world struggle with ways to reverse plunging birth rates, new US studies suggest they have ignored a key culprit -- the smartphone.

"Is the iPhone Birth Control?" asked a paper published Monday by the National Bureau of Economic Research, delving into why US fertility rates have fallen by 22 percent since 2007.

For a while, experts linked the decline to the recession that struck in 2008 when the global financial system nearly imploded, driving millions of people into hardship. But when the economy picked up, a rebound in births never came.

Myriad other reasons have been posited, such as increased use of contraception, more female education, and growing housing or childcare costs. However, no clear cause has been established.

So Middlebury College economist Caitlin Myers and her student Ezekiel Hooper tested a hypothesis that smartphones -- which emerged with the arrival of the first iPhone in 2007 -- might have something to do with it.

Until 2011, iPhones were available from a single US cellular network, AT&T, so they compared US counties that had near-universal AT&T coverage with those that had little or none during those years.

And they found that access to the iPhone correlated with reductions in births by 4.5–8.0 percent at ages 15–19 and by 3.2–6.6 percent at ages 20–24.

There were also statistically significant but smaller declines among older women.

While they stress that iPhones are not the "sole cause," the introduction of the smartphone "played a sizable role in the decline in US births" after 2007 as it shaped people's behavior with less in-person contact.

"As modern smartphones diffused, time spent with friends in person and sexual activity fell sharply alongside rising consumption of pornography, a possible substitute for partnered sex," they concluded.

'Technology shock'

Another study published in May by University of Cincinnati economists Nathan Hudson and Hernan Moscoso Boedo found evidence of similar trends and on a global scale since 2007.

They analyzed World Bank data measuring smartphone penetration and teenage fertility rates in 128 countries.

They found that the decline in birth rates accelerated once smartphones became widely available -- a phenomenon found across countries "with fundamentally different healthcare, welfare, economic, and cultural environments."

This, they concluded, points to "a common global technology shock."

Some academics remain skeptical. For example, teenage births in the United States have been falling since the early 1990s, long before the dawn of the smartphone.

Neither study examines how governments might utilize their findings.

Both rich and poorer countries are grappling with declining birth rates which lead to aging societies and a shrinking workforce -- putting strain on social security systems and potentially dampening economic growth and productivity.

The Centers for Disease Control says US fertility rates are at an all-time low, while leading Asian economies all face the prospect of shrinking populations in the coming years.

China's communist government abandoned its decades-long one-child policy in 2016, while Japan and South Korea have invested heavily in pro-natal policies with little impact.

The world's poorest countries, such as in sub-Saharan Africa, often still have high birth rates, but middle-income countries like India and Brazil also face a fast-dropping fertility rates.