Researchers used data from the Programme for International Student Assessment, a survey of over a million 15- and 16-year-old students. The survey included a six-item measure of loneliness at school in 2000, 2003, 2012, 2015 and 2018.

Before 2012, the trends had stayed relatively flat. But between 2012 and 2018, nearly twice as many teens displayed high elevated levels of "school loneliness," an established predictor of depression and mental health issues.

Jean Twenge, a psychology professor at San Diego State University and the study's lead author. "If this trend is caused by smartphones or electronic communication, a worldwide increase is exactly what you'd expect to see."

In an earlier study, Twenge had identified 2012 as the year when smartphone ownership passed 50 per cent in the United States.

In the worldwide study, school loneliness was not correlated with factors such as income inequality, gross domestic product and family size, but it did correlate with increases in smartphone and Internet use.

By 2012, most of the countries in the study had reached a point where at least half of teens had access to smartphones, and that is when teen loneliness levels began to rise, Twenge said.

"When it got to that saturation point where social media was virtually mandatory and practically everybody had a phone, it changed things", she said.

As smartphone adoption spread in the 2010s, adolescents spent less time interacting in person and more time using digital media, the paper said, adding, "Given that digital media does not produce as much emotional closeness as in-person interaction, the result may be more loneliness in recent years".