Teens are lonelier than a decade ago. The reason may be smartphones.

In 36 out of 37 countries, feelings of loneliness among teenagers rose sharply between 2012 and 2018, with higher increases among girls, according to a report released Tuesday in the Journal of Adolescence.

Researchers used data from the Programme for International Student Assessment, a survey of over 1 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. (The study did not cover the period of the coronavirus pandemic, which also may have affected teen well-being.)

“It’s surprising that the trend would be so similar across so many different countries,” said Jean Twenge, a psychology professor at San Diego State University and the study’s lead author. “On the other hand, if this trend is caused by smartphones or electronic communication, a worldwide increase is exactly what you’d expect to see.”