So what does the PISA survey show? In a paper we just published in The Journal of Adolescence, we report that in 36 out of 37 countries, loneliness at school has increased since 2012. We grouped the 37 countries into four geographic and cultural regions, and we found the same pattern in all regions: Teenage loneliness was relatively stable between 2000 and 2012, with fewer than 18 percent reporting high levels of loneliness. But in the six years after 2012, rates increased dramatically. They roughly doubled in Europe, Latin America and the English-speaking countries, and rose by about 50 percent in the East Asian countries.

This synchronized global increase in teenage loneliness suggests a global cause, and the timing is right for smartphones and social media to be major contributors. But couldn’t the timing just be coincidental? To test our hypothesis, we sought data on many global trends that might have an impact on teenage loneliness, including declines in family size, changes in G.D.P., rising income inequality and increases in unemployment, as well as more smartphone access and more hours of internet use. The results were clear: Only smartphone access and internet use increased in lock step with teenage loneliness. The other factors were unrelated or inversely correlated.

These analyses don’t prove that smartphones and social media are major causes of the increase in teenage loneliness, but they do show that several other causes are less plausible. If anyone has another explanation for the global increase in loneliness at school, we’d love to hear it.