When teen depression started to increase in the U.S. survey datasets in 2012, I had no idea why. At first, I thought it might be a blip. Then depression kept rising with each year, into 2013, 2014, and then 2015. At that point it seemed imperative to figure out the cause.

Poor economic conditions were easy to eliminate: Unemployment was going down at this time as the U.S. economy finally improved after the Great Recession. Increases in income inequality had leveled off after rising the most between 1980 and 2000. So the increase in teen depression was a mystery.

Then, sometime in 2016, I found a Pew poll: The end of 2012 to the beginning of 2013 was the first time the majority of Americans owned a smartphone. It was also around the time teens’ social media use moved from optional (with about 50% of teens using it daily) to almost mandatory (80%+ of teens using it daily). Then the datasets yielded further secrets: Teens were also spending less time with their friends in person and sleeping less. In other words, the way teens spent their time outside of school had fundamentally changed. It would be surprising if that didn’t have an impact on mental health. That was the argument I made in iGen in 2017, and found further support for in Generations.

So how do we know that something else didn’t cause the increase in teen mental health issues? Let’s break down the alternatives and see if they fit the data.

A. Teens are more depressed because of the COVID pandemic. Given that teen depression had already doubled by 2019, the COVID pandemic (which did not impact the U.S. until March 2020) is clearly not the original cause. Depression kept increasing after 2019, but at about the same pace as it did 2012-2019.

B. Teens became more likely to self-report symptoms of depression due to lessened stigma or some other factor. If this explained the increase, you’d expect no changes in behaviors linked to depression, since behaviors aren’t self-reported. However, there have been large changes in behaviors: Self-harm, suicide attempts, and suicides all rose among teens at the same time self-reports of symptoms rose. We’ve known about the concurrent increases in behaviors since at least 2017, but this argument is still made despite the strong evidence against it.

C. Worries around climate change caused the increase. If that were true, we’d expect the increase in teen depression to be steady since at least the late 1980s or early 1990s when the issue gained major prominence (TIME magazine made “Endangered Earth” the Planet of the Year in 1989”. Or perhaps it would start in 2006 when Al Gore’s documentary “An Inconvenient Truth” was widely discussed. But neither was true: Depressive symptoms among teens were fairly steady 1991-2011, and teen suicide rates declined 1994-~2010. Plus, if climate change worries were the cause, you’d expect the largest increases among older teens, who are more aware of global issues. Instead, the largest increases in depression, self-harm, and suicide are among 10- to 14-year-olds. And if anxiety around climate change was the primary driver, why would teen loneliness also increase? It’s difficult to explain why climate change worries would cause loneliness, but easy to explain why spending more time online and less time with friends in person would cause loneliness.

D. Worries about school shootings caused the increase. If that were true, we’d expect the increases to begin in the mid-1990s when school shootings first attracted major news coverage, especially Columbine in 1999. Again, though, that’s not when the increases started. Plus, if school shootings were the cause, teen depression would not increase in countries with much lower rates of gun violence than the U.S. But depression does rise elsewhere, including in the UK, Canada, and Norway. In addition, teen loneliness increased 2012-2018 in 36 countries around the world.

E. Teens have more homework and are under more academic pressure than in previous eras. Actually, teens don’t have more homework (they have less) and don’t report more academic pressure.

F. More marijuana use among teens caused the increase. Except marijuana use among teens was stable to declining at the same time teen depression was rising after 2012. The declines were largest among the youngest teens, the population with the biggest increases in depression and self-harm.

In my view, the question is this: What, other than new technology, had such a big impact on teens’ lives between 2012 and 2019?