Every parent I know wants to raise kids who are at least as successful and happy as we are--more so, if we can pull it off. But something disturbing has happened in the last few years to radically reduce how happy kids in middle and high school are.

There are a lot of potential culprits, but researchers think they've figured the likely culprit behind this sad decline: It's that the rise in unhappiness began just as smartphone adoption in the United States began to skyrocket around 2012, with the entry of lower-cost phones and plans. Among teens, we've gone from almost zero adoption in 2009, to 37 percent in 2012, and 73 percent in 2015. 

It's nearing ubiquity now, and a new study using longitudinal data from more than 1 million U.S. teenagers says it's likely this sudden and dramatic spike in the use of smartphones that has negatively impacted happiness. But before you dismiss this as just-another-study, I think there's something that sets it apart.

That's that the researchers doesn't preach the unrealistic goal of total digital abstinence. Instead, they crunch the numbers and concludes that there's potentially a reasonable amount of smartphone use--above zero--that correlates with greater happiness.

The sweet spot? 

Writing in the journal Emotion (.pdf link, may require purchase), researchers "found that teens who spent more time in front of screen devices--playing computer games, using social media, texting and video chatting--were less happy than those who invested more time in non-screen activities like sports, reading newspapers and magazines, and face-to-face social interaction."

That makes sense and is consistent with many other recent studies. However, the difference here is the recognition that teens can report less happiness if they use smartphones too rarely, too.

In general, the sweet spot for reasonable screen time that doesn't inhibit happiness is somewhere between one and two hours per day. This may be in part because so much legitimate social interaction now does take place online.

"The key to digital media use and happiness is limited use," study co-author Jean M. Twenge, a psychology professor at San Diego State University, said in a university press release. "Aim to spend no more than two hours a day on digital media, and try to increase the amount of time you spend seeing friends face-to-face and exercising--two activities reliably linked to greater happiness."

Kids today, right?

This study, like many others recently, starts with the assumption that teenagers' happiness has in fact suffered during the past few years. It's worth digging a bit into why and how they reach that assumption.

Twenge, along with co-authors Gabrielle N. Martin of San Diego State University and W. Keith Campbell of the University of Georgia, used data from the Monitoring the Future study out of the University of Michigan.

But they also reported on other studies that have found general levels of happiness dropping among teenagers since 2012--a decline that comes after a nearly four-decade rise in reported happiness in this age group between the 1960s and the 2000s.

There is also the causation question, of course. Maybe it's not that smartphone use leads to unhappiness; maybe it's the other way around, and kids are already less happy--which might lead them to use phones more often. Or, perhaps there could be unrelated factors: a rise in academic pressure, for example, or a general lowering of happiness among all Americans, regardless of age.

Twenge, Martin, and Campbell say they can't be sure, but they really do seem to think it's likely the smartphones.

"With current research methods, it is very challenging to confidently pinpoint causal forces in cultural change," they wrote in the article. "Given the data and theory we have, the most likely culprit for a cultural force leading to lower well-being among adolescents since 2012 is the increase in electronic communication."