Social media use is ubiquitous. Young people, especially, are relying on it for social interaction, sometimes to the near exclusion of offline friendships.

While social media provides tools to connect people from all over the world, it can also bring out the worst in human nature. People who are polite in their everyday dealings can turn vicious online.

The combination of cell phones and social media can be an especially toxic witches’ brew for young people. For one thing, it can amplify problems with bullying.

Online bullying is hard for parents and teachers to detect. Moreover, young people tend to be nearly constantly online, meaning that targets of social media bullying get little respite.

Over the past decade, mental health data have shown ever-worsening rates of anxiety and depression in young people. This has coincided with the growth of social media and cell phone usage.

A correlation alone cannot prove that social media or phones are to blame for the wave of mental health problems in young people. But recent research by Sara Abrahamsson from the Institutt for Samsfunnsøkonomi (Institute of Economics) in Norway comes close to establishing a causal link.

Abrahamsson compared academic achievement and mental health at schools following the adoption of cell phone bans, and at schools that did not ban phones. She found a marked decline in bullying at schools implementing bans.

For girls at schools banning phones, there was a 29% reduction in seeking specialist help for mental symptoms. Girls’ academic results also improved. These benefits were especially pronounced for girls from socioeconomically disadvantaged families. There were no reliable effects of bans on boys’ mental health or academic results.

Abrahamsson suggested that the lack of effects for boys may be attributable to their lower use of social media in the first place. Alternatively, as social psychologist Jonathan Haidt has noted, social media exacerbates traditionally female forms of bullying. On the academic front, boys may simply have found other ways to be distracted in class.

The deterioration in teenagers’ mental health alongside the growth of social media and phone usage has been especially serious for girls. Putting that together with Abrahamsson’s findings of positive effects of bans for girls helps establish a causal link between excessive social media use and poor mental health. Her results support the blanket ban on cell phones in New Zealand schools, which came into effect last week.