Spending too much time on “social media” sites like Facebook is making people more than just miserable. It may also be making them depressed.

A new study looked at 504 millennials who actively use Facebook, Twitter, Instagram, and/or Snapchat, individuals who met the criteria for a major depressive disorder scored higher on the “Social Media Addiction” scale. These individuals were more likely to compare themselves to others better off than they were, and indicated that they would be more bothered by being tagged in unflattering pictures, it found.

Those with major depressive disorder were less likely to post pictures of themselves along with other people and reported fewer followers. The findings are published in the latest edition of the peer-reviewed Journal of Applied Biobehavioural Research.

“While this study highlights social media behaviors that are associated with major depression, it is important to recognize that social media use can offer many positive benefits, including fostering social support,” co-author Krista Howard, an associate professor of psychology at Texas State University, said in a statement.

“The key is for individuals to develop an awareness of how they currently use social media and to determine what changes could be made in their social media use to reduce the behaviors associated with psychological distress,” she added. (The companies mentioned in this study were not immediately available for comment.)

A separate recent study conducted by psychologists at the University of Pennsylvania has shown — for the first time, researchers claimed — a causal link between time spent on social media and depression and loneliness.

It concluded that those who drastically cut back their use of sites like Facebook, Instagram FB, +0.56% and Snapchat SNAP, -0.70% often saw a marked improvement in their mood and in how they felt about their lives.

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“It was striking,” says Melissa Hunt, psychology professor at University of Pennsylvania, who led the study. “What we found over the course of three weeks was that rates of depression and loneliness went down significantly for people who limited their (social media) use.”

“ Many of those who began the study with moderate clinical depression finished just a few weeks later with very mild symptoms. ”

Many of those who began the study with moderate clinical depression finished just a few weeks later with very mild symptoms, she says.

The study, “No More FOMO: Limiting Social Media Decreases Loneliness and Depression,” was conducted by Melissa Hunt, Rachel Marx, Courtney Lipson and Jordyn Young, is being published by the peer-reviewed Journal of Social and Clinical Psychology.

For the study, Hunt and her team studied 143 undergraduates at the University of Pennsylvania over a number of weeks. They tested their mood and sense of well-being using seven different established scales. Half of the participants carried on using social media sites as normal. (Facebook, Instagram and Snapchat did not respond to request for comment.)

The other half were restricted to ten minutes per day for each of the three sites studied: Facebook, Instagram and Snapchat, the most popular sites for the age group. (Use was tracked through regular screen shots from the participants’ phones showing battery data.)

Net result: Those who cut back on social media use saw “clinically significant” falls in depression and in loneliness over the course of the study. Their rates of both measures fell sharply, while those among the so-called “control” group, who did not change their behavior, saw no improvement.

This isn’t the first study to find a link between social media use, on the one hand, and depression and loneliness on the other. But previous studies have mainly just shown there is a correlation, and the researchers allege that this shows a “causal connection.”

“ It’s possible — even likely — that lonely and depressed people use sites like Facebook more because they are seeking social connections. ”

It’s possible — even likely — that lonely and depressed people use sites like Facebook more because they are seeking social connections, says Hunt. The new study suggests that Facebook, Instagram and Snapchat aren’t just popular with the lonely and depressed: They’re also making people more lonely, and more depressed.

Why does social media make so many people feel bad? The study didn’t analyze this, but Hunt offers two explanations. The first is “downward social comparison.” You read your friends’ timelines. They’re deliberately putting on a show to make their lives look wonderful. The result: “You’re more likely to think your life sucks in comparison,” says Hunt. The second reason: FOMO, or Fear of Missing Out.

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Social media sites have become such an integral part of the modern world that many people simply can’t cut them out altogether, Hunt says. That’s why the study focused just on cutting back. It’s significant that restricting use to ten minutes per site per day helped those with depression so much. You don’t have to give it up altogether to feel better.

The main caveat is that the study was restricted to undergraduates. Whether the same sites affect older groups, who may be less susceptible to social pressure, is another matter for another day.

Correlation and causation are two issues that researchers grapple with and typically only make claims for the former. In an increasingly polarized and heated political climate, for example,Twitter TWTR, +1.43% may be making older Americans miserable and angry — or angry Americans may use Twitter.

(Quentin Fottrell contributed to this story.)