Ever wonder what’s going through teenagers’ heads as they stare at their phones? A group of researchers at UCLA’s Ahmanson-Lovelace Brain Mapping Center decided to find out. They scanned the brains of teens as they used a social media network similar to Instagram.

In a 12-minute period, the researchers showed 32 teens — ages 13 to 18 — each a set of 148 photos on a computer screen. Each group of photos included 40 from the adolescents’ own Instagram accounts that they had submitted for the experiment beforehand.

The photos already had a number of “likes,” which the researchers — unbeknownst to the teens — had assigned in order to examine the effect of perceived peer approval or disapproval.

While the subjects viewed the images, the scientists analyzed their brain activity using functional magnetic resonance imaging. “When the teens saw their own photos with a large number of likes, we saw activity across a wide variety of regions in the brain,” says lead author Lauren Sherman M.A. ’11.

The brain areas associated with social cognition and motivation came alive when the subject’s photo got likes from peers. The brain’s reward circuitry — thought to be more sensitive during adolescence — was especially active.

The researchers say that the same brain circuits that are activated by eating chocolate or winning money came alive when teens saw large numbers of likes.

The study also showed that the teens were much more prone to like an image that already had a large number of likes. “We showed the exact same photo with a lot of likes to half of the teens and to the other half with just a few likes,” Sherman says. “When they saw a photo with more likes, they were significantly more likely to like it themselves, even if [the likes came from] strangers.”

The study highlights a new way in which peer influence occurs. On social media, teens are not left to guesswork or interpretation to find out what their friends like; the evidence is right on the screen — and that number influences their behavior.

This effect was even true of photos that depicted risky behavior such as drinking, smoking or wearing provocative clothing. However, the study showed that “participants demonstrated significantly less activation in a network of regions implicated in cognitive control and response inhibition.”

Should parents be worried about the effects of social media? “If your teen’s friends are displaying positive behavior, then it’s fabulous that your teen will … be influenced by it,” says Patricia Greenfield, director of UCLA’s Children’s Digital Media Center and the study’s co-author.

But if your child is influenced by those displaying risky behavior, she says, then not so much.