FOR the first six months after teenagers in Colorado pass their driving test, the state bans them from carrying non-sibling teenage passengers unless someone over 21 is also in the car. It is not alone in this ageist approach. Fourteen other American states impose similar restrictions. The reason is that mountains of data show teenagers take risks more readily in the presence of their peers.

But, in today’s virtually enabled world, “presence” is a slippery concept. As a study published in Psychological Science by Lauren Sherman of the University of California, Los Angeles, and her colleagues shows, peers’ influence can travel even through something as apparently trivial as the “like” button in social media.

Dr Sherman, an expert on digital-media use and the development of the adolescent brain, was aware of many studies suggesting peer pressure can be exerted digitally, through chat rooms and messaging systems. But she also knew that little work had been done on what actually happens in the brain as teenagers engage with the technology. She knew, too, there was no understanding of how they respond to more subtle aspects of social media that do not involve typed communication. To study both at the same time, she collaborated with a team of colleagues to look at how use of the “like” button affects the brains of teenagers lying in body scanners.

To do so, she recruited 32 healthy teens (14 boys and 18 girls) who had Instagram accounts and asked them to provide photographs from their accounts for use in an internal social network that her team was building for an examination of the brain’s responses during the use of social media. That was indeed the truth. But not exactly the whole truth.

Participants were asked to lie down in a functional magnetic resonance imaging (fMRI) scanner. This let Dr Sherman monitor their brain activity while they were perusing both their own Instagram photos and photos that they were told had been added by other teenagers in the experiment. In reality, Dr Sherman had collected all the other photos, which included neutral images of food and friends as well as many depicting risky behaviours like drinking (including a driver having an open container of alcohol behind the wheel), smoking and drug use, from other peoples’ Instagram accounts.

To maintain the illusion that there really was a social network behind all this, the researchers told participants they were viewing photographs that approximately 50 other teenagers had already seen and potentially endorsed with a “like” in the laboratory. The photos themselves were each assigned a specific number of “likes”, ranging between zero and 45. For half of each category (ie, neutral and risky) this number ranged from 23 to 45. The other half were assigned one below 23. In total, each participant saw and had a chance to “like” 40 images from his own Instagram account, 42 risky images and 66 neutral images.

As Dr Sherman expected, the participants were significantly more likely themselves to “like” photos already depicted as having been “liked” a lot than they were photos depicted with fewer previous “likes”. Although this effect was strongest when an individual was viewing his own photographs on the system, it proved true in all categories. When she looked at the fMRI results, Dr Sherman found that activity in the nucleus accumbens, a hub of reward circuitry in the brain, increased with the number of “likes” that a photo had, as did activity in the social-cognition sections of the brain such as the pre-cuneus and prefrontal cortex.

She also discovered that levels of activity in the cognitive-control regions of the brain, like the dorsal anterior cingulate cortex, were considerably lower when the teenagers viewed risky photos than when they viewed other images. Thus, photos depicting risky behaviours with many “likes” lowered adolescent self control while simultaneously triggering a reward response—a result that applied equally to boys and girls.

This discovery is not entirely unexpected. Many theoretical models suggest that adolescent risk-taking (or showing off, as it might better be described) arises as a result of heightened neural sensitivity to rewards combined with an inability to maintain cognitive control in the presence of others. Such inability might be caused by neurological immaturity, as psychologists tend to assume, or it might be adaptive, as evolutionary biologists could argue, with young adults wrestling to establish their positions in the social hierarchy and to look good in the eyes of the opposite sex. Either way, though, natural selection could not have foreseen the consequences of mixing such behaviour with either pocket-sized supercomputers or massive, fast-moving metal objects.