Marijuana And The Brain Mechanisms of Peer Pressure

Gary Boas
September 6, 2016

The findings could help healthcare providers and others working with young adult marijuana users

Social factors play a critical role in marijuana use in young adults. Young adult users are often introduced to the drug by their peers, and they typically smoke or otherwise partake in it in social settings. But while the role of social factors generally is well established, neural correlations between marijuana use and susceptibility to social influence have remained elusive.

In a study reported in August in the journal Cognitive, Affective, & Behavioral Neuroscience, a team of researchers with the Martinos Center for Biomedical Imaging and the Department of Psychiatry, Center for Addiction Medicine, both at Massachusetts General Hospital, sought to establish such a relationship.

The study offered new insights into the neural mechanisms of peer influence in marijuana users, said Jodi Gilman, an investigator in both the Martinos Center and the Center for Addiction Medicine, an assistant professor of psychiatry at Harvard Medical School, and the lead author of the paper. An extensive body of research, compiled over the past several decades, suggests that young adults who engage in recreational drug use are more susceptible to peer influence than those who do not, but this work is generally based on self-report surveys and questionnaires. The present study is one of the first to use experimental psychology and neuroimaging techniques to understand the brain mechanisms underlying the differences between the two groups.

The researchers enrolled 43 subjects in the study—20 young adults who used marijuana at least once a week and 23 age-matched controls—and asked them to perform a decision-making task while undergoing functional MRI scans. The task measured the likelihood of an individual either following the decisions of a group or acting independently. In it, subjects were asked to make a visual determination—which of two lines presented side by side is longer, for example—after they were shown the choices of unknown peers.

While there were no differences between the marijuana and control groups in the overall number of choices that agreed or disagreed with those of peers, the experiments did uncover important distinctions between the two. First, the marijuana users exhibited longer reaction times and greater prefrontal activation when they opposed group choices, indicating that they spent more effort than the controls when trying to resist peer influence. At the same time, they showed greater activation in reward regions of the brain than did subjects in the control group, suggesting that those regions may be more responsive to social information in marijuana users than in nonusers.

These findings shed light on the brain mechanisms associated with marijuana use, but they also have other important implications, Gilman said. They could help healthcare providers and social workers working with marijuana users, particularly with respect to treatments such as lifestyle management.

“Research has shown us that social network support is vitally important in promoting healthier lifestyle behaviors,” she said. “If it is in fact the case that marijuana users are more susceptible to peer influence, we may find that positive peer interactions can be an especially effective intervention.”

New research is under way to determine whether there are subsets of marijuana users who are especially susceptible to peer influence, and whether these users show more severe patterns of problematic marijuana use than their peers.

The Martinos Center’s Andre van der Kouwe also contributed to the Cognitive, Affective, & Behavioral Neuroscience study.