Researchers at Washington University School of Medicine in St. Louis are reporting in an upcoming issue of the journal Addiction, now available online, that people who hang out with marijuana, cigarette or alcohol users are not only more likely to do the same, but that exposure allows genetic tendencies for substance use to become more robust.
“We think of our peers as an external part of the environment in which we live, but the peers we select are a reflection of our
selves,” says first author Arpana Agrawal, PhD. “ In addition, the more substance-using friends an individual has, the more the genetic influences on that person’s own substance use increase.”
Agrawal and her colleagues studied more than 2,100 female twins, looking for links between those who regularly use alcohol, tobacco or drugs and the be
havior of their friends.
All were part of the Missouri Adolescent Female Twin Study, made up of twins born between 1975-1985. They filled out questionnaires between 1996-99 that included a series of questions about peer substance use. Five years later, they participated in follow-up interviews, which also included questions about peer substance use and about their own use of marijuana, alcohol and cigarettes.
The study found that 42 percent of the variation in regular substance use among women is due to genetic factors, indicating that genes play an important role in substance use. Using statistical methods, Agrawal’s team was able to determine that the influence of those genes is diminished to about 10 percent in those with the fewest substance-using friends, but that increases to nearly 60 percent in those who have many friends who frequently use alcohol and other drugs.
It already was known that adolescents often select peer groups that engage in behaviors similar to their own, but Agrawal’s team found that peer selection also has a genetic basis. The genetic factors that influence our own risk of using drugs also modify our likelihood of associating with friends who do the same. Agrawal’s group determined that 19 percent of the genetic factors that affect a woman’s tendency to regularly use alcohol, cigarettes and marijuana also influence her choice of peers who use drugs.
“It’s like making a bad thing worse,” explains Agrawal, an assistant professor in the Department of Psychiatry. “A predisposition to substance use tends to bring a person closer to peers who also use alcohol or drugs, and in their company, the genetic factors involved in drug use become more pronounced.”
Studying twins offers advantages in learning about genetic and environmental influences on substance use, Agrawal says. Since identical twins share 100 percent of their DNA, differences in behavior between those twins must come from environmental factors. Similarities between identical twins tend to be influenced by genes and family environment.
Non-identical twins are about 50 percent similar genetically. And by identifying differences between identical and non-identical twins, researchers can ascribe risk for certain behaviors more to genes or to environmental factors, in accordance with the statistical differences observed between the identical and non-identical twin pairs.
But as this study suggests, Agrawal says, it’s an oversimplification to blame either genes or environmental factors for particular behaviors. Although the nature-nurture debate often looks to determine how either genes or environment influence specific behaviors, she says in the case of substance use, genes and environment tend to interact and influence each other.
“It’s almost as if everything is heritable,” she says. ”Our genes influence the environment in which we live as much as they influence things like height and weight. A classic example would be an intelligent child who inherits genes for intelligence from her parents, but then the parents also provide a lot of books to read, meaning that the child grows up in an enriched home. So in a sense, her genes are putting that child into an environment that enhances her genetic tendency to be highly intelligent. The same sort of thing goes on with peer groups and substance use.”
All of the women included in the study were Americans of European descent. Agrawal says African-American and Asian women tend to have different substance-use patterns, and further study of those groups would require a sample much larger than is available in the Missouri twin database.
Agrawal A, Balasubramanian S, Smith EK, Madden PAF, Bucholz KK, Heath AC, Lynskey MT, Peer substance involvement modifies genetic influences on regular substance involvement in young women. Addiction 2010; vol. 105.
This study was funded by grants from the National Institute on Alcohol Abuse and Alcoholism and the National Institute on Drug Abuse of the National Institutes of Health.
Washington University School of Medicine’s 2,100 employed and volunteer faculty physicians also are the medical staff of Barnes-Jewish and St. Louis Children’s hospitals. The School of Medicine is one of the leading medical research, teaching and patient care institutions in the nation, currently ranked fourth in the nation by U.S. News & World Report. Through its affiliations with Barnes-Jewish and St. Louis Children’s hospitals, the School of Medicine is linked to BJC HealthCare.