Friendships and peer groups provide more than simple companionship. Strong friendships may become the most important relationships you have, particularly during times of stress and change.

According to Dr. Kenneth Dodge, director of the Duke Transdisciplinary Prevention Research Center, peer groups play a significant role in setting the social norms in a child’s life. They help children develop good social skills. Acceptance into a peer group helps you learn to cooperate, interpret social cues, solve problems with other people, and see things from someone else’s perspective.

But peer groups can have a negative impact, too. Dr. Thomas Dishion, director of research for the Child and Family Center at the University of Oregon, says, “Peer groups have the strongest influence on attitudes and behavior, especially problem behavior.”

People often call the influence of peers on one another peer pressure. Dishion prefers to call it peer contagion, because it is more complicated than a group applying pressure to one person. The influences run back and forth between everyone in the group. It is the sum of all these interactions that affects each member. Dishion says, “It is a subtle social process, difficult to track and control.”

Being part of a group affects your tendency to make your own decisions. “Some kids abstain from decision making when they are in the context of their friends; they just go with the group,” Dishion says. For instance, you are less likely to stop smoking or drinking too much if those around you are doing it.

Adults aren’t immune to social effects, either. A recent NIH-funded study found that adults are more likely to become obese if a close friend or family member has put on some pounds. The effect was most striking when the 2 considered each other close friends. The risk of obesity rose by 171% after one of the friends had become obese.

“We didn’t find that people who were overweight simply flocked together,” explains Dr. Nicholas Christakis of Harvard Medical School, one of the study’s researchers. Rather, when people became obese, those close to them seemed more likely to become obese as well.

Researchers don’t understand exactly how this might work. One possibility is that norms shift within a social group when one person gains weight. Close friends and family might then find it more acceptable to put on extra pounds. Scientists hope that future research will help them develop strategies for harnessing the power of social relationships to encourage healthier habits.

NIH currently funds several studies designed to examine how social factors (such as families, friends, peers, schools, neighborhoods and communities) influence teens’ health and risk behaviors. Researchers hope to use this growing body of knowledge to develop strategies to encourage healthier behaviors.

One sure way to affect the health behavior of your children is to help them as they choose a social group. Though children in particular may identify with a certain peer group for a period of time, they often move from group to group.

“Which peer group children end up staying in is dependent on how readily accepted they are into the group,” Dodge says. “When they are looking for a peer group to join, they seek other kids who are similar to them.” If the common ground for the peer group is deviant behavior, it will most likely reinforce problem behaviors in the future.

But Dodge says that parents of kids and teens don’t have to take a back-seat in their children’s choice of peer group. Parents play a unique role in developing opportunities for their children. They make decisions about where to live, where to send their children to school, what after-school activities their children attend and how much contact they have with other family members.

Dishion suggests that parents try to help their children find a positive social niche early on. It becomes more difficult to guide children into a specific niche once they’ve reached adolescence.

He also suggests that parents stay involved in their children’s lives. Research has shown, he says, that the earlier parents stop monitoring their children, the higher their children’s risk for problem behaviors.

“The most important thing parents can do is place controls over the environments their children are placed in,” Dodge adds. “The exposure that kids have to other kids is determined largely by the parent. Knowledge of your children’s activities and better communication between parent and child will create more favorable outcomes for your child.”