FRIDAY, Jan. 14 (HealthDay News) -- Young adults who are overweight or obese tend to befriend and date people who are also overweight or obese, new research indicates.

The finding echoes previous research that found health behaviors, and their results, "cluster" within social networks, said study author Dr. Tricia M. Leahey, an assistant professor of psychiatry and human behavior at Brown Medical School and Miriam Hospital's Weight Control and Diabetes Research Center in Providence, R.I.

"We found that overweight people do have more social contacts who are overweight and are more likely to have an overweight romantic partner or best friend," Leahey said.

Overweight and obese youth in the same age group also tend to have more overweight relatives, although not more overweight classmates or colleagues. The study was published in the Jan. 11 issue of the journal Obesity.

Previous research from Harvard Medical School has found that a person's chance of becoming obese increases 57 percent if a friend becomes obese, 40 percent if a sibling becomes obese, and 37 percent if a spouse becomes obese.

While Leahey found that overweight young adults tend to have more overweight casual friends and social contacts then do normal weight young adults, there was a bright spot in her research. The results "suggest if [overweight young adults] have more social contacts trying to lose weight, they have greater intentions to lose weight" as well.

For the study, which included 151 participants of normal weight and 137 overweight or obese men and women, Leahey's team asked the volunteers to complete questionnaires about their weight, height, the number of overweight social contacts and their perceived social norms for obesity and obesity-related behaviors.

Interestingly, she and her colleagues found that both the normal weight and overweight participants reported similarly low levels of social acceptability for being overweight, eating unhealthy foods and being inactive.

Why did those who had social contacts trying to lose weight say they were trying themselves to lose? Leahey says social norms for weight loss, such as encouragement from others and their approval for weight loss, account for the association.

The question about why overweight young adults have more overweight social contacts is less clear-cut. Researchers are not sure whether overweight people seek out other overweight people, or whether normal weight people who become friends with overweight people put on weight.

"It could go both ways," Leahey said. "It could be overweight people tend to attract overweight [peers], or someone of normal weight gets into a relationship with someone overweight and they tend to gain weight," she said, as they adopt the other person's habits. In her opinion, "there is more evidence to suggest there is a social contagion, it's contagious -- [that is], "they get heavier once they develop a friendship or romance" with someone who is overweight.

The findings make sense to Michelle van Dellen, a visiting assistant professor of psychology at the University of Georgia, Athens, who has found in her own research a direct effect of peoples' behavior on the eating habits of those around them. For instance, van Dellen found that participants who watched someone eat cookies instead of carrots did less well on self-control tests taken later than the participants who observed people choosing the carrots over the cookies.

The new research by Leahey, she said, suggests that it's not only the behavior of social contacts that influence people's own behavior, but also the goals of friends and partners. The contacts of the participants, she said, were overweight but some also were trying to lose weight. "That goal...appears to influence the person's own behavior and their goals," she said.

Goals are known to affect behavior, added van Dellen, whose own research suggests that self-control itself is contagious.

Leahy's take-home message for overweight young adults, in fact, echoes this sentiment. "If you are an overweight or obese young adult, you might want to try losing weight with your social contacts who are also overweight or obese," she said. "This age range seems to be more influenced [than older people] by the social network factor."

More information

To learn more about social contagion theory, visit the Edge.