Obesity can spread like a virus among people who interact with each other. Clusters of obesity have been shown to be contagious among friends and in certain areas, but researchers haven’t been sure exactly why this is. A study of military families tells us more about how people “catch” obesity.

One explanation is that people who are prone to certain weights, or at least to certain habits, congregate in the same areas. Or it could be that people living in a community are subject to the same environmental effects — living in food deserts without easy access to fresh fruits and vegetables or places to exercise or in communities and cultures favoring high calorie foods — which serve as triggers for obesity.

“[L]iving in a community where obesity is more common can make sedentary lifestyles, unhealthy eating, and overweight or obesity more socially acceptable.”

Another possibility, which is what the recent study ultimately confirmed, is that obesity is spread through social circles.

Researchers from the University of Southern California and the RAND Corporation used data on military families, reasoning that since they don’t choose their locations, this would rule out self-selection. Some of the locations to which families were posted had higher obesity rates and some lower. They tracked the families’ weights over time, taking into account all the environmental factors, like the abundance of gyms and types of grocery stores.

The team found that when a family moved to a place with a relatively high obesity rate, both the parents’ and the children’s odds of obesity increased. The effect was particularly strong for families that lived off-base, which suggests that they may be more vulnerable to the influences of the larger community. It was also stronger for families who had lived in a given place for longer.

“If you move a family from a typical county to one with a higher rate of obesity, such as Vernon County in Louisiana where 38 percent of adults are obese, that would increase the parent's chances of being obese by 25 percent,” said study author, Ashlesha Datar, in a statement. “It would also increase the chances of the child being overweight or obese by 19 percent.”

The opposite effect was seen for families that moved to low-obesity locations.

Although it can’t prove it definitively, the study does offer evidence for the idea that obesity is spread through social contact, rather than through the environment itself. This effect is known as “social contagion,” and has also been seen in other types of health and mental health issues. It's the reason why it is a bad idea to eat out when you are trying to lose weight.

“Social contagion in obesity means that if more people around you are obese, then that may increase your own chances of becoming obese,” said Datar. “In other words, living in a community where obesity is more common can make sedentary lifestyles, unhealthy eating, and overweight or obesity more socially acceptable.”

More work will be needed to sort out all the effects of environment, genes and social influence. But it definitely makes one think about the influence of a person’s social circle, and the habits and norms that exist around them.

The study is published in JAMA Pediatrics.