Weight Gain Is Contagious and You Could ‘Catch’ Obesity From Your Neighbors, Study Finds

Updated| In the long list of things that you can spread to your friends—the flu, yawning, bad moods—a few extra pounds usually doesn’t rank. But researchers have found that much like a sexually transmitted disease, obesity could also be spread via close proximity.

Related: Weight loss saves obese adults nearly $30,000, according to study

As the Los Angeles Times reported, researchers from the University of Southern California and think tank Rand Corp. delved into whether obesity could spread through social contagion. This phenomenon encompasses the idea that certain behaviors and beliefs, such as smoking or happiness, can be spread just like a pathogen. As a stastician explained to Science magazine in a story last year about the theory, people who exercise, eat healthy and refrain from smoking generally don’t run in the same circles as those who have opposite behaviors.

The theory that weight problems could be spread via social networks isn’t new. In 2007, researchers found that people were up to 57 percent more likely to become obese if a friend or family member did the same during a certain period of time.

For this study, the team wanted to answer whether living in a community with higher obesity rates increased an individual’s risk of gaining weight. The researchers pulled information from another study analyzing exercise and nutrition among teens living on military bases, choosing to focus on these locations because they tend to be tight-knit communities—the exact type of environment that would lend itself to this kind of social contagion, if it exists.

Data from a total of 1,111 adolescents and more than 1,300 parents from 12 U.S.-based military installations were culled in order to determine whether someone’s chances of being overweight were greater in counties with more obese people. According to Science Alert, obesity rates at the bases ranged from 21 percent at the Fort Carson base in El Paso County, Colorado, to 38 percent in Fort Polk, based in Louisiana.

The study, newly published in JAMA Pediatrics, found that both parents and kids who lived in counties with higher obesity rates were more likely to be obese or overweight. And the longer families lived in these overweight areas, the higher their own risk of weight gain. Those who lived off base were particularly vulnerable; the researchers speculated that this link could be due to more time spent within the general population. For example, living in a community where few people exercise or frequently eat fast food might have influenced a military members’ behaviors.

This study may not be representative of the general population, but it does advance the research on spreading obesity socially. In that study from 2007, researchers found that certain relationships could impact weight gain more than others. Friends and siblings of the same sex seemed to impact weight gain more significantly than the opposite sex, which made scientists conclude that we’re more influenced by those who are like us. Just something to keep in mind the next time your sister tries to persuade you to eat that doughnut.

This story was updated to include more information about the social contagion theory.