Warning: Obesity is Contagious

It’s no secret that people tend to be similar to the people around them.  You’ve probably heard the now-cliched adage that you’ll be the average of the five people you spend the most time around. And yet, most people don’t want to discuss the uncomfortable implications of this.

So I’ll lay it out there: If you spend a lot of time around obese people, you’re likely to become obese yourself.  If you’re surrounded by images of obesity in the media, you’re likely to become obese yourself.

I hate to be the bearer of bad news, but obesity is contagious- not in the way a disease is contagious, by physical contact or proximity to others, but more in the way that ideas and attitudes are contagious, through sheer cultural exposure.

In this article, I’ll tell you how obesity spreads from one person to the next, give you an estimate of just how contagious obesity is, and then I’ll share some practical tips for protecting yourself from obesity contagion and fighting back against the normalization of unhealthy lifestyle choices. 

The social science of health and fitness

It’s well-established at this point that people are more likely to be fat if their friends, family, and coworkers are fat, and if they’re in an environment where obese people and imaged of obese people are common. 

However, the exact reasons for this are still debated.  There are many candidates, including:

  1. Food pushers, i.e. people actively influencing their friends to eat like themselves.
  2. Homophily.  That is, people choose to be friends with people who look like them.
  3. Confounding by socioeconomic factors.  In other words, poor people are more likely to be obese, and people to to be of the same social class as their friends.
  4. Behavior copying, i.e. you eat like the people around you even when they don’t push you to do so.
  5. Normalization- seeing obese people and being led to view obesity as normal and/or acceptable.
  6. Offense avoidance- you overeat around obese people to avoid offending them.

Let’s run down the list one at a time.

First off, we have the food pushers, those aggravating fuckers who demand that you eat pizza and candy because they just can’t stand that other people don’t eat the same way they do.  I’ve encountered them once or twice in my life, and I’ve heard countless stories from friends and readers about being pressured to eat unhealthily by friends, family and coworkers.

But what does science say about food pushers?  Actually, I wasn’t able to find any studies on the subject.  As you’ll see, I did find plenty of studies that looked at how obese people can influence their friends to become obese, but what I haven’t seen is a study that specifically examined to what extent obese people actively exert social pressure to overeat.

Now, widespread anecdotes are enough to tell me that food pushers are real.  But without studies, I can’t say just how pervasive of a problem they are.  My (unscientific) read is that it varies a lot; some people are surrounded by food pushers, while others never have that problem, despite having a lot of fat friends.

Next let’s look at homophily.  Your gut intuition probably tells you that this is a big factor, and your intuition would be correct.  A 2011 study in the Journal of Adolescent Health which followed teens for the first 2 years of high school found that friend selection was the primary driver of weight-based similarities among friends.  It did not find significant contagion effects- that is, it found no evidence that friends influenced each other to gain or lose weight. 1

That would seem to suggest that obesity is not contagious.  But leaving aside the fact that it’s just one study, there are also a few big limitations here.  First, the subjects were all adolescents- maybe adults are different?  Second, they all went to the same school, so beyond their immediate friend group, they were in the same overall environment.  Third, being in school limited their choice of friends, more than an adult’s choices would be limited.  Indeed, kids at that age are mostly hanging out with the friends they already have; it would have been better to look at people who were in the process of completely replacing their friend group, like college freshmen for instance.  As we’ll see, studies on adults using other methodologies have found different results. 

Next up, let’s look at socioeconomic confounds.  A 2012 population study- again of adolescents- found that socioeconomic status did have a sizable confounding effect, but even after controlling for it, children’s eating behaviors were strongly correlated with those of their friends. Specifically, correlations were found for breakfast eating, as well as grain, dairy and vegetable, but not fruit intake.  This study did not attempt to establish causation, nor did it analyze attitudes towards health and fitness.  2

Both of these studies have a huge limitation: because they looked at existing friendships without using an experimental design, they were unable to establish causality. A 2011 study by Carrell et al looked at subjects in a setting in which they were randomly assigned to peer groups.  To quote the abstracts, “We find statistically significant positive peer effects that are roughly half as large as the own effect of prior fitness on current fitness. Evidence suggests that the effects are caused primarily by friends who were the least fit, thus supporting the provocative notion that poor physical fitness spreads on a person-to-person basis.”  3  This study verified that obesity is contagious, but did not investigate the mechanisms by which obesity spreads from one person to another. 

In layman’s terms, how fit or obese your peers are affects your weight about half as much as your own prior attitudes towards fitness.  That’s huge. 

A study published in 2007 used longitudinal data from 5124 subjects, collected by another research project between the years 1971 and 2003, to analyze the spread of obesity through social networks.  By using time-lagged variables and accounting for the type and directionality of social relationships, the authors were able to establish that when one person gains weight, it does indeed cause people they now to gain weight as well. 

Interestingly, the strength of this effect was dependent on the closeness of the relationship, but not geographic distance- that suggests that actually eating with people has very little to do with the spread of obesity.  The researchers found that the effect was strongest among male-male friendships, same-sex siblings and married couples, with a non-significant (but almost certainly still very real) effect among female-female and opposite-sex friendships, and no effect at all among neighbors who weren’t friends.  Also interesting: the network effect remained significant through three degrees of separation.  That is, you’re significantly influenced by your friends, your friends’ friends, and your friends’ friend’s friends.

Based on the data, the researchers concluded “the psychosocial mechanisms of the spread of obesity may rely less on behavioral imitation than on a change in an ego’s general perception of the social norms regarding the acceptability of obesity.”  4

A study on female Japanese college students living in New York City supports this interpretation.  The students were interviewed within one month of moving to NYC, then again two months later.  Between the two interviews, participants gained weight on average, and their ideal body size- the weight they wanted to be at- went up.  At the same time, they actually started to view themselves as thinner. 5   

One weakness of this study is that it did not look at who the students hung out with and the degree to which they may have been copying the food choices of those around them.  I suspect that subjects who made more American friends probably gained more weight than those who stuck with other Japanese students, but the study didn’t ask this question.

A pair of experiments by Lily Lin and Brent McFerran supportthe notion that much of this normalization of obesity may come from media exposure, rather than real-life interactions.  In the first experiment, subjects were offered a cup of chocolates (Hershey Kisses, it sounds like).  Women who saw an ad featuring a plus-sized model with the tagline “For normal women” ate more chocolates than women who saw the same model with the tagline “For women.” 

In the second experiment, subjects were asked to design their ideal meal from a list of foods.  Women who saw an ad with a plus-sized model with the tagline “For real women” chose less healthy, more calorie-dense foods than women who saw an ad with the same model and no tagline.  The authors concluded that fat-positive ad campaigns, such as Dove’s “Campaign for Real Beauty” or The Body Shop’s “Love Your Body” campaign normalize obesity and cause people to engage in less healthy eating behavior. 6

An experiment by Shimizu et al demonstrated a mixed effect of normalization and food choice copying.  Subjects were asked to serve themselves a mix of pasta and salad after seeing a confederate serve herself.  The confederate, a thin woman, either wore a fat suit or didn’t, and either served herself a lot of salad and a little pasta, or a lot of pasta and a little salad.

The subjects served themselves more healthily when the actress served herself healthily, but also when she was skinny vs wearing the fat suit.  In other words, they were partially copying her behavior, and partially copying her body type. 7

Finally, a novel series of studies out of Duke University found that, people will sometimes match their eating behavior to that of an obese companion out of a desire to avoid giving offense.  In experiments where subjects were invited to choose foods for both themselves and a dining companion, subjects were more likely to choose the same food for both themselves and the other person when the other person was obese.  When questioned, subjects acknowledged that their decisions were driven by the desire to avoid giving offense.  8

This study may explain the finding of the Carrell study that peer effects are mainly exerted by the friends who are least fit- that obese people spread obesity far more than fit people spread fitness.  It does have a major limitation in that subjects were choosing food for the other person as well as themselves.  This obviously isn’t how people usually eat, but there are real-life scenarios where people have to choose food for others- when cooking for a group of people, planning a party or bringing food to one, or selecting a restaurant for a group to go to. 

To summarize: yes, obesity is contagious, via a few different pathways.  Sometimes people actively copy each other’s food choices, either of their own accord or because the other person pressures them to do so.  However the primary means by which obesity spreads is through normalization- seeing obese people everywhere, both in person and in the media, and being told that obesity is normal and acceptable, causes people to stop taking care of their health and thereby become overweight themselves.

Disturbingly, this also suggests that anti-obesity messages can backfire- by reminding people how prevalent obesity really is, they can inadvertently contribute to the normalization of obesity and unhealthy eating themselves. 

How to avoid catching (or spreading) obesity

I realize that reading all of that can be a real downer.  Do you really need to completely avoid overweight people, people who eat unhealthy food, and even images of overweight people?

To be perfectly honest, the research suggests that that would be ideal.  However, I don’t recommend going that far.

First, completely avoiding obese people is a non-started in a society where 70% of the population is overweight and 33% is obese.  I’m honestly not sure how you’d do it.

Second, it isn’t necessary.  Plenty of people get into amazing shape without going nearly that far.  Instead, here’s what I would do to avoid obesity contagion.

Push out the food pushers

The research suggests that food pushers aren’t the main driver behind obesity contagion.  And yet, if you do encounter them, they will absolutely wreck your diet.

If anyone you know regularly pushes you to break your diet, skip workouts, or otherwise neglect your own health, you need to address that.  First, tell them you’re trying to be healthier and make it clear how important that is to you.  If they persist, tell them flat out that they need to stop.

If they keep sabotaging you after that, they need to go.  Remove them from your life if possible, but at the very least, avoid spending time around them, and cut back contact with them to a bare minimum.  More on that in a bit. 

Have a clear plan and habits

People are vulnerable to obesity contagion in large part because they don’t have a clear diet plan.  When they go out to eat, they decide on the spot what they’re going to eat.  But the fittest people in the world are different; they go into a restaurant with a clear idea of what they’re going to eat.

If your diet plan is to “eat healthy,” “cut back on carbs,” or “eat more vegetables,” you need to get more specific.  Have clear rules about what you eat at each meal, and you won’t be as susceptible to the subliminal effects of behavior copying and obesity normalization. 

Build a fitness environment

Spend more time around people who are in shape.  This can mean taking a group exercise class, joining a recreational sports league, moving to a more walkable neighborhood where people are in better shape, or it can be as simple as spending more time at the gym.

Surround yourself with images of healthy food and people who are in good shape.  Get a cookbook and a few fitness magazines and leave them sitting around your home. 

On the flip side, remove images of junk food and the like from your home and work environments.  Design your environment to normalize fitness.

Use Facebook to reprogram your mind for fitness

You can extend this strategy online by reprograming your Facebook feed to show you pro-fitness messages.  You have four tools for doing this: following, friend lists, pages and groups.

For those who don’t know, you can “follow” people so that their posts show up on your wall.  You don’t follow all of your friends, and you can also follow people you aren’t friends with- such as fitness coaches who have already reached the 5000-person friend limit.  Most importantly, nobody knows if you’re following them or not.

You can use this to your advantage.  Unfollow anyone who regularly posts messages that normalize obesity, such as talking about pigging out on junk food, not working out, “normal people/real women” messages that shame and dehumanize fit people, or general “woe is me, I’m out of shape but it’s not my fault and there’s nothing I can do about it” posts. 

After you’ve done that, follow everyone you know who is in really good shape or posts pro-fitness messages.

Next, join a few fitness groups.  Make sure they’re quality groups though, with motivated members and proper moderation.  Facebook fitness groups tend towards one of two extremes- either they’re full of good content and very motivated people, or they’re dominated by spam posts and populated by people looking for the magic supplement that will melt 50 pounds off their body while they eat cake. 

While you’re at it, follow a few fitness pages- fitness coaches, bloggers, gyms, and the like.  Not supplement companies though- see above about spammy magic pill bullshit. 

Finally, organize your friends into friends lists.  Put any strongly anti-fitness people into the acquaintances group so they see your posts less often, or even the “work friends” group, so that they wont see any posts you label as for friends only.  Put pro-fitness people into the close friends group so they’ll see everything you post.  As with following, people don’t know you’ve done this.

Reprogramming your Facebook feed is a great way to build a fitness environment for yourself, but it has another, less obvious benefit: it can change your social life.  You’ll naturally grow closer to the people who you follow and label as close friends, as you start to see each other’s posts more and more.  At the same time, you’ll grow a little more distant from the people who you unfollow and label as acquaintances or work friends.

Set a good example for others

Once you immunized yourself from obesity contagion, it’s time to look in the mirror and make sure that you’re not contributing to the problem yourself.  Are you the person who always suggests junk food when the group needs to decide where to eat?  Do you refer to people who are out of shape as “real people?”  Do you encourage your friends to drink too much?

Start setting a good example for others.  Get into shape.  Model healthy behaviors.  Eat a healthy diet based around whole foods and caloric restriction, and make sure your friends know (without being obnoxious about it- vegans and Crossfitters, I’m looking at you).  Casually mention going to the gym once in a while.

Do your part to normalize fitness.  The food pushers won’t like it, but by setting a good example, you’ll be helping the people around you to become healthier and happier.

Now sound off- how does your environment help or hurt you in your quest for health, abs, and superhuman strength?  Let’s hear it in the comments.


  1. de la Haye et al.  2011.  Homophily and Contagion as Explanations for Weight Similarities Among Adolescent Friends.  Journal of Adolescent Health.  49(4):421-427.
  2. Bruening et al.  2012. Relationship between Adolescents’ and Their Friends’ Eating Behaviors: Breakfast, Fruit, Vegetable, Whole-Grain, and Dairy Intake.  Journal of the Academy of Nutrition and Dietetics. 112(10):1608-1613.
  3. Carrell et al.  2011.  Is poor fitness contagious?: Evidence from randomly assigned friends.  Journal of Public Economics.  95(7-8):657-663.
  4. Christakis, Nicholas, and Fowler, James.  2007.  The Spread of Obesity in a Large Social Network over 32 Years.  New England Journal of Medicine.  357:370-379
  5. Bagrowicz et al. 2013. Is Obesity Contagious by Way of Body Image? A Study on Japanese Female Students in the United States.  Journal of Community Health. 28(5):834-837.
  6. Lin, Lilly, and McFerran, Brent.  The (Ironic) Dove Effect: How Normalizing Overweight Body Types Increases Unhealthy Food Consumption and Lowers Motivation to Engage in Healthy Behaviors.  Association for Consumer Research.
  7. Shimizu et al.  2014. In good company. The effect of an eating companion’s appearance on food intake.  Appetite.  83:263-268
  8. Liu et al. 2013.  Matching choices to avoid offending stigmatized group members.  Organizational Behavior and Human Decision Processes. 122(2):291-304.