Making Fatter Humans

Are social cues tricking us into eating more food than our ancestors did?

Posted Aug 20, 2012

Worldwide, human beings are fatter than ever. And problem starts early, with many kids packing on extra pounds during the first few years of elementary school.

Why is this happening? Researchers have named many possible culprits, ranging from stress and poor sleep habits to a virus that seems to speed up the rate at which fat cells develop.

But no matter how exotic the explanation, the basic problem still comes down to an energy imbalance—eating more calories than the body burns. And so many evolutionary psychologists have blamed the obesity epidemic on a mismatch between human nature and our modern environment.

Our ancestors had to cope with food scarcity, the story goes, and so they evolved efficient metabolisms and fitness-enhancing preferences for energy-dense foods. Nowadays many people live with an abundance of food. They don’t know when to stop eating.

There’s the curse of inactivity, too. But as many frustrated people have learned, increasing physical activity levels doesn’t always lead to weight loss. The trouble, it seems, is that many people compensate for that increased activity by eating more. And now a new study by Herman Pontzer and colleagues offers surprising evidence that eating matters more than being physically active.

Hadza mother and child by Idobi / wikimedia commons

Hadza mother & child by Idobi

When the researchers measured the total daily energy expenditure of Hadza hunter-gatherers, they found that these African foragers didn’t burn any more energy each day than Westerners do.

Yes, that’s right. Although people living in subsistence farming societies show higher daily energy expenditures, the hunter-gatherer Hadza don’t. After controlling for differences in body size, it doesn’t take any more calories to run a Hadza forager than it does to run the average Western urbanite.

Pontzer’s group found this to be the case despite the fact that the Hadza have higher activity levels. And no, the Hadza didn’t show evidence of having slower metabolisms. They appeared to burn as many calories walking as Western people do.

This is ground-breaking research, the first study of its kind. We can’t know if this pattern will hold for other hunter-gatherer groups. But the study adds fuel to the speculative fire that many modern people are eating more food than the human body is designed to eat.

So it seems that – whatever else is going on – we have developed highly unrealistic ideas about how much food we should be eating each day. And unfortunately, we’re passing those ideas onto our children.

In many Western countries, one in four school kids is already overweight, and many others will become overweight as young adults. For the first time in history, the majority of adults in these countries are overweight or obese. Studies suggest that people who gain excess weight tend to be more impulsive. Have people suddenly become less capable of self-restraint? Maybe, but it seems more plausible that the environment is working against us.

Not only are we bombarded by advertisements about food, there also seems to be a cultural shift in recent years. Like hobbits with their second breakfasts, elevensees, lunches, and afternoon teas, we seem to be increasing the number of occasions where we have a social meal.

image by Dan Brian Geron

American blogger Betsy Shaw commented on this after she returned to the U.S. from a lengthy stay in France. Compared to business as usual in France, American parents seemed to be pushing high-calorie treats – like cupcakes – at children’s play-dates and after-school activities.

I’d noticed this myself, after becoming a parent. So much had changed since I was a kid. Back then kids were far more in charge of their own social activities. Adults didn’t organize “play-dates” for us. And if someone offered us a snack, it was some graham crackers or apple slices. Not a fancy desert or second lunch.  

It happens in the workplace too. Business meetings serving catered meals or party food. Whenever I go to the bank, someone offers my kids candy. I don’t have any objective, systematic data to back these claims up. But I’m betting we could test the idea in a number of ways – like tracking the rise of large “economy packs” of baked treats in grocery stores. People barely seem capable of gathering anymore without turning the event into a social meal.

And that’s key, because one of the most powerful feeding cues in the animal world is the observation of a companion eating.

Humans are no exception to the rule, though they are perhaps more subtle. If we see that others are eating with restraint, we nibble. If we feel we are among friends, or we perceive a feast in progress, we dig in.

In an interesting study of school kids – aged 9 to 15 – researchers found that youngsters given snacks ate more when in the presence of a friend than they did in the presence of an unfamiliar peer. But what was really remarkable was the behavior of overweight kids. Like everyone else, they ate more with friends. But their intake also depended on the weight status of their companions.

When the kids ate with overweight friends – as opposed to slim friends – they consumed 300 additional calories.

There’s more going on here than impulsive eating. It seems likely that social meals shift our reference points for making decisions about food.

Instead of asking “Am I hungry? Do I need these calories?” we ask “Am I eating as much as the other guy?”

And of course eating with enthusiastic friends and colleagues is just one of many social cues leading us astray. Large portion sizes are another social cue that shift our reference points. Instead of asking “Am I full now?” we ask “Have I eaten everything on my plate?”

So if we’re really interested in avoiding a major public health disaster, we’d better start thinking about the powerful social messages we’re sending about food. Have we created a social environment that virtually guarantees people will overeat?




Dewar G. 2012. Obese kids and AD-36: Is obesity caused by a virus? Parenting Science.

Pontzer H, Raichlen DA, Wood BM, Mabulla AZP, Racette SB, et al. 2012. Hunter-gatherer energetics and human obesity. PLoS ONE 7(7): e40503. DOI: 10.1371/journal.pone.0040503

Salvy S-J, Howard M, Read M, and Mele E. 2009. The presence of friends increases food intake in youth. Am J Clin Nutr 90(2):282-287.

Schlam TR, Wilson NL, Shoda Y, Mischel W, and Ayduk O. 2012. Preschoolers' delay of gratification predicts their body mass 30 years later. The Journal of Pediatrics, DOI: 10.1016/j.jpeds.2012.06.049

Sutin AR,  Ferrucci L, Zonderman AB, and Terracciano A. 2011. Personality and obesity across the adult life span. Journal of Personality and Social Psychology, DOI: 10.1037/a0024286


Text © 2012 Gwen Dewar, Ph.D., all rights reserved

About the Author

Gwen Dewar, Ph.D.

Gwen Dewar, Ph.D., is a writer and anthropologist interested in how parents, peers, evolution, and cultural forces combine to shape the way kids learn and grow.

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