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As I was walking through the gym the other day, I caught a glimpse of an overweight woman across the room. But then I did a double take, and then another. The woman was me — I had seen my own reflection in a distant mirror and, for a split second, hadn’t recognized myself.

This moment of mistaken identity was disconcerting, but it wasn’t all that unusual. Many of us are surprised by our size when reflected in the mirror or a store window — it’s like thinking that a recording of your own voice sounds off. And while psychologists have worried for years that media images of superslim starlets would put the nation’s collective self-esteem at risk, it turns out that something altogether different has happened. As the population becomes fatter, study after study shows that instead of feeling bad about ourselves, we have entered a collective state of denial about how big we’re actually getting.

Can You Tell Your Actual Size? Look at a group of silhouettes and see if you can choose which one matches your body. See the chart»

A team of researchers led by a group from the University of Illinois at Urbana-Champaign recently asked 3,622 young men and women in Mexico to estimate their body size based on categories ranging from very underweight to obese. People in the normal weight range selected the correct category about 80 percent of the time, but 58 percent of overweight students incorrectly described themselves as normal weight. Among the obese, 75 percent placed themselves in the overweight category, and only 10 percent accurately described their body size. (Notably, a sizable minority who were at a healthy weight described themselves as being underweight.)

The tendency for people to underestimate their body sizes, according to studies in the United States, Canada, Europe and elsewhere, is remarkably consistent across cultures and age groups. So why are so many people in fat denial? Scientists are only now beginning to understand the complicated process in which the brain (in particular, the posterior parietal cortex) integrates signals from all the senses to form our body images. Because our bodies change over time, the brain must constantly adjust its perception. Scientists believe that this internal calibration system can sometimes go haywire, notably for sufferers of anorexia, bulimia and body dysmorphic disorder, and possibly for obese people too.

In the meantime, they certainly know that the brain’s body-perception center isn’t foolproof. In an experiment called the Pinocchio Illusion, a person can be fooled into thinking that his nose is growing. This happens when someone touching his own nose with closed eyes has his biceps stimulated to feel as if his forearm is moving forward. The brain senses the arm movement but also knows that the fingers are still touching the nose. For both sensations to be true, the brain decides that the nose must be growing.

A few years ago, researchers at University College London, conducted a similar experiment regarding waist size. While a person’s hands were resting on his waist, his wrist tendons were stimulated to create a sensation that they were moving inward — to feel, in other words, as if his waist were shrinking. Brain scans conducted during the experiment showed a marked increase in activity in the posterior parietal cortex, which gave the researchers a glimpse of the brain trying to tweak its perceived body size in real time. “The relative size of our body parts needs to be continuously updated or recalibrated,” said Henrik Ehrsson, lead author of the study, now associate professor of cognitive neuroscience at the Karolinska Institute in Stockholm. “One possibility is that, in people who get obese or who have body-image disorders, something goes wrong with that process.”

While researchers admit that some denial may have to do with personal embarrassment, the consistency of the findings suggests that neural processing and psychology probably both play a role. It is also possible that a few extra pounds isn’t an urgent priority for the brain to acknowledge. Researchers at University of Texas Medical Branch in Galveston found that one in three women did not know when they had gained 5 pounds, and about 15 percent weren’t aware when they had gained more than 10.

But part of the explanation may have to do with perspective. In a recent study, 3,665 children and adolescents in Quebec were given a series of silhouettes showing body sizes ranging from underweight to obese. When asked to describe their own body, nearly 70 percent of the overweight and obese children chose a slimmer silhouette. But the researchers discovered that children with the heaviest parents and peers were far more likely to underestimate their weight than those with healthy-weight parents and friends. “When kids live in an environment in which they see, on a daily basis, parents or school peers who are overweight, they may develop inaccurate perceptions of what constitutes a healthy weight,” says Katerina Maximova, assistant professor of epidemiology at the University of Alberta. “Their own overweight seems normal by comparison.” Now that health officials estimate that two out of every three adults in the United States are overweight, future generations may not see the difference, either.