The bodies of U.S. teenagers have been expanding, and so has the gap between their perceptions of their weight and reality. Compared with adolescents of about 20 years ago, today's overweight or obese teens are 29% less likely to accurately perceive themselves as having a weight problem.

Researchers compared data on adolescents ages 12 to 16 years who participated in the National Health and Nutrition Examination Survey from 1988 to 1994 with data on adolescents who participated from 2007 to 2012. They were asked whether they saw themselves as overweight, underweight, or about the right weight. Overweight and obesity were calculated both as standard body mass index (BMI) and BMI z scores, which are determined according to the number of standard deviations the weight differs from the mean. Teens were categorized as obese, overweight, or normal. The mismatch between scores and self-perceptions was greatest among white teens and least among black teens.

Over the past 20 years, the number of obese U.S. teens has doubled. Rather than comparing themselves with a scientific standard like BMI, teens compare themselves with peers. This changing view reduces psychological distress and social pressure to maintain lower weights. However, overweight and obese teens who don't see themselves as such-or are less worried about weight because of the size of their peers-are less likely to want to lose weight and adopt weight loss behaviors.

This generational shift in body weight perception highlights an urgent need for novel strategies to delicately point out misconceptions and spur lifestyle changes. "We should discuss weight issues with youths from a healthy lifestyle perspective and empower them to engage in a prohealth, rather than an antiobesity, campaign," says study leader Jian Zhang, associate professor of epidemiology at Georgia Southern University in Statesboro, Georgia.

"A good way to initiate a conversation is to point out to a child that [she or he is] overweight based on BMI curves," says pediatric obesity specialist Rita John, director of the Pediatric Primary Care Nurse Practitioner Program at Columbia University in New York City. She suggests motivating youths with questions such as, Is being overweight an issue for you? How motivated are you to lose weight? Are you confident that you can start lifestyle changes?

Such questions are vital, says John, because "kids won't do anything if it's not their issue." But change doesn't come overnight. She advises helping youths to make one lifestyle change at a time, like walking each day or cutting out sugary drinks.-Carol Potera

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