Sylvester Williams, a Denver Broncos nose guard, nabs Isaiah Crowell of the Cleveland Browns last October. If measured by BMI, Williams is obese. AAron Ontiveroz/Denver Post via Getty Images hide caption

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Government workers exercise at their office in Mexico City, August 2013. To counter the obesity epidemic, the city requires all government employees to do at least 20 minutes of exercise each day. Tomas Bravo/Reuters /Landov hide caption

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Posting a picture like this on the fridge might seem like good motivation for weight loss. But scientists say it might instead inspire weight gain. iStockphoto.com hide caption

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