Anne Becker has been studying eating disorders for nearly three decades, but it was from her twin 13-year-old daughters that she learned the term “thigh gap.” Her daughters got their Seventeen magazine and pulled up Web site images to show Becker, a psychiatrist and eating disorders specialist at Harvard Medical School, what a thigh gap looks like. “They said kids at school talk about it offhandedly like, ‘Well, you have a thigh gap, so you can have the extra ice cream,’ ” Becker says.