Slender women worry about packing on extra weight, say Brigham Young University researchers who used MRI technology to observe what happened in the brain when people viewed images of complete strangers.

If the stranger happened to be overweight and female, it surprisingly activated an area in women's brains that processes identity and self-reflection. Men did not show signs of any self-reflection in similar situations.

"These women have no history of eating disorders and project an attitude that they don't care about body image," said Mark Allen, a BYU neuroscientist. "Yet under the surface is an anxiety about getting fat and the centrality of body image to self."

The results appear in Personality and Individual Differences.

When anorexic and bulimic women view an overweight stranger, the brain's self-reflection center – known as the medial prefrontal cortex – lights up in ways that suggest extreme unhappiness and in some cases, self-loathing.

The motivation for this new study was to establish a point of reference among a control group of women who scored in the healthy range on eating disorder diagnostic tests. Surprisingly, even this control group exhibited what the authors call "sub-clinical" issues with body image.

"Although these women's brain activity doesn't look like full-blown eating disorders, they are much closer to it than men are," Allen said.

The authors note that women are bombarded with messages that perpetuate the thin ideal, and the barrage changes how they view themselves.

"Many women learn that bodily appearance and thinness constitute what is important about them, and their brain responding reflects that," Spangler said. "I think it is an unfortunate and false idea to learn about oneself and does put one at greater risk for eating and mood disorders."

Tyler E. Owens, Mark D. Allen, Diane L. Spangler, 'An fMRI study of self-reflection about body image: Sex differences', Personality and Individual Differences, May 2010 48(7), 849-854; doi:10.1016/j.paid.2010.02.012