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"I'm five feet tall and I'm curvy. I feel good about how I look," says Deanna Melluso, a New York City-based makeup artist who dolls up models for magazine shoots and runway shows. "But when I'm around tall, thin women all day, I start to feel fat. As soon as I walk outside, I feel normal again—I see that I've been in a fake world."

Perhaps because their social status is often contingent upon their faces and bodies, women are particularly susceptible to this effect. "When women evaluate their physical attractiveness, they compare themselves with an idealized standard of beauty, such as a fashion model," says Richard Robins, professor of psychology at the University of California, Davis. "In contrast, when both men and women evaluate their intelligence, they do not compare themselves to Einstein, but rather to a more mundane standard."

In a study where people were asked to solve math problems, there was no difference in how well men and women scored—when everyone was fully dressed. But when subjects were required to perform the calculations in their bathing suits, the women suddenly fared worse than their male counterparts. They were too busy wondering how they looked to crunch numbers correctly.

Everyone judges his or her own appearance more critically when self-aware, as when giving a presentation to coworkers. But people who score high on measures of a personality trait called "public self-consciousness" feel that way all the time. We all know someone like this—a friend who never runs out of the house to grab coffee without fixing herself up first. Strangers generally consider such people to be more physically attractive than average, says William Thornton, professor of psychology at the University of Maine. But that extra personal care doesn't correct their internal funhouse mirrors: They tend to compare themselves exclusively with very good-looking people—and feel especially down after doing so.

As our faces and figures evolve during childhood and adolescence, we create a picture of ourselves that is hard to get out of our minds in adulthood, however outdated or wrong it may be. Not all people who grow up disliking their appearance were ugly children, says James Rosen, emeritus professor of psychology at the University of Vermont. Some were perfectly cute as kids, but had an exceptional trait, like being very tall or heavily freckled, which drew comments and stares.

Our "internal mirrors" are often shaped by our parents, contends psychoanalyst Vivian Diller. A child whose parents tell him he's ugly will have to overcome that perception, but that's uncommon. More subtle is the effect of "the gleam in their eye," says Diller—whether parents sincerely light up at the sight of us and appreciate our individual charms.