Body of lies: Patients aren't 100% honest with doctors

Not included in those figures would be patients who "lie" without knowing they do so by withholding information because it slips their mind or they have no idea it could be useful. (Maybe Aunt Agnes would gladly tell about the time she snored so loud she woke the neighbors if she knew that a diagnosis of sleep apnea could depend on it.)

In the WebMD survey, 38% of respondents said they lied about following doctors' orders and 32% about diet or exercise. Doctor reports bear this out.

"Patients are strongly motivated to have their doctors think they're good patients," says Dr. Steven Hahn, professor of clinical medicine at Albert Einstein College and an internist at Jacobi Medical Center in New York City.

It's hard to make a good impression when you're on an examining table in a flimsy, open-backed gown -- a fact that might make lying that much more tempting. But even fully clothed, talking face to face across a desk, a patient cedes authority to the doctor. And people generally like to please those in authority, says Emanuel Maidenberg, clinical professor of psychiatry at UCLA.

Patients also are prone to lying about the fact that they engage in social taboos, things their doctor might not approve of. In the WebMD survey, 22% lied about smoking, 17% about sex, 16% about drinking and 12% about recreational drug use.

"When you're studying psychiatry, you're taught that if a patient says, 'I use cocaine once a month,' you figure it's twice a month," says Dr. Robert Klitzman, professor of clinical psychiatry at Columbia University. "We were taught to double."

Patients lie because they don't want to be judged, embarrassed or misunderstood. They lie about pursuing alternative health remedies because they disagree with their doctor or because they think an item is none of their doctor's business.

Doctors, of course, make the case that even deeply personal matters such as sexual orientation or having an extramarital affair can affect the care doctors give (how to interpret symptoms, what tests to order, exams that might be important). Patients may see only unpleasant invasions of their privacy -- and a risk that somehow their co-workers, parents or spouses will find out too.

"We live in complex social webs," Klitzman says. "Someone will see the forms. . . . People talk."

But co-workers, parents and spouses aren't the only threats hanging over a patient's head. Health insurance is another. And so -- not surprisingly -- sometimes people lie in order to keep something out of their medical records or out of the hands of their insurance companies.