Patients are only human, and humans are born to lie -- at least they start doing it by age 3. By age 4 they're already good enough to fool people.
By the time they're adults, they're lying one or two times every day, one well-known study showed. In another, 60% of undergraduate students, when trying to make a good impression, lied at least once, and on average two or three times, during a 10-minute conversation.
And to doctors?
"It's an important area we don't really know much about," says Dr. Robert Klitzman, professor of clinical psychiatry at Columbia University.
Still, a number of studies have tried to chronicle how much patients lie about doing what the doctor ordered.
In one study, published in the journal Chest in 2000, 40 patients said they had quit smoking. But saliva tests indicated that, in fact, 10 of them -- 25% -- had smoked recently.