An old joke observes that 98 percent of people masturbate, and the other 2 percent are . But according to a recent study based on a representative sample of American adults, only 38 percent of women said they'd masturbated at all during the past year. The figure for men was 61 percent.

The study by University of Chicago sociologists analyzed data from 3,116 Americans aged 18 to 60 (1,769 women and 1,347 men) gathered during face-to-face interviews as part of the National Health and Survey. The interviewers asked, "On average, over the past 12 months, how often did you masturbate?" It's possible that the face-to-face format suppressed the response. Some people might not have admitted masturbating to an interviewer. But even allowing for this possibility, it seems clear that is by no means as prevalent as the old joke suggests, or as many people believe.

Previous studies have shown that men are most likely to masturbate from their teens into middle age. That was partly true in this study. Men's masturbation rate fell somewhat after age 50. But on the whole, men who masturbate continue to do so into later life.

However, it apparently takes young women some time to warm up to masturbation. In this study, women aged 20 to 39 were the most likely to masturbate, with lower rates among women 18 to 20 and those over 40.

Previous research has shown that masturbation becomes more likely with increased , greater frequency of sexual thoughts, sexual experimentation before , and a larger number of lifetime sexual relationships. This study agreed on both sexes.

Previous studies have suggested that poor health reduces masturbation. In this study, that was true for women, but not for men. Men who masturbate keep doing it regardless of their health.

Previous studies have shown that compared with people raised in families that are religiously liberal or non-observant, those raised in fundamentalist families report less masturbation. This study agreed.

Traditionally, masturbation has been regarded as a convenient sexual outlet for people who lack a lover. In this study, that held true for both men and women.

In this study, white people were the most enthusiastic masturbators. African-Americans, Asian-Americans, and Pacific Islanders reported less masturbation.

In American culture, masturbation is often viewed as a sexual refuge for singles, as a way to compensate for a lack of in a relationship. In this survey, that turned out not to be the case. In both genders, a sexless relationship suppressed masturbation. Respondents who masturbated the most were usually involved in a sexual relationship. Having partner sex, it appears, piques interest in solo sex.

Finally, sex involves both physical and emotional closeness. In this study, any disconnect between these two elements, i.e., physical contact but no emotional closeness or visa versa, was associated with increased masturbation. In fact, for women, one of the best predictors of masturbation was a relationship that lacked emotional .

Sadly, like most sex surveys, this one did not include people over 60, despite a growing research literature that most of those over 60 remain sexual.

Bottom line: Masturbation is considerably less prevalent than many people believe, and its frequency depends on many personal and socio-economic factors.