Most women have nights where fun under the sheets just isn't so much fun. Nearly half of American women, according to a 1999 survey, regularly have some type of sexual dysfunction—lack of desire, pain during intercourse, or difficulty achieving orgasm. Female sexual dysfunction is often talked about but has not been rigorously studied scientifically. British researchers decided to look at whether women's ability to achieve orgasm, long thought a product of both practice and a woman's comfort, might instead be related to her genetics.

What the researchers wanted to know: Does genetics play a role in a woman's ability to achieve orgasm during sex?

What they did: The researchers sent surveys to twins, both monozygotic (one embryo that split in two in utero) and dizygotic (two separate embryos). They asked a variety of women's health questions, including "How frequently do you experience an orgasm during intercourse?" and "How frequently do you experience an orgasm during masturbation by yourself or a partner?" Monozygotic twins, because they are from the same sperm/egg combination, have identical genes, while dizygotic (fraternal) twins are as closely related as any other sibling pair. The researchers expected that if genes contributed to the frequency of orgasms, the identical twins would have more similar answers.

What they found: As they suspected, when the researchers compared answers by identical and fraternal twins, the former had similar answers about a third of the time, while the latter answered similarly only about one sixth of the time. The researchers estimate that between 34 and 45 percent of a woman's ability to reach orgasm during sexual contact is due to her genetic makeup.

What it means to you: Sexual problems have many sources, and most of us are used to blaming stress, the kids, or the way we were raised. But this study shows that there could be a strong genetic element to orgasm frequency. The researchers say that anatomical differences or variations in the levels of mood chemicals in the brain could be potentially gene-altered and contribute to variations in orgasm frequency among different women.

Caveats: This study researched only the effects of genetics on women's orgasms. It did not look at factors such as interest in sex or arousal, which could be less influenced by genetic factors.

Find out more: The American Academy of Family Physicians has a Web page with information about what causes sexual dysfunction along with some ways to handle it.

The American Association for Marriage and Family Therapy also has a website that describes female sexual dysfunction.

Read the article: Dunn, K.M. et al. "Genetic Influences on Variation in Female Orgasmic Function: A Twin Study." Proceedings of the Royal Society of London: Biology Letters. Published online June 7, 2005, DOI. 10.1098/rsbl.2005.0308.

Abstract online: http://www.journals.royalsoc.ac.uk