Who orgasms most and least, and why

More and more studies are shedding light on the "orgasm gap," which refers to how men orgasm during sex more frequently than women.

But why does this gender gap exist, and what can be done to achieve orgasm equality? After all, about 40% of women experience sexual dysfunction, associated with a chronic difficulty in achieving orgasm.

Experts are offering some answers.

"All groups of men -- gay, bisexual, heterosexual -- orgasm more than all groups of women," said David Frederick, assistant professor of psychology at Chapman University, who has studied human sexuality.

"Lesbian women orgasm more often than heterosexual women but less often than men," he said. "What makes women orgasm is the focus of pretty intense speculation. Every month, dozens of magazines and online articles highlight different ways to help women achieve orgasm more easily. It is the focus of entire books. For many people, orgasm is an important part of sexual relationships."

The reason for the orgasm gap could be sociocultural or evolutionary, Frederick said.

"Women have higher body dissatisfaction than men, and it interferes with their sex life more. This can impact sexual satisfaction and ability to orgasm if people are focusing more on these concerns than on the sexual experience," he said.

"There is more stigma against women initiating sex and expressing what they want sexually," he said, adding, "one thing we know is that in many couples, there is a desire discrepancy: One partner wants sex more often than the other. In heterosexual couples, that person is usually the man."

Therefore, a woman might engage in sex with her partner when she isn't necessarily in the mood, and then she may be less likely to orgasm, Frederick said.

There is also the idea that male orgasm helps facilitate reproduction, as it is invariably required for ejaculation. For women, however, there is no clear tie between orgasm and reproduction, Frederick said.

"But millions of years ago, there might have been," he added.

"One theory is that in ancestors of humans, orgasm occurred more easily because its function was to cause ovulation to occur. This happens in many animals," Frederick said. "Once the monthly menstrual cycle began regulating ovulation, orgasm was no longer coupled with reproduction for women. This allowed ability and ease of orgasm becoming more variable in women over millions of years and is why orgasm frequency is much more variable in women than in men."

Some women might be anatomically predisposed to regularly orgasm, said Elisabeth Lloyd, a professor of biology and philosophy at Indiana University-Bloomington who co-authored a study about genital anatomy and orgasm in intercourse.

A shorter distance between the clitoris and the urinary opening, where urine is released, may increase a woman's likelihood to orgasm, according to the study, published in the journal Hormones and Behavior in 2011.

Lloyd analyzed data from two studies on the relationship between anatomy and orgasm in women with her co-author Kim Wallen, a professor of psychology and behavioral neuroendocrinology at Emory University.

"We found that the distance between the clitoris and the urinary opening, which is called CUMD, indicates whether or not a woman is inclined to have an orgasm with intercourse or not, and if it's below 2 centimeters, then she likely is going to have an orgasm with intercourse," Lloyd said.

"If it's above, if it's around 3, then she's likely not going to have an orgasm with intercourse," she said. "Those were our findings, that have since been confirmed through other tests. So what that means is that if a woman doesn't have an orgasm from intercourse, it's not her fault or it's not his fault. It's not anybody's fault. It likely has to do with her anatomy."

For a woman with such anatomy to achieve orgasm, Lloyd recommended trying "manual stimulation of the clitoris during intercourse."

She added that studying orgasm frequency remains an important area of research since orgasms have been linked to greater satisfaction with personal relationships.

"Women who have better sexual relationships with their partners also have more satisfied relationships in general, and it improves the quality of their relationships," Lloyd said. "So in general, a better sex life leads to a better relationship, which leads to a better sex life. It's kind of circular."