During intercourse, the female orgasm can be elusive. What frustrated woman hasn’t wondered: Am I simply, um, put together differently than other women?

Kim Wallen, professor of psychology and behavioral neuroendocrinology at Emory University, is busy doing the math to find out. And, yes, he says, simple physiology may have a lot to do with orgasm ease -- specifically, how far a woman’s clitoris lies from her vagina.

That number might predict how easily a woman can experience orgasms from penile stimulation alone -- without help from fingers, toys or tongue -- during sexual intercourse.

In fact, there’s even an easy “rule of thumb,” Wallen says: Clitoris-vagina distances less than 2.5 cm -- that’s roughly from the tip of your thumb to your first knuckle -- tend to yield reliable orgasms during sex. More than a thumb’s length? Regular intercourse alone typically might not do the trick.

Wallen is not the first to check into this “C-V distance.” In the 1920s, Princess Marie Bonaparte, a French psychoanalyst and close friend of Sigmund Freud, grew fed up with her own lack of orgasmic response. In her professional practice, she saw plenty of patients with the same complaint (“frigidity,” in the parlance of the day).

She blamed physiology, not psyche.

Bonaparte collected C-V and orgasm data from her patients and in 1924 delicately published her observations under a pseudonym. (She also persuaded an Austrian surgeon to experiment on her, by cutting around her clitoris and stretching it closer to her vagina -- with disappointing results.)

Recently, Wallen dug up Bonaparte’s measurements and analyzed them with modern statistical techniques. Sure enough, he found a striking correlation. Now he is hoping to do his own measurement study.

Preliminary work has revealed that only about 7% of women always have orgasms with sex alone, he says, while 27% say they never do. The current research hold-up: developing a reliable, at-home technique for measuring C-V distance, especially one that can deal with stretchy skin.

Women with a large C-V distance should not be discouraged, Wallen says. “Personally, I don’t think the inability to experience no-hands, penis-only intercourse with orgasm says anything about a happy sex life,” he says. “Maybe it could allow couples to be a bit more inventive in how they have sex.”

He acknowledges that the measure might become one more standard women feel they need to live up to, like breast size. “People would ask, ‘Is your distance really small?’ ”