But the preliminary factor in how easy, often woman can climax, or not, is the distance between the vagina and the clitoris according to Kim Wallen, professor of psychology and behavioral neuroendocrinology at Emory University.

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.

This confirms previous research in the 1920s by Princess Marie Bonaparte. So guys, you now have a counter question for when women ask you for your size.

Via LA Times.