The intromission function of the foreskin.

Masters and Johnson observed that the foreskin unrolled with intercourse. However, they overlooked a prior observation that intromission was thereby made easier. To evaluate this observation an artificial introitus was mounted on a scales. Repeated measurements showed a 10-fold reduction of force on entry with an initially unretracted foreskin as compared to entry with a retracted foreskin. For the foreskin to reduce the force required it must cover most of the glans when the penis is erect. This may occur only in humans. These observations mean that foreskin and prepuce can no longer be considered synonymous. Why the penises of humans and chimpanzees evolved so differently should be addressed in light of these findings.