MEDICAL HYPOTHESES, Volume 59, Number 2: Page 180-182,
August 2002.

The intromission function of the foreskin.

Donald R. Taves

Redmond, Washington, USA


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.


Morgan [1 and 2] stated that vaginal intromission is easier with a (retractable foreskin in place. However, no one has experimentally tested his contention or clearly stated the mechanism. There are no contradictory claims in any of the citations to Morgan's letters. Harnes [3] published a `fruitless' attempt with a questionnaire to evaluate Morgan's claim of greater sexual pleasure with a foreskin. Articles about circumcision and evolutionary antecedents of human sexually have almost totally neglected the even more general question: the function of the foreskin. The recent position paper of the American Pediatrics Association is silent on this issue [4]. A heated debate in the New England Journal of Medicine produced no references and only a single sentence regarding the function, ` of the sensitive glans' [5]. Porter and Bunker [6] in an article entitled, The dysfunctional foreskin, say only `...its function-physically, physiologically and immunologically protective of the glans and external urethral orifice.'

Several recent publications have considered evolutionary antecedents of human sexual behavior and unusual primate male genitalia but they do not mention the foreskin or prepuce [7, 8 and 9]. Money [10] cites a survey finding that the foreskin is a `visual and tactile erotic organ.' Cox [11] postulates, without evidence, that the foreskin retards the age of sexual reproduction. There is little in the older literature they missed. Even an article entitled, `Observations on the evolution of the genitalia and copulatory behaviour in male primates, [12] there is no mention of the foreskin or prepuce! All the Textbook of Sexual Medicine [13] says, without references, is `There is much controversy and little data surrounding the effect of circumcision on male function.'


Preliminary measurements of the force required for intromission with and without the benefit of the foreskin have been made with a simple device. The device consisted of a Styrofoam cup the bottom of which was cut to make an artificial introitus. The cup was mounted on diet scales so that the force being applied could be noted (Fig. 1). Twelve measurements were made with the glans penis alternately exposed or covered by the foreskin. Intromission with the glans exposed required a minimum of 500 g of force and was uncomfortable. In contrast only 40 to 50 g were required for complete insertion of the glans when it was covered, and there was no discomfort. Thus, intiating intromission with the foreskin covering the glans resulted in a greater than 10-fold reduction in the force needed to penetrate an `introitus' and make an otherwise uncomfortable intromission comfortable.


Fig. 1. Device for measuring penile force during intromission. (Diet Scales, Hanson Scale Co., Division of Sunbeam Corporation, P. O. Box 38, Highway 45 North, Shubuta, Mississippi 39360.) Lawer left: an artificial introitus was made by cutting a quarter size piece out of the bottom of the an 8 1/3 ounce foam beverage cup (Western Family Space Saver Inulated FOAM CUPS, Western Family Foods, Inc. Portland, Oregon 97223.) and making 8 equidistance cuts from the margin to the thickened ring of the cup. The length and number of cuts determined the stiffness and expansibility of the 'introitus' and hence the ease of intromission.

Several factors can be expected to affect the required force: The degree of exposure of the glans with an erection, the size of the penis and the length of the foreskin. If the artificial orifice is smaller than the size of the exposed glans then there will be more friction. With a large glans more friction can be expected and the force required would increase after the foreskin has completely unrolled. In this trial the exposure of the glans was just under 1 cm with erection of the penis while the opening of the articficial orifice was 2.4 cm. The lateral diameter of the erect penis was 4.1 cm. The maximum distance the foreskin moved up the shaft (not including the glans) was 6-7 cm, depending inversely on the amount of turgor in the glans. The limiting factor was tension in the area of the frenulum, which decreased with loss of turgor in the glans.


These preliminary figures support the claim by Morgan [1 and 2] that vaginal intromission is easier with a (retractable) foreskin in place. The mechanism is simple. The interposed foreskin decreases the friction between the introitus and the glans. The unretracted foreskin consists of a thin dermis that is folded on itself with very little friction between the layers. As the penis advances, the foreskin unrolls so that the portion that makes initial contact with the introitus is 6 cm. Up the shaft before any friction occurs between the device and the skin.

These observations were limited to a single subject with an artificial introitus. However, this function of the foreskin is likely to be a general phenomenon. Masters and Johnson report [14], `Only 6 of the 35 uncircumcised study subjects failed to demonstrate significant exposure of the glans during or immediately subsequent to active coition.' They also note that this happened occasionally during auto manipulation. Thes statements imply that none of the subjects significantly exposed the glans by an erection and that the act of intromission generally rolled the foreskin back.

Current dictionaries use the term foreskin and prepuce interchangeably. It is now possible to make a clear distinction between them. According to Roberts [15], `the prepuce, or sheath, is a double invagination of skin which contains and covers the free portion of the penis when not erect (italics mine) and covers the body of the penis behind the glans when the penis is erect.' A foreskin is a double invagination of skin that covers the glans when the penis is erect and is retracted over the body of the glans with intromission.

Homo sapiens may be the only species that has a foreskin. Farm animals have prepuces [15]. There are no published statements on non-human primates that allow classification. Personal communication indicates that many if not all Macaque species of monkeys do not have foreskins [16]. There is a prosimian species described as having an "...ample prepuce which narrows over the apex glandis in singularly human fashion' [17] but the size of the flaccid penis looks small and seems unlikely to remain covered during an erection. The flaccid human penis is said to be uniquely large in relative size [7]. Also Hafez [18] says, `Man is exceptional in attachment of the prepuce very close to the glans.' Both features may have been important in the evolution of the foreskin.

Primates other than chimpanzees generally have glans but with highly variable shapes [12 and 19]. Chimpanzees lack glans [20 and 21] and their prepuces may fail to fully cover the tip of a very pointed penis even when it is flaccid [see picture, [20]]. The marked difference between early and late primates and between the late primates themselves raise interesting questions. What were the reproductive advantages to cause the evolution of such varied penies? How did these evolutionary forces tie in with the increasing use of sex for social purposes seen in chimpanzees and Homo sapiens? Thornhill and Palmer [22] make a case for rape as an important ancestral strategy to allow males to obtain a higher number of mates. However, neither they nor Palmer [23], who reviews the evidence for rape in non-human species, can point to anything besides the notal organ of the scorpion-fly as `...a trait being designed specifically to promote successful reproduction by rape.' Recognition that the foreskin makes intromission easier raises the possibility that it is an organ specifically designed for rape. The pointed penis of the chimpanzees might have been an alternative adaption for the same purpose. However, unlike the common chimpanzee [23], it would appear that it is no longer useful in the pigmy chimpanzee, since they have no longer been observed to rape in the wild [9].


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  2. W.K.C. Morgan, Penile plunder. Med. J. Aust. (1967), pp 1102-1103.
  3. J.R. Harnes, The foreskin saga, JAMA 217 (1971), pp. 1241-1242
  4. American Academy of Pediatrics. Circumcision Policy Statement. Pediatics 1999; 103:686-693.
  5. M. R. Soper, Neonatal circumcision, NEJM 323 (1990), pp. 1204-1205.
  6. W.M. Porter and C.B. Bunker, the dysfunctional foreskin, Int. J. of STD & AIDS 12 (2001), pp. 216-220.
  7. J. Diamond, The Third Chimpanzee: The Evolution of Future of the Human Animal, Harper Collins, New York, NY (1992).
  8. R. Thornhill and S.W. Gangestad, The evolution of human sexuality. Trends in Ecology and Evolution 11 (1996), pp. 98-102.
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  10. J. Money, Sexology, body image, foreskin restoration and bisexual status. Journal of Sex Research. 28 (1991), pp. 145-156.
  11. G. Cox, De Virginibus Puerisque: The function of the human foreskin considered from an evolutionary perspective. Medical Hypotheses 45 (1995), pp. 617-621.
  12. A. F. Dixson, Observations on the evolution of the genitalia and copulatory behaviour in male primates. Journal of Zoology (London) 213 (1987), pp. 423-443.
  13. R.C. Kolodny, W.H. Masters and V.E. Johnson In: Textbook of Sexual Medicine, Little, Brown and Company, Boston (1979), pp. 5-6.
  14. W.H. Masters and V.E. Johnson In: Human Sexual Response, Little, Brown and Company, Boston (1966), p. 190.
  15. S.J. Roberts in: Veterinary Obstetrics and Genital Diseases (Thereriogenology), Ithaca, New York, Edwards Brothers, Inc., Ann Arbor, MI (1971), p. 610.
  16. Kelley S. Personal 2002 communication, Head of Veterinary Medicine and Surgery, Regional Primate Center, University of Washington, Seattle WA, 98195.
  17. W.C.O. Hill In: Primates: Comparative Anatomy and Taxonomy: II Haplorhini: Tarsioidea, The University Press, Edinburgh (1955), pp. 136-137.
  18. E.S.E. Hafez, Comparative Reproduction of Nonhuman Primates., Charles Thomas, Springfield, Ill (1971).
  19. R.M. Harrison and R.W. Lewis, The male reproductive tract and its fluids. In: W.R. Dukelow and J. Erwin, Editors, Comparative Primate Biology Reproduction and Development vol. 3, National Geographic Society, Washington, DC (1986), pp. 101-148.
  20. A.F. Dixson and N.I. Mundy, Sexual behavior, sexual swelling, and penile evolution in chimpanzees (Pan troglodytes). Arch. Sex Behav. 233 (1994), pp. 267-280.
  21. R. J. Izor, S.L. Walchuk and l. Wilkins, Anatomy and systematic significiance of the penis of the pigmy chimpanzee, Pan panicus. Folia primatol. 35 (1981), pp. 218-224.
  22. R. Thornhill and C.T. Palmer In: A Natural History of Rape Biological Bases of Sexual Coercion, The MIT Press, Rape in non-human animal species: definition, evidence and implications. J. Sex Res. 26 (1989), pp. 355-374.
  23. C.T. Palmer, Rape in non-human animal species: definition, evidence and implications. J. Sex Res. 26 (1989), pp. 355-374.

Correspondence to: Donald R. Taves MD, PhD, 10901 176th Circle NE #3516 Redmond, WA 98052 USA. Phone: +415-885-7027 (home)/360-336 3721 (office); Fax: 425-556 8128; email:

(File revised 14 January 2003)