In a memorable article yesterday in the Guardian.co.uk, science writer Carole Jahme explains women’s fascination with the size of a man’s penis: Evolution!
Interestingly, several male friends of mine who read this article were quite...well...uncomfortable. I can understand completely. Women have been judged by their body parts ever since Eve lived in the garden – albeit briefly.
So, read all about it, as the entire article from “Ask Carole” appears below, or you can read it directly at the newspaper’s website:
Penis size: An evolutionary perspective
Carole Jahme shines the cold light of evolutionary psychology on readers' problems. This week: penis size
Penis size is sexually selected only in ape species like chimps and humans where the female exercises mate choice. Above photo depicts a silverback male gorilla. Silverbacks by contrast, monopolize a harem of females and are poorly endowed. Photo: Getty Images/Dorling Kindersley
Anonymous, age and sex unspecified
Dear Carole, Why are women so obsessed with the size of a man's cock – wanting ones 6 inches and over and kicking others aside when they really should be concentrating on the emotional connection and love being shared, putting the size of the man's cock right out of her mind?
The origins of the primate sex drive go back more than 60m years to the late Mesozoic era when the first primate evolved. A lot of sex has taken place since then, and a significant proportion has been motivated by female choice between rival males.  Female primates can experience multiple orgasms, and it has been theorised that ancestral hominid females sought out males who would sexually satisfy them. Through the mechanism of sexual selection, this will have increased penis size and altered structure.
Today, the average erect gorilla penis is 3cm (1.25 inches) long, the average chimp or bonobo penis comes in at around 8cm and the average human penis stands at around 13cm. Most primates, including chimpanzees, have a penis bone and achieve erections through muscle contraction. The human penis has evolved the unusual system of vasocongestion to achieve erection, making the erect organ far more flexible than that of other primate species.
This unique adaptation is thought to have been selected through female mate choice, and by the time Homo erectus arrived on the scene, the hominid penis was significantly longer, fatter and more bendy than our ape cousins'. It has even been theorised that bipedalism evolved in humans to allow the fashionably new, larger, flexible penis to be displayed to discerning females.
Interestingly, while the human penis is the biggest of all the ape species in length and girth, both in absolute terms and as a proportion of body size, the human testes are not. As a proportion of overall body size, chimp and bonobo testes are twice as large as human testes, whereas gorilla testes are half the size. Why?
Correlations can be found between primate mating systems and male genital anatomy. In multi-male/multi-female groups, males must compete to reproduce and frequently the competition takes place inside the female reproductive tract. The more sperm a male produces and ejaculates inside a female the greater the probability that one of his will fertilise the ovum. Female chimps or bonobos in oestrus often mate with several different individuals, so males must reproductively compete in this way and larger testes will therefore confer greater reproductive fitness.
By contrast, female gorillas live in harems and don't often get a chance to exercise a choice between mates, though occasionally a female and a male from outside the group may risk it. The impressive 200kg (400lb) silverback gorilla does have the smallest penis and testes of all male apes, but his massive canines and biceps and his controlling, jealous temper allow him to intimidate and fight off potential competitors.
Human testis size indicates that males evolved under conditions in which their sperm competed inside females, but perhaps not to the same extent as chimp sperm. But the larger human penis suggests that hominids needed to keep females with choice sexually satisfied. Ancestral females would have experienced a sexual freedom denied in Western cultures today and it has been suggested that our ancestors went through a period of matriarchy and enhanced female choice.
When compared with patriarchal chimps, the matriarchal bonobo is a far more sex-oriented ape. Enthusiastic females initiate both hetero- and homosexual activity, particularly when aggression begins to surface, resulting in satisfied, contented and peaceful bonobos. Patriarchy, on the other hand, correlates with a lack of openly displayed female choice.
Women with choice are not all "obsessed with the size of a man's cock". Women are as aware as men that to build a stable relationship you need trust, shared interests and the ability to keep each other amused. But a woman is not going to "put the size of a man's cock right out of her mind", because she can't. Females have an evolved interest in the size of a man's penis, which has been sexually selected for its size and shape. But humans are also selected for creativity – we are highly innovative, imaginative apes. Accordingly, women's minds can be aroused by creativity and being sexually imaginative can be physically arousing, adding satisfying metaphorical inches to one's love life.
 Dixson, A (2003) Sexual selection by cryptic female choice and the evolution of primate sexuality. Evolutionary Anthropology; 11 (S1): 195-199.
 Diamond, M (1980) The biosocial evolution of human sexuality. Behavioral and Brain Sciences; 3: 184-186. Sheets-Johnstone M. (1990) The Roots of Thinking. Temple University Press.
 Harcourt, A, Gardiner, J (1994) Sexual selection and genital anatomy of male primates. Proceedings. Biological Sciences/The Royal Society; 255 (1342): 47-53.
 de Waal, F B M (1995) Bonobo sex and society, the behavior of a close relative challenges assumptions about male supremacy in human evolution. Scientific American, March 1995, 82-88.
Carole Jahme has a master's degree in evolutionary psychology and is the author of Beauty and the Beast: Woman, Ape and Evolution. In 2004 she won the Wellcome Trust's Award for Communication of Science to the Public.
— The Curator