The average adult woman's "wick of Eros", from base to glans, measures 16 millimeters, contains 8,000 nerve fibres and when insanguinated inflates to twice its size. This atavistic penis is the booby prize of the DNA delivery-van, not measuring up - no way - to the ejaculatory penis. And it is much smaller than it used to be, shrinking slowly over the millennia and if trends continue - well, I'm not going to spell it out.
The clitoris just ain't what it used to be. Pulitzer Prize-winning science writer, Natalie Angier, ex plains why: "Women once were promiscuous, appetitive, roving diplomats, as many female primates are. They caroused with as many consorts as was practical, and took on the risks that come with multiple matings to quench . . . the far more dire and pervasive threat of infanticide - the tendency of males to kill babies they think are not their own. Well might our ancestresses have shuffled their Latin and cried: `Vidi, veni, vici!'
"In today's world it is hardly adaptive for a woman to flit about like a Barbary macaque, and in some cultures such wanton behaviour is punishable by death. As a result, the clitoris may no longer be considered a woman's best appendage."
Many women would agree the fickle clitoris fails as often as it gives pleasure - although to admit this would be heresy. At the height of the 1970s feminist movement, knowing the location of one's "wick" was a political act. Twenty years later, "location, location, location" is a mantra applicable only to the property market. "Why do we have to work much harder for our finale than men do? The clitoris is an idiot savant: it can be brilliant, and so stupid. Or is it a Cassandra, telling us something that we ignore to our grief?" writes Angier in her groundbreaking, new book, Woman: an Intimate Geography. In her view, "the apparent fickleness and mulishness of the clitoris, its asynchronicity with male responsiveness, and the variability of its performance from one woman to the next can be explained by making a simple assumption - that the clitoris is designed to encourage its bearer to take control of her sexuality. Yes, this idea sounds like a rank political tract, and body tissue has no party affiliation. But it can vote with its behaviour, working best when you treat it right, faltering when it's abused or misunderstood.
"In truth, the clitoris operates at peak performance when a woman feels a-thunder with life and strength . . . women who are easily and multiply orgasmic have one trait on common: they take responsibility for their pleasure." How revolutionary! How sensible! Perhaps this is why Gloria Steinhem has described Woman as "nothing less than liberation biology", adding that: "Anyone living in or near a female body should read this book." Simon Andreae, author of Anatomy of Desire, has described Angier's work as "essential reading for men as well as women". And Fay Weldon declared: "How did we ever get by without it? This book explains your life."
The mass conspiracy between society and DNA to adapt, through natural selection, the clitoris as a smaller and less fearsome object - and thus to de-energise women's sexuality - is more than just a theory. In the US, 2,000 female infants a year with abnormally large clitorises are subjected to clitorectomies. (The total of such operations worldwide is two million annually.) A baby can be born with a clitoris of 1.5 centimetres, growing to three centimetres - a prospect so objectionable that "clitorial reduction" is practised in the West purely for aesthetic purposes. "A big clitoris doesn't hurt anybody, certainly not the baby. But it looks funny, boyish, obscene and parents are advised to fix it while the child is young enough to escape any putative psychological trauma that might accompany uncertainty about her sex," says Angier. A "beefy" clitoris - so large that in a child it may be mistaken for a penis - is but one mode of inter-sexuality, a biological condition more common than you might think. Meanwhile, some of the world's most gorgeous and desirable models and actresses - tall women with long legs, larger-than-average breasts, perfect skin and thick, wavy hair - have a condition known as AIS, which affects one in 20,000 births. They are born with testicles where their ovaries should be, and short vaginas of about one-third the normal length that end in skin, leading nowhere, since these women have no uteruses. Yet these women appear curvaceous and sexier than average. They developed this way when they were little more than embryos.
All embryos are created female. At conception males and females are then differentiated by a mere 30 genes on one Y chromosome, which gives the embryo potential to be male. Around the eighth week of gestation, hormones kick in telling the XY foetus to begin growing testes. (It's pretty clear Adam was made from Eve's rib, rather than the other way around.)
Each X chromosome has between 3,500 and 5,000 genes, but the man's lone Y chromosome has only about 30 genes. A man, therefore, is lacking a few thousand genes compared with a woman. In women with AIS (androgen insensitivity syndrome) something unusual occurred during that crucial eighth week of gestation. These women, like men, have one X and one Y chromosome on the 23rd pairing. Early in gestation, they began to develop testes within the body. The testes duly released androgens to "turn off" the genetic signals which had already begun to form the uterus and vagina - so the "girl" could become a "boy". But by a genetic fluke, these genetic boys had an insensitivity to androgens and the girl signals were not turned off. So they literally were conceived as females, began to turn male, then became physically female again in external appearance, although not internally. These women can be gorgeous because they are insensitive to androgens - the hormones that cause acne and hair loss. So they have perfect skin and long, thick hair. And despite the possession of one X and one Y chromosome - a combination which is irrefutably male - such women feel "feminine" and yearn for child, which they cannot have because they lack ovaries and a uterus.
AIS illustrates the biological fact that the difference between men and women is minute and that a "man" can literally feel like a woman. All men are genetically more like their mothers than their fathers, leading Angier to theorise that when men instinctively reject their mothers, they can only do so by rejecting large parts of their own, inner selves. Women are more like their fathers - with both male and female genetic "voices" influencing their development.
Angier writes that women who "hear voices" may literally have two competing genetic personalities - one male and one female.
The minuscule genetic differences between men and women make nonsense of talk about women naturally being nest-builders and men being emotionally-stunted grunters who cannot find the butter in the fridge. Mothers are mythologised as having natural instincts for nurturing and self-sacrifice, whereas the truth is that we mothers have to work at having supposedly "innate" qualities such as patience and the willingness to repeat endlessly the small, routine chores of socialising a little human being. Men can love their babies too and enjoy the closeness of cuddling them close to their bare skin, but most are not allowed to do what comes naturally because women tend to monopolise parenthood, Angier dares to suggest. More men would campaign for paternity leave and family-friendly work-schedules if they were encouraged, rather than discouraged, to be close to their children.
`I don't buy the argument that men are inevitably less invested in their children than women are, that because there is always a chance to do better reproductively, to conquer new wombs, their feet are always shod and halfway out the door," she says. "In this murderously competitive habitat of ours, this teeming global agora, men's reproductive success may well hinge on their capacity to do just the opposite, to pay attention to every offspring, to shower each child with every possible advantage. Men need women and children now, just as women and children are always thought to need their men."
There is, indeed, a growing movement of men who sense the need to nurture children. They have it within their emotional capacity to "mother" as much as any mother does - apart from pregnancy, child-bearing and breast-feeding. Likewise, women are genetically strong and aggressive but the "feminist" movement which began to acknowledge these "male" qualities has fallen flat. It is not women's genetics, but their socialisation which has made women give up their autonomy, Angier believes.
"We are not fools and we want our families. We want what is best for our children, and for thousands of years we have needed the help and love of men to keep our children safe. Many of us still do.
"When a couple with children divorces, the woman usually gets poorer than she was during her marriage, while the man gets richer. It is still too costly to behave in a way that risks the investment and tolerance of a man, of the greater male coalition that is our post-tuber planet. And so at times we perform little clitoridectomy equivalents on ourselves. We reject the ideas of sisterhood and of female solidarity. We scorn the words `feminist' and `independent' in them."
Women, given the selfish, expedient choice, side with men before they'll side with other women. Men are from Mars, women from Venus? Not at all. We are all from the same planet, made of the same stuff, give or take a few genes out of a total human genome of, perhaps, 100,000. We create our differences.
If Angier is right, we are at a crucial moment of choice: maintain the differences, or give our children the freedom to access both their male and female sides. Women are great adaptors, remaining nurturing nesters while also becoming breadwinners and boardroom battlers. This has put women and families under intense and potentially destructive pressure. How far can women stretch - taking on all the roles - before men decide to balance the inequality by taking on more so-called "feminine" roles? Is it in men's genetic make-up to adapt in this way? Time will tell.
My guess is that the more men perceive themselves as being under threat - as many now do - the more they will start opening themselves to the possibilities of their female sides. That could ultimately be good for both men and women - if we don't destroy each other in the divorce courts in the process.
Natalie Angier's Woman, an Intimate Geography, is published on March 8th by Virago, price £17.99