The Clitoris and the Orgasm

In most species in which males have a penis, females have a homologous organ called the clitoris. "Homologous" means that both organs grow from the same kinds of cells in the fetus. Anatomically, the human clitoris has the same three-part columnar structure as the penis: a glans, a shaft, and bifurcating roots. The main differences are that the penis is much larger overall, its shaft protrudes much more from the pelvis, it keeps blood from flowing back out when aroused, and it has a tube down the axis for urine and semen.

The human clitoris shows no apparent signs of having evolved directly through male mate choice. It is not especially large, brightly colored, specially shaped, or selectively displayed during courtship. By contrast, in spider monkeys the clitoris is almost as large as the penis, protruding nearly an inch. In hyenas, the female clitoris is larger than the male penis, and seems to play a role in female competition. The human clitoris could easily have evolved to be much more conspicuous if males had preferred sexual partners with larger, brighter clitorises. Its inconspicuous design combined with its exquisite sensitivity suggests that the clitoris is important not as an object of male mate choice, but as a mechanism of female choice. It helps to select for males who provide pleasurable foreplay, copulation, and orgasms, and such discriminative power is just what we should expect from an organ of female choice. Yet this has led to all sorts of confusion among evolutionists.

Some male scientists, such as Stephen Jay Gould and Donald Symons, have viewed the female clitoral orgasm as an evolutionary side-effect of the male capacity for penile orgasm. They suggested that clitoral orgasm cannot be an adaptation because it is too hard to achieve. Sigmund Freud suggested that clitoral orgasm was a sign of mental disorder, and counseled his female clients to learn how to have purely vaginal orgasms. Other male scientists such as Irenaus Eibl-Eibesfelt and Desmond Morris have viewed female orgasm as a reinforcement mechanism for promoting long-term pair-bonding that keeps a female faithful to her mate. They also wondered why clitorises have such trouble provoking orgasm. They assumed that if clitorises worked properly like penises, they should just do their job of promoting marital satisfaction without so much copulatory effort.

These men seem to have overlooked the possibility that clitoral orgasm is a mechanism for female choice rather than pair-bonding. Mechanisms for choice have to be discriminating: they must fire off excitedly when given the right stimulation, and emphatically must not fire off when given inferior input: As a mechanism for female choice, we would not expect female clitoral orgasm to respond to every male copulation attempt, however inept, lazy, inattentive, brief, and selfish. It is possible for a woman's vagina to become lubricated during unwanted sex to avoid injury, but women under such conditions practically never have orgasms. This is strong evidence of clitoral orgasm's role in female choice.

From a sexual selection viewpoint, clitorises should respond only to men who demonstrate high fitness, including the physical fitness necessary for long, energetic sex, and the mental fitness necessary to understand what women want and how to deliver it. The choosy clitoris should produce orgasm only when the woman feels genuinely attracted to a man's body, mind, and personality, and when the man proves his attentiveness and fitness through the right stimulation.

Not surprisingly, female scientists have held the clitoris in higher regard than have male scientists. Helen Fisher, Meredith Small, and Sarah Blaffer Hrdy have viewed the clitoral orgasm as a legitimate adaptation in its own right, with major implications for female sexual behavior and sexual evolution. Lynn Margulis has pointed out that female orgasm leads to female choice, and female choice is how females influence the evolutionary trajectory of their species. Natalie Angier's recent book Woman: An Intimate Geography stressed the clitoral orgasm's role in sexual choice: "She is likely to have sex with men she finds attractive, men with whom she feels comfortable for any number of reasons, and thus to further her personal, political, and genetic designs." I agree that the clitoris is an adaptation for sexual choice, and want to go one step further in considering its design within a sexual selection framework.

The sex difference between penis and clitoris can be viewed as a physical manifestation of Fisher's runaway process: a highly developed male trait (the penis) designed to stimulate, and a highly discerning female preference (the clitoral orgasm) designed to respond selectively to skillful stimulation. If this runaway model is right, then there was a sort of stimulatory arms race between the human penis and the human clitoris. The penis evolved to deliver more and more stimulation, while the clitoris evolved to demand more and more.

This tension explains why women and men are not well adapted to giving each other easy, simultaneous, repeated orgasms. If the function of orgasm were simply to reinforce monogamous pair-bonds, why should evolution make female orgasm so difficult and male orgasm so easy during vaginal intercourse? If female orgasm is a side-effect of male orgasm, why does it just happen to work when an attractive man provides a lot of foreplay and deep, slow copulatory thrusting, but not so well when sex is hurried or the partner is undesirable? Surely, sexual selection theory offers insight into this ancient human mystery. Female orgasm seems poorly designed as a pair-bonding mechanism, but it is perfectly designed as a discriminatory system that separates the men from the boys.

Yet the image of an evolutionary arms race between penis and clitoris is not quite accurate. The female mechanism for assessing penis size is not the clitoris itself, but the ring of nerves around the entrance to the vagina, which sense circumference. The clitoris does something more sophisticated, assessing the male's ability to move in pleasurable, rhythmic ways during copulation. Also, clitoral stimulation usually leads to orgasm only when the female mind is feeling erotic about the man and the situation. Human female orgasm depends on an interaction between the clitoris, the hypothalamus (the brain's emotional center), and the cerebral cortex (the brain's cognitive center). The clitoris is only the tip of the psychological iceberg in female choice. Having a mate with a large penis is not enough. To be fair, the penis is not just an insensate stimulator either. It is also a mechanism for male mate choice. If it is happy, its owner may be more likely to stay in a long-term relationship with a woman.

Tragically, while scientists in developed countries spent decades debating whether clitorises are legitimate adaptations, over a hundred million clitorises were cut out of African girls by village women precisely so that the girls would not be tempted to exercise their powers of sexual choice. Currently, another two million girls a year are genitally mutilated in countries such as Egypt, Sudan, Somalia, and Ethiopia. To my mind, sexual selection theory offers a powerful scientific rebuttal to the argument that we should accept female genital mutilation in such countries as part of "traditional tribal practice."

Just as the penis can be seen as a metaphor for the mind's sexually selected entertainment abilities, the clitoris can be seen as a metaphor for the mind's judgment and discrimination abilities. When we see a human perceptual or cognitive ability that looks curiously sensitive to stimulation yet resistant to satisfaction, we should not assume that it is a poorly designed information processing system. It may be part of a system for sexual or social discrimination. Consider humor. Some theories of humor have proposed that laughter evolved to promote group bonding, discharge nervous tension, or keep us healthy. The more laughter the better. Such theories predict that we should laugh at any joke, however stupid, however many times we have heard it before, yet we do not. A good sense of humor means a discriminating sense of humor, not a hyena-like shriek at every repetitive pratfall. Such discrimination is easy to understand if our sense of humor evolved in the service of sexual choice, to assess the joke-telling ability of others.

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