If men have sex on the brain, they are not alone. Recent research found that women’s sensory cortex has three distinct areas corresponding to stimulation of the clitoris, vagina, and cervix (1). To their surprise, researchers found that self stimulation of the nipples lights up the same areas. This sheds further light on the sexual importance of breasts.
In an earlier post, I discussed some evolutionary reasons that men are fascinated by women’s breasts and pointed out that stimulation of the breast plays a key role in women’s sexual arousal and satisfaction.
The permanently enlarged human breast is a peculiarity of our species (2). It may have some signal value in communicating fertility and plays a role in physical attractiveness.
Breasts are less eroticized in subsistence societies where women go topless than in our own where they are exploited in advertising, and in pornography. Even in subsistence societies, breasts are not entirely lacking in sexual significance and are generally stimulated in foreplay according to ethnographic accounts (3).
Moreover, the breasts play a key role in female sexual arousal and we are beginning to understand why in terms of hormones and neuroscience. In their classic report on the female sexual response, Masters and Johnson (4) pointed out that breast volume increases during sexual arousal in addition to changes in the areola and erection of the nipples.
The breast and bonding
The function of the breast in sexual behavior is sometimes attributed to face-to-face copulation that is unusual amongst mammals. If the breast is already used for mother- infant bonding, the argument goes, then it is a small step for it to be used in facilitating bonding between lovers. After all, it is in easy reach!
Stimulation of the nipple during breast feeding increases the amount of the hormone oxytocin that circulates. Oxytocin is often referred to as the “cuddling hormone” because it is released by male and female mammals during close social encounters of various kinds (5).
In addition to its general social effects, whereby a mother feels closeness for the baby she is feeding (and vice versa), there are other more specialized functions of oxytocin. One is that milk flows, a reflex known as the “milk let-down response” familiar to mothers and dairy farmers alike.
Another is sexual arousal, and orgasm. Some women experiencing intense pleasure - even orgasm - from breast feeding. This phenomenon was long written off as a mere oddity but neuroscientists are beginning to understand why it happens.
Sexual wiring of women’s brains
The great complexity of the female sexual response may be attributable to the fact that there is not one, but three sensory maps in the parietal cortex that light up in functional MRI images when the genitals are (self) stimulated. One represents the clitoris, another the vagina and the third represents the cervix.
All three of these maps also receive input when the nipple is stimulated. From a functional perspective, this means that the breast doubles as a truly sexual organ. It is not just an exciting visual stimulus for (most) men but also a key source of sexual pleasure for most women.
As to the wiring of men’s nipples the jury is out. Some men’s nipples are also responsive to sexual stimulation but the brain response has yet to be mapped.
1. Komisaruk, B. R., et al. (2011). Women’s clitoris, vagina, and cervix mapped on the sensory cortex: fMRI evidence. The Journal of Sexual Medicine, 8, 2822-2830.
2. Barber, N. (1995). The evolutionary psychology of physical attractiveness: Sexual selection and human morphology. Ethology and Sociobiology, 16, 395-424.
3. Ford, C. S., & Beach, F. A. (1951). Patterns of sexual behavior. New York: Harper.
4. Masters, W. H., & Johnson, V. E. (1966). Human sexual response. Boston: Little Brown.
5. Uvnas-Moberg, K. (1998). Oxytocin may mediate the benefits of positive social interaction and emotions. Psychoneuroendocrinology 23: 819-835.