Jokes about breasts, and men looking at breasts, are such a comedy staple they've become a kind of go-to cliché. How many times have we seen a man talking to a curvaceous woman only to have her point to her own eyes and say "Hey, buddy, up here!"?
It's funny -- or, at least, it was funny the first dozen times we saw it -- because it's true. The male eye does have a way of drifting south. But why? Why are heterosexual men so fascinated by women's breasts that we sometimes act as if the breasts are the seat of the soul?
Well, we happen to be heterosexual men. We also happen to be men interested in biology -- one of us, Larry, is one of the world's leading experts in the neuroscience of social bonding. So we've been thinking about this, and, in our new book, The Chemistry Between Us: Love, Sex, and the Science of Attraction, we propose an answer.
Biologically speaking, this human male breast obsession is pretty weird. Men are the only male mammals fascinated by breasts in a sexual context. Women are the only female mammals whose breasts become enlarged at puberty, independent of pregnancy. We are also the only species in which males caress, massage and even orally stimulate the female breasts during foreplay and sex.
Women do seem to enjoy the attention, at least at the right moments. When Roy Levin, of the University of Sheffield, and Cindy Meston, of the University of Texas, polled 301 people -- including 153 women -- they found that stimulating the breasts or nipples enhanced sexual arousal in about 82 percent of the women. Nearly 60 percent explicitly asked to have their nipples touched.
Men are generally pretty happy to oblige. As the success of Hooters, "men's" magazines, a kajillion websites, and about 10,000 years of art tell us, men are extremely drawn to breasts, and not because boys learn on the playground that breasts are something that they should be interested in. It's biological and deeply engrained in our brain. In fact, research indicates that when we're confronted with breasts, or even breast-related stimuli, like bras, we'll start making bad decisions (and not just to eat at Hooters).
For example, in one study, men were offered money payouts. They could have a few Euros right away, or, if they agreed to wait a few days, more Euros later. In this version of a classic "delayed gratification" (also called intertemporal choice by behavioral economists) experiment, some men watched videos of pastoral scenes while others watched videos of attractive women with lots of skin exposed running in slo-mo, "Baywatch" style. The men who watched the women's breasts doing what women's breasts do opted for the smaller-sooner payouts significantly more often then men who watched the pastoral scene.
This likely indicates that parts of their brains associated with "reward," the pleasure centers, and the sites of goal-directed motivation, were shouting down the reasoning centers of their brains, primarily the pre-frontal cortex. Neurochemicals were activating those reward and motivational circuits to drive men toward taking the short money.
So breasts are mighty tempting. But what purpose could this possibly serve?
Some evolutionary biologists have suggested that full breasts store needed fat, which, in turn, signals to a man that a woman is in good health and therefore a top-notch prospect to bear and raise children. But men aren't known for being particularly choosy about sex partners. After all, sperm is cheap. Since we don't get pregnant, and bear children, it doesn't cost us much to spread it around. If the main goal of sex -- evolutionarily speaking -- is to pass along one's genes, it would make more sense to have sex with as many women as possible, regardless of whether or not they looked like last month's Playmate.
Another hypothesis is based on the idea that most primates have sex with the male entering from behind. This may explain why some female monkeys display elaborate rear-end advertising. In humans, goes the argument, breasts became larger to mimic the contours of a woman's rear.
We think both of these explanations are bunk! Rather, there's only one neurological explanation, and it has to do with brain mechanisms that promote the powerful bond of a mother to her infant.
When a woman gives birth, her newborn will engage in some pretty elaborate manipulations of its mother's breasts. This stimulation sends signals along nerves and into the brain. There, the signals trigger the release of a neurochemical called oxytocin from the brain's hypothalamus. This oxytocin release eventually stimulates smooth muscles in a woman's breasts to eject milk, making it available to her nursing baby.
But oxytocin release has other effects, too. When released at the baby's instigation, the attention of the mother focuses on her baby. The infant becomes the most important thing in the world. Oxytocin, acting in concert with dopamine, also helps imprint the newborn's face, smell and sounds in the mother's reward circuitry, making nursing and nurturing a feel-good experience, motivating her to keep doing it and forging the mother-infant bond. This bond is not only the most beautiful of all social bonds, it can also be the most enduring, lasting a lifetime.
Another human oddity is that we're among the very rare animals that have sex face-to-face, looking into each other's eyes. We believe this quirk of human sexuality has evolved to exploit the ancient mother-infant bonding brain circuitry as a way to help form bonds between lovers.
When a partner touches, massages or nibbles a woman's breasts, it sparks the same series of brain events as nursing. Oxytocin focuses the brain's attention to the partner's face, smell, and voice. The combination of oxytocin release from breast stimulation, and the surge of dopamine from the excitement of foreplay and face-to-face sex, help create an association of the lover's face and eyes with the pleasurable feelings, building a bond in the women's brain.
So joke all you want, but our fascination with your breasts, far from being creepy, is an unconscious evolutionary drive prompting us to activate powerful bonding circuits that help create a loving, nurturing bond.
For more, including the male side of this equation, see our book, "The Chemistry Between Us."
ALSO ON HUFFPOST WOMEN: 7 Steps To Mind-blowing Sex
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