Excerpt: 'You: Being Beautiful' on the Biology of Sex

Perhaps that's because men have 2.5 times the amount of brain space devoted to sexual drive that women do (or because women have more important things to think about).

Sex, of course, is more than just thinking about it; it's also about craving it. That craving originates in a part of the brain called the insula. Blocking messages to the insula is one of the ways that cigarette cessation techniques work—good news for many, they don't block sexual craving messages; in fact, bupropion, the drug we most often use in our breathe-free program with nicotine, actually increases libido in most people. The insula (remember it from chapter 8?), a primitive area of the brain, is especially active in women who have more frequent orgasms.

Let's now look at the way men and women biologically work when it come to sex:

WOMEN: During sex, your pupils dilate, nostrils flare, heart rate increases, oxytocin level increases, sweat glands open for cooling, breasts enlarge by 25 percent, and nipples increase in height by half an inch. Infrared cameras also show increased blood flow to the lips, nose, and labia. All of these things happen as the sexual stimuli build up to the almighty orgasm (see Figure 10.3).

A good question to ask right about now: Why do women have orgasms?

Evolutionarily, it was one of the ways that women could tell whether a man would be a good lifelong partner, because it could help women distinguish between a caring, patient male and a selfish or impatient one. Nevertheless, female orgasm can be so subtle that some women don't even know when they've had one.

Here's what happens:

During intercourse, your vaginal walls make fluids that let your partner's penis slide with just the right amount of friction. Together with the sights, sounds, and smells of sex, the stimulation to the clitoris, labia, and breasts all builds up a crescendo of intense physical sensation.

This is about the time when your brain tells your vagina and nearby muscles to contract. Why? To bring his penis in deeper and increase the chance of his sperm hitting its target—the egg.

In the process, some women even ejaculate. During orgasm, the uterus dips in like an anteater and sucks up the semen into the uterus to further increase the chance of fertilization.

The female orgasm also causes hormones to increase contractions in the vagina and uterus and help move semen into the uterus (women who orgasm between 1 minute before and 45 minutes after their partner's ejaculation have a higher tendency to retain sperm compared to those who don't have an orgasm).

The female orgasm, of course, isn't an easy thing to describe.

The brain serves as the main conductor in this symphony, but it might involve many different instruments, sometimes including the area known as the G-spot, which is parallel to a gathering of nerves on the male prostate.

Women usually do not have a single spot like some magic sex-me- here button but rather a region of nerves like those spread over the surface of the male prostate. That's because as a woman's reproductive organs develop in utero, her rudimentary prostate moves away so these nerves end up on the vaginal wall. So if you insert your index finger upward into the vagina and make the "come here" movement, you will touch the G-spot region that exists in some women.

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