Bonk: The Curious Coupling of Science and Sex

by Mary Roach

and straight, will show immediate genital arousal (as measured by a photoplethysmograph) in response to films of sexual activity, regardless of who is engaging in it—male, female, gay, straight, good hair or bad. Men, contrary to stereotype, tend to respond in a limited manner; they are aroused only by footage that fits their sexual orientation and interests.

While straight women—and gay men—become physically aroused by footage of two men having sex, straight men generally do not. (A straight man will, however, respond to footage of women having sex, partly because he’s looking at two naked women.)

Women’s genitals may respond indiscriminately to images of sex, but the women themselves will often report being totally unaffected by what they’ve viewed. Based upon how they feel, women are quite picky about pornography.

Be all that as it may, it is the mind that speaks a woman’s heart, not the vaginal walls.

Rape offers a plangent illustration of this fact. I learned in a paper by Roy Levin that rape victims occasionally report having responded physically, even though their emotional state was a mixture of fear, anger, and revulsion.

Lubrication from “reflex arousal” (physical stimulation of the genitals) can occur with absolutely no subjective emotional arousal.

Regardless of the mechanisms that may or may not explain a rape victim’s physical state, a rapist’s defense based upon evidence of arousal has, to quote Levin, “no intrinsic validity and should be disregarded.”

Experiments had

shown that people’s pupils enlarge when they’re interested in what they’re looking at.

When a penis hits a cervix in a certain way, for instance, this is a stimulus. In response, a woman’s adductor muscles reflexively contract, pulling her thighs together and—in what might be a protective mechanism—limiting the depth of the man’s thrusts.

When the lower third of a woman’s vagina widens—as it does during penetration—several reflexes get triggered. The vaginocavernosus reflex may sound dry or arcane on paper, but it is the basis of what appears to be a remarkable physical synergy between male and female anatomy during sex. When the cavernosus muscles reflexively contract—as they do upon entry—this boosts blood flow to

the cli...

At the same time as the vaginocavernosus reflex is affecting the clitoris, Shafik found, it’s also putting the squeeze on the man’s dorsal vein, helping trap blood in the penis and keeping it firm.

When the tip of a penis is stimulated—by bumping against a cervix, say, or the opening to a vagina (or any other orifice, for that matter)—several muscles contract reflexively. Among them are an anal and a urethral sphincter. The closing of the latter prevents urine from mixing with semen in the urethra during ejaculation. The closing of the two together—let’s let Dr. Shafik say it—“prevents leak of urine or stools” during sex.

Hormones can act as the invisible puppet strings behind the discomfiting one-night stand, the shameless flirtation with the bellboy, the unexpected and regrettable kiss between friends. Your genes want you to get pregnant, and hormones are their magic wand.

Women who are part of couples will initiate sex more often at mid-cycle than during the rest of the month—provided they’re using a reliable birth-control method (and don’t wish to become pregnant); if they’re not, then they typically avoid mid-cycle sex. Women also masturbate significantly more often around ovulation than at other times. Take the hormones away,

as menopause does, and these mid-cycle spikes in libido level out.

Among women too, socially risky sex tends to happen when the hormones hit their peak. In a study by M. A. Bellis and R. R. Baker, the sex that cheating women were having with their lovers closely mirrored their monthly cycle, peaking on the day of maximum fertility.

U.K. sex physiologist Roy Levin puts two minutes at the fringes of normal; his figure for an average male’s thrusting time is two to five minutes (or, if you prefer, 100 to 500 thrusts).

This is what primate

sex hormones do: “They make individuals perceive other individuals as more attractive than they’d normally perceive them.” Hormones are nature’s three bottles of beer.

In humans, a hormone-based contraceptive levels out the monthly peaks and troughs of one’s natural hormone levels—and, in consequence, those of libido. The Pill supplies a steady daily dose of hormones, enough that your body stops supplying its own unsteady, cyclically fluctuating dose. While the Pill’s estrogen levels...

The Pill contains estrogen and progesterone, but it also affects testosterone. And it is testosterone, more than any other hormone, that influences a woman’s libido. What the Pill does, specifically, is raise levels of sex-hormone-binding globulin (SHBG), a protein in the blood that binds itself to testosterone, taking the hormone out of commission. And going off the Pill might not restore libido.

For many women, the freedom from worrying about pregnancy cancels out any mid-cycle dip in libido; they’re having more sex then, not less. The Pill doesn’t make women enjoy sex less, it doesn’t change their responsiveness; it just mutes their drive. A lot of them don’t even notice, and for some, it’s a price worth paying.

Menopause is a natural, more

exaggerated version of being on the Pill. Estrogen and testosterone levels fall, taki...

“Men’s colognes actually reduced vaginal blood flow.”