9 most provocative sex studies of 2009

We've loved. We've learned. And we've had some of our sexual suspicions confirmed by scientific research. As 2009 comes to a close, LiveScience looks back at the year's nine most intriguing sex lessons.

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Sex smells. A man's sweat smells different when he's sexually aroused — and women can tell the difference between the smell of sexual sweat and the regular stuff, according to a study in The Journal of Neuroscience.

Pulling out works. Well, most of the time. In a paper published in the June issue of Conception magazine, researchers claimed that withdrawal was "almost as effective as the male condom" when it came to pregnancy prevention (a failure rate of 18 percent, vs. the 17 percent failure rate of condoms).

Child's play. An Iowa State University study found that 25 percent of children — ranging in ages from 11 to 16 — in low-income households reported having sex. The average age of first intercourse for that group was 12.77.

Growing pains. According to a study from the University of Turin, penis extenders might work — a particular brand that used traction to gradually stretch the penis over time was found to increase flaccid members' length by almost one inch.

10 surprising sex statistics

The pursuit of pleasure. Men who are very sexually active in their 20s and 30s — especially those who masturbate frequently — are at higher risk for prostate cancer, said researchers at the University of Nottingham. But that risk decreases as a man ages, and once he's in his 50s, even small levels of sexual activity can help protect him from the disease.

Pill popping. In February, the Federal Drug Administration mandated that Bayer, the manufacturer of Yaz birth-control pills, fix their commercials that promoted Yaz as a weapon against acne and PMS and downplayed its potential health risks.

This is a test. The American College of Obstetricians and Gynecologists released new guidelines for cervical-cancer screenings: Women should wait until age 21 to get their first Pap smear, and should be checked every three years (instead of annually) if they have a history of normal test results.

Not yet. According to research published in the Journal of Theoretical Biology, women who hold out on sex are acting on a biological impulse to find more suitable providers — and men are waiting to prove that they're up to the challenge.

Role reversal. Men feel guiltier following sexual infidelity, while women feel worse after an emotional transgression—and both are incredibly self-involved. "If an individual assumes that everyone, regardless of their sex, is most concerned with the same form of infidelity that they themselves are most concerned about, this person would consequently make false inferences leading to feelings of guilt," said researcher Maryanne Fisher, a professor at St. Mary's University in Halifax, Canada.