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What's your number? 10 surprising sex statistics

Check out these stats to see if you are well within the sexual mean — or if you're off the charts.
/ Source: TODAY

Whether it's penis size, papillomavirus risk, or profligate pregnancies, it's good to know the numbers. Check out these stats to see if you are well within the sexual mean — or if you're off the charts.

Why do you do it?

There's the obvious. But there's also an argument for men's biological drive to perpetuate their genes: An 18th-century Russian woman holds the world record for having birthed the most children: 69, which she had over the course of 27 pregnancies that included sixteen pairs of twins, seven sets of triplets, and four sets of quadruplets. But she's outdone by the male record-holder for most kids, a Moroccan emperor who, according to the Guinness Book of World Records, sired "at least 342 daughters and 525 sons, and by 1721, he was reputed to have 700 male descendants."

RELATED: This is what happy couples do to stay together (sex optional)

Source: "Why Evolution Is True" (Viking, 2009), by Jerry Coyne

— By Sally Law, LiveScience's Science of Sex columnist.

Does size matter?

Relax. No matter what movies might suggest, in the United States, the average erect penis is five to seven inches long, and four to six inches in circumference.

Source: Kinsey Institute

Do you need assistance?

Approximately 5 percent of 40-year-old men and between 15 to 25 percent of 65-year-old men experience erectile dysfunction.

Source: National Institute of Diabetes and Digestive and Kidney Diseases

When did you lose your virginity?

The average male loses his virginity at age 16.9; females average slightly older, at 17.4. And a new study shows that genetics may be a factor: inherited traits, such as impulsivity, can make a person more or less willing to have sex at an earlier age.

Sources: Kinsey Institute; California State University

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Where do you sleep?

About one out of 10 married adults — 12 percent — say that they typically sleep alone.

Source: National Sleep Foundation

Do you reach orgasm every time?

While 75 percent of men always reach orgasm during sex, only 29 percent of women report the same. In addition, most women are unable to climax through vaginal intercourse, instead needing clitoral stimulation.

Source: National Health and Social Life Survey

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Do you get fringe (friend) benefits?

Two-thirds of college students have been in a "friends with benefits" relationship, citing the lack of commitment required as the main advantage to such an arrangement. More than half of those who had sex with a friend said they had engaged in all forms of sex; 22.7 percent said they had intercourse only, while 8 percent said they did everything but have intercourse.

Source: Wayne State University and Michigan State University

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How many sex partners have you had?

Couple having sex
Is he the only one, ever? Probably not. Getty Images stock

What's your number? According to a survey of adults aged 20 to 59, women have an average of four sex partners during their lifetime; men have an average of seven.

Source: National Center for Health Statistics

Did you take maternity leave?

Two-thirds of women who had their first baby between 2001 and 2003 worked during their pregnancy, and 80 percent of those women worked within one month or less of giving birth. Compare this to the period between 1961 and 1965, when 44 percent of women worked during their pregnancy (35 percent worked one month or less before delivering).

Source: U.S. Census

Are you infected?

The Center for Disease Control and Prevention estimates there are nearly 20 million new sexually transmitted infections every year in the U.S., half among young people aged 15–24.

In 2014, there were increases in all three nationally reported STDs — chlamydia, syphillis and gonorrhea. The approximately 1.4 million cases of chlamydia were the highest number of annual cases of any condition ever reported to CDC.

RELATED: The 6 biggest health mistakes women make in their 20s

Source: Centers for Disease Control and Prevention

This updated story was originally published in 2009 on Live Science