That three-letter word that can make you both blush and say "aahhh" is more than a simple act of lust. Sex has been around since, oh yeah, nearly the beginning of time. And since then scientists and enthusiasts alike have delved into understanding and even improving the act and its outcomes. Here's a look at some of the coolest facts about S-E-X.
Let's get down to business about the facts of life: Sex is made for mixing and mingling genes. These genes are carried by the male and female gamete cells, the sperm and eggs. Most of the time, a woman releases one egg per month, but men are much more prolific. In a single ejaculation, a guy sends between 30 million and 750 million sperm swimming toward that egg.
Human males have nothing on pigs, though. A single swine ejaculate contains about 8 billion sperm cells.
Humans start to think about sex relatively early in our life spans. By age 19, about 70 percent of American teenagers have had sex.
Unintended pregnancies are all too common, but the stars do have to align to get a single sperm to a ready egg. Women are fertile for about three to six days each month, depending on their menstrual cycle. Sperm can hang around in the reproductive tract for up to five days and survive, but it's rare for them to remain viable for more than two days or so.
Trying to keep these stars from aligning may be tough, though. With typical use, fertility-awareness methods of birth control (when a couple avoids sex during fertile days) result in 24 pregnancies for every 100 couples who use these methods each year.
Ovulation heats up a woman's body by as much as half a degree Fahrenheit. Before ovulation, most women run between 96 and 98 degrees F (35.5 to 36.6 degrees Celsius). Right after ovulation, body temperature goes up to around 97 to 99 degrees F (36.1 to 37.2 degrees C). The most effective fertility-awareness methods of birth control require daily temperature-taking to detect ovulation.
Bestiality is generally frowned upon. Here's one more reason to abstain: A 2011 study found that sex with animals, such as chickens, pigs and horses, is linked to penile cancer, perhaps because microtrauma to the penis during these acts lets foreign microorganisms in. Some microorganisms, such as the human papilloma virus (HPV), can cause cancer in humans.
Humans sometimes go to strange lengths for sex. Take the alleged aphrodisiac Spanish fly. It's a ground-up bug called a blister beetle that contains the acid cantharidin. When taken and excreted, it causes a burning sensation in the urethra that apparently passes for sexual excitement in some circles. Oh, and the powder is toxic. [Top 10 Aphrodisiacs]
Learning about sex makes you more likely to go out and do it, right? Nope. According research published in 2012 by the nonprofit Guttmacher Institute, any sex education at all delays teen sex.
Group sex and adolescents?! Yes, according a 2011 study, one in 13 teen girls, ages 14 to 20, reported having a group-sex experience, with these gals also being more likely to have been exposed to pornography and childhood sexual abuse than their peers. The study results, detailed in the Journal of Urban Health: Bulletin of the New York Academy of Medicine, showed that 7.3 percent of the 328 teen girls surveyed said they'd had group sex. More than half of that group reported being pressured into the multiple-sex-partner situation; 45 percent indicated no condom was used during their most recent group-sex encounter.
Sex is increasingly in-your-face, according to an analysis of magazine covers from 1983 to 2003. During that period, the number of sexy ads in mainstream publications rose from 15 percent to 27 percent. Alcohol, entertainment and beauty ads drove the sexiness trend.
Are we boring you? At least 12 percent of moms aren't always into sex, according to a nationally representative survey that found 12 percent of women admitted to using their cellphones during the deed. (Twenty-one percent of the nationally representative sample of more than 1,000 mothers also said they bring their smartphones to the toilet.)
Ignore Hollywood: Sex is not just for the young and skinny. Unsurprisingly, loving your body has benefits in the sack. A small study of obese women found that those who accepted their bodies reported better sex, while those who hated their size said their sex lives suffered.
Guys and gals lose their virginity at about the same time, on average, with the Kinsey Institute estimating the average age of first intercourse for guys is 16.9 and gals 17.4.
Putting on a condom may not be as simple as it seems, really. A review of 50 relevant studies on condom use, published Feb. 17, 2012, in the journal Sexual Health, found that condom-use errors are common. For instance, the various studies showed between 17 and 51 percent of those queried said they put on a condom partway through intercourse, negating any disease-controlling benefits. And about 75 percent of men and 82 percent of women failed to check condoms for damage before using them. [14 Common Condom Use Errors]
At the very least, be glad you aren't a sea slug. These hermaphroditic creatures' sexual repertoire involves stabbing with a syringe-like penile appendage to inject prostate fluids into the body. Oh, and sea slug penises come complete with spines to anchor the organ during sex.
Don't be fooled, those safe-sex practitioners are still having some fun. Responsible sex has gotten a bad rap, with one survey showing a quarter of young men and women consider sex with a condom a "hassle." But there's hope yet, as other research finds safe sex can be fun. A study published in 2012 in the journal Perspectives on Sexual and Reproductive Health found that certain factors, such as a partner's comfort and oral and genital stimulation, were linked with pleasurable safe-sex experiences.
What else makes sex fun? Talking about it. People who are more comfortable with sexual communication are happier with their sex lives, according to research published in September 2012. You don't even have to use words: The happiest participants were those who used nonverbal communication to tell their partners what they liked in bed.
Speaking of condoms, who's using them? Not college gals, as a 2012 study in the Journal of Sex Research found women use condoms less and less frequently over the course of their freshman year. And perhaps surprisingly, their younger, and male, counterparts are more apt to practice safe sex; a higher proportion than ever (80 percent) of teenage guys are using a condom the first time they have sex, according to a Centers for Disease Control and Prevention (CDC) report that compared 2002 data with that from 2006-2010.
Now to more pleasurable thoughts. Women may not need a guy, a vibrator, or any other direct sexual stimulation to have an orgasm, as exercise may do the trick. A study involving women who had experienced exercise-induced orgasms and a group who reported exercise-caused sexual pleasure found that 40 percent of these women had these experiences on more than 11 occasions in their lives.
Even so, most of the women in the "orgasm" group said they felt some level of embarrassment when exercising in public places, the researchers say in a 2012 issue of the journal Sexual and Relationship Therapy.
While some women can orgasm with a little physical activity, about one in four women find reaching orgasm an elusive goal. Research detailed in August 2010 in the Journal of the British Association of Urological Surgeons found that for women ages 18 to 30 the No. 1 sex complaint is trouble reaching orgasm, while older women mostly complain about lack of sexual desire.
Unfortunately, science has not yet come to the rescue. A review of 101 studies on female orgasm disorder, in which a woman has trouble climaxing or reaching orgasm at all, showed treatments for the disorder are inadequate. In addition, the Food and Drug Administration reported in 2010 the drug flibanserin (female Viagra) didn't increase a woman's sexual desire.
But perhaps women can take things into their own hands, or minds, as other research has found a lack of erotic thoughts, and even negative thoughts, during sex are linked to the trouble some women have reaching orgasm. These negative or distracting thoughts fell into six categories: thoughts of sexual abuse, sexual failure, a partner's lack of affection, sexual passivity and control, things that had no erotic nature, and body shortcomings.
"There is no easy way to avoid negative or distractive thoughts," study researcher Marta Xavier Cuntim, a clinical psychologist in Portugal, told LiveScience at the time the study was published in 2011 in the journal Sexologies. "However, if we know that they exist, it is easier to learn to deal with them."
Taking sex outside the species is generally frowned upon. But back when humans had other hominin cousins to hang out with, things got cozy. Up until about 47,000 years ago, modern humans and Neanderthals likely interbed. However, research suggests that these liasons only occasionally resulted in offspring.
Wait a minute. What is sex, anyway? According to the famous Kinsey Reports, opinions vary. In a 2010 study by the Kinsey Institute, 45 percent said manual stimulation of the genitals is sex, 71 percent said oral sex is sex and 80 percent said anal intercourse is sex.
Equality pays off between the sheets: Research published in October 2012 in the journal Sex Roles found that young men who believe that men should take charge in the bedroom were actually less likely to feel confident in sexual situations compared with guys who were less concerned with gender roles. The belief that men need to dominate may prevent them from communicating about sex with their partners or asking questions about things they don't know.
Does size matter? For some. A study published Sept. 24, 2012 in the Journal of Sexual Medicine found that women who report having frequent orgasms with vaginal stimulation are more likely to say they prefer sex with men with larger penises.
For what it's worth, the same study considered an "average" penis to be about the length of a U.K. 20-pound note or a U.S. dollar bill, which are 5.9 inches (14.9 centimeters) and 6.1 inches (15.5 cm) long, respectively. Across multiple studies, average erect or flaccid-but-stretched penis length ranged from 4.7 inches (12 cm) to 6.5 inches (16.7 cm).
On the whole, however, women worry less about penis size than men, according to a 2007 review of research published in the British Journal of Urology International. Personality and grooming rank way higher on the list of concerns, the researchers found. Eighty-five percent of women were happy with their partner's penis size, compared with only 55 percent of men who were satisfied with their own penis.
Vaginal size gets less play than the male equivalent. According to a 2006 study in the journal Human Reproduction, the average vagina is about 2.44 inches (6.2 cm) long, but vaginal length ranges wildly from 1.5 inches (4 cm) to nearly 4 inches (9.5 cm). As any woman who has given birth can attest, the vagina is very stretchy, and its length can change across a woman's menstrual cycle as her cervix changes position.
Ladies and gentlemen, we seem to have a failure to communicate. Results from the 2010 National Survey of Sexual Health and Behavior (NSSHB) revealed that 85 percent of men said their partner orgasmed the last time they had sex. Only 64 percent of women said the same. The gap between what women and men reported is too large to be explained by some men having had male partners.
But there's good news for women, too. The same NSSHB report found that for women and men ages 18 to 59, getting older was associated with a decreased likelihood of orgasming for men. For women, it was the opposite: Older women were more likely to say they'd orgasmed during their last sexual encounter than younger women.
Can oral sex kill? It's incredibly rare, but there are a few reported cases of mostly pregnant women ending up with venous air embolisms after their partners blew air into their vaginas during sex. When they enter the circulatory system, air bubbles can block vessels, causing pain, tissue damage and, in extreme cases, death.
Researchers reported a case of venous air embolism after oral sex in the Journal of Emergency Medicine in 1993, but the danger is nevertheless slim. A 1998 paper published in the Postgraduate Medical Journal reviewed data on more than 20 million pregnancies, and found only 18 cases where a venous embolism caused death. In 2007, a similar review found only 22 cases of sex-related venous air embolism, 19 of which were in pregnant women.
In some mammals, semen may trigger ovulation. According to research published in August 2012 in the journal PNAS, a compound in semen can trigger an egg to be released from the ovary in animals that ovulate only when they've had sex. Not only did the finding explain how this "induced ovulation" works, it raised questions about human fertility. The same protein is present in human semen, but because humans ovulate on a regular schedule, it's not clear whether it has any function.
Not having sex seems to be gaining steam. A study conducted between 2006 and 2008 with 13,500 men and women found some 27 percent of male 15- to 24-year-olds had never had sexual contact, of any form, with another person; in 2002, 22 percent of this age group said the same. For females in that same age group, 29 percent had never had sex in the 2006-08 survey, compared with 22 percent who said the same in 2002. Not to worry, these are not your 40-year-old virgins. The study, carried out by the Centers for Disease Control and Prevention, found almost everyone ages 25 to 44 had taken the plunge and had sex.
Just as people can sleep-walk, sleep-eat and sleep-drive, they can have sex during a snooze. The condition is called "sexsomnia," and it occurs when people engage in sexual behavior while asleep. A 2010 study found that of 832 patients at a sleep disorders clinic, 7.6 percent reported sleep-sex.
Speaking of sleep and sex, men have a leg up here. When their heads hit the pillow, women tend to have more nightmares, while men dream of sex more often. The study, which included nearly 200 male and female young adults, found women's nightmares could be broadly divided into three categories: fearful dreams (being chased or life threatened), losing a loved one, and confused dreams. When the ladies' dreams did head into the naughty direction, they reported more kissing and sexual fantasies about other dream characters; men, meanwhile, reported more actual intercourse.
Guys' tendency to snooze after sex may be based in science. Most practically, couples often do the deed at night, in bed, and sex takes physical energy. Then there's biology: When guys orgasm, their brains release a mix of chemicals, including prolactin, which spikes during shut-eye in animals. In addition, injections of it have been shown to make animals sleepy. Brain-imaging research has also suggested to reach orgasm, guys and gals must let go of anxiety, something that would seem to put you in a relaxed — even sleepy — state. That means lights out for guys.
Some of us even use sex like we would a cigarette or a stiff drink — sex is an addiction, scientists say. While they haven't agreed on a definition of what is formally called hypersexual disorder, scientists say it's not normal sexual behavior, even frequent sex or pornography watching. Rather, those with the disorder tend to feel out of control and act on sexual urges with no consideration of consequences. Sex addicts may also use sexual activity to deal with stress; and like other addictions, this one interferes with the person's ability to function day-to-day.
Here's one to throw into the no-duh category, but nevertheless, science has found one way to better ensure risky sex: Get drunk. The study, detailed in the January 2012 issue of the journal Addiction, found the more alcohol participants consumed, the higher their willingness to engage in unsafe sex. The researchers noted the finding is an important one, as unsafe sex is a major pathway to HIV infection and other diseases. Despite this risk, HIV incidence in most high-income countries like the United States hasn't changed much over the past decade, they said.
Oh yeah, meth use can also fuel risky sexual behaviors. A study published in the Aug. 1 issue of the Archives of Pediatrics & Adolescent Medicine found participants (young men who have sex with other men) who had recently used methamphetamine were also more likely than others to have sex with an HIV-infected other and an injection drug user.
Sex is supposed to bring ecstasy, but for some women it can bring the blues. Called postcoital dysphoria, this experience of negative feelings following otherwise satisfactory intercourse is not so uncommon. About 33 percent of 200 young women participating in a study by the Queensland University of Technology said they had experienced post-sex blues at some point in their lives. While scientists had thought the melancholy following sex was linked to past sexual abuse, the new study found only a modest association with psychological distress. "This suggests other factors such as biological predisposition may be more important in understanding the phenomenon and identifying women at risk of experiencing postcoital dysphoria," researcher Robert Schweitzer said in a statement.
If sex in space doesn't blow your mind, this will — Sex can trigger transient global amnesia, a rare condition in which memory suddenly, temporarily, disappears. Researchers describe one instance in which a 54-year-old woman showed up in the emergency room at Georgetown University Hospital unable to remember the past 24 hours. Her amnesia, it seems, had started right after having sex with her husband just an hour before. The mind-blowing sex affects just three to five people per 100,000 each year. But what makes transient global amnesia so eerie is that researchers aren't sure what causes it, or why patients remain otherwise chatty and alert while missing large chunks of their memories.
A far-out thought: Could you have sex in zero-gravity? Well, the idea has yet to be tested as far as anyone knows, though if the human race were to colonize another orb it'd be key, albeit, tricky. "Sex is very difficult in zero gravity, apparently, because you have no traction and you keep bumping against the walls," biologist Athena Andreadis of the University of Massachusetts Medical School told SPACE.com, a sister site to LiveScience. "Think about it: you have no friction, you have no resistance."
Why do we have sex in the first place? One of the strongest hypotheses for why organisms have two sexes that have sex is the so-called Red Queen Hypothesis, which is named after the monarch in Lewis Carroll's "Through the Looking Glass." The hypothesis suggests organisms are in a constant race with the parasites that live on them; as parasites evolve genetic mutations to take advantage of organisms' weaknesses, those organisms with rare versions of genes, called alleles, are less susceptible and more likely to survive the parasitic onslought. Sex, it seems, gives such organisms a leg up, according to a study in the journal Science.
"This continual selection for rarity favors sexual reproduction over asexual reproduction; sexual recombination allows hosts to reshuffle their pack of alleles and generate new, rare combinations in their offspring," writes Michael Brothurst of the University of Liverpool in the research article.
Are you clingy? How about aloof? Turns out, both types can wreak havoc on sex, according to a review of research detailed July 3, 2012, in The Journal of Sexual Medicine. The study researchers suggest those who are clingy (also called anxious attachment) may put a lot of effort into encouraging a partner to have sex, placing too much importance on sex in a relationship and being hypervigilant about sexual rejection. At the other end of the spectrum, those with avoidant attachment styles may represssexual desire, arousal and pleasure from orgasm, and distance themselves from a partner who is interested in sex. The result: less sexual satisfaction.
The icky bodily fluids being swapped during sexual intercourse are enough to take the pleasure out of a roll in the hay. But research detailed online Sept. 12, 2012, suggests we don't gross out because sexual arousal somehow dampens our disgust response. In the study of heterosexuals, the researchers found women primed to be sexually aroused found activities that might otherwise seem sexually disgusting (think touching "used" condoms) much less gross than other participants. The results may shed light on problems of sexual arousal andsexual pain disorders; perhaps these women arousal doesn't dampen their disgust responses.
Some sexual dysfunction may stem from how a woman feels about the appearance of her genitals. The anxiety over "large" labia or lips has led women to seek female genital cosmetic surgery. An analysis of 10 websites for practices offering these procedures suggests they provide confusing information on the actual procedures, lack short- and long-term risk information and promote unsubstantiated claims of the psychological, physical and sexual enhancements tied to the procedures sometimes referred to as designer vaginas.
Self-sexualization starts early, with girls as young as 6 already thinking of themselves as sex objects of sorts, according to a study detailed July 6, 2012, in the journal Sex Roles. Using paper dolls to assess self-sexualization in a group of 6- to 9-year-old girls, the researchers found girls chose the "sexy" doll most often. For instance, 68 percent of the girls said the doll looked how she wanted to look, while 72 percent said the sexy dolls were more popular than the non-sexy ones.
People who are well-off have better sex lives than their poorer counterparts, research finds.
The study, published in the journal Annals of Epidemiology in October 2013, surveyed almost 1,000 citizens of Spain. Overall sexual satisfaction was high, with 90 percent of men and women saying they were quite or very satisfied with their sex lives. But both men and women (but especially women) were less satisfied the lower their socioeconomic status. The reason may be that poorer people have less control over contraception and have a higher experience of past sexual abuse.