There are a lot of myths about sex out there. In their book Don't Put That in There!: And 69 Other Sex Myths Debunked, Indiana University doctors Aaron Carroll and Rachel Vreeman look deep into the scientific and medical literature to show that many popular ideas about sex are flat wrong.
I spoke to Carroll about some of the most enduring misconceptions about sex.
Myth 1: Men want sex way, way more than women do
"There's been lots of research on this," Carroll said. "This idea that men are obsessed with sex and that women don't think about it is false."
Here's what research does find: "More than half of men report that they think about sex every day or maybe several times a day. But that means that almost half of men don't think about sex even every day. Granted, fewer women [19 percent] think about sex every day than men do, but again, we're talking about once a day or every day.
"This idea that men are having much more sex than women is false, as well. If you look at single people, just over half of men ages 18 to 24 haven't had sex in the last year. And fewer women — it's like 57 percent of men versus 51 percent of women — haven't had sex in the year before.
"If you look at the other side of the spectrum, of having the most sex, 5 percent of single women in that age group had sex four or more times per week, versus 2 percent of men. They [women] start to fall behind at 30."
Myth 2: Oysters and chocolate are aphrodisiacs
Carroll is very skeptical of this one: "As with all sexual arousal, there is some sort of placebo effect, where if people believe that they're going to get sexually aroused by something, it's probably more likely to occur."
"You can probably find small, biased studies" that suggest some foods are aphrodisiacs, he said. "But when they try to do double-blind placebo-controlled trials, there's very little evidence that any of that works. Some people will point to lab studies with rats where they fed them different kinds of stuff. Well, that's great for rats, but human sexuality is a little more complicated. There's just a total lack of evidence."
"The question is if I somehow gave this to you, and you were not aware of it, would you be more willing to be aroused? And there's no evidence of that for any food."
Myth 3: Circumcision dampens sexual sensation
"I've always heard that circumcision is a negative thing because it's going to reduce people's sensation," Carroll said. "That the rationale behind that is that the foreskin is going to be very sensitive. And if you take the foreskin off, then the head of the penis just rubs against things its entire life, and therefore might get calloused or not be as sensitive.
"In some countries where HIV is much more prevalent, this [circumcision] became a potential way to try to reduce men's chance of getting HIV. And they now have studies where they took men and got them to agree to be randomized to be circumcised or not, to see if it would change the rates of HIV being transmitted.
"As a side note, they could also ask them if it changed sensation. It turns out that the opposite of what most people might have thought occurred. Men who had circumcision didn't have any problems with their penises becoming less sensitive. In fact, there was some evidence that their penises became more sensitive.
"Whether or not that is sustained over time, we don't know. We don't have long-term studies on the order of years to see if it would decrease sensation over time."
(Note: Since this story was first published, a study testing the penis sensitivity of 62 total men found no differences between those who had been circumcised and those who hadn't. The study was too small to necessarily settle the case for good, but as Vox's Brian Resnick noted, "That no effect was found in this small group probably means if a difference does exists, it's likely very small.")
Myth 4: Female ejaculation only exists in porn
Carroll said the research on this one is surprisingly thin. "Why don't we have any studies that are conclusively determining what the fluid is, or does it actually happen? That would be easy to do, you would think. And yet no one has done that study. You just have to go to the internet, and there's huge amounts of evidence that this is happening."
"There was only one real study that did it through close observation, and it was very small," he says. "It was 38 women, and they didn't see a release of fluid, so they said it doesn't exist. But it's like, really? Could they not turn on the internet and find someone who does report to squirt all the time, and go study them?"
Myth 5: The average penis is 7 inches long
"There's a big difference between studies that ask men how big their penises are and those that actually measure how big their penises are," Carroll said.
"When men report it, they find that the average erect penis size is like 5.6 to 6.4 inches. But in studies where urologists actually measure stretched penile length, the average size actually comes out less. In most of the studies, the average size is like 4.7 to 5.1 inches. So the idea that the average penis is 7 inches seems a little inflated."
Myth 6: Women peak sexually later than men do
"I had always been told that men peak way earlier than women — that women sort of hit the peak of their sexuality in their 30s and that men were much earlier," Carroll said. Still, the research doesn't seem to bear this out.
"Part of the issue is: How do you define peak? Is it when you're having the most sex? Is it when you're having the most pleasurable sex? But it almost didn't matter what metric you picked —the idea of women peaking way later than men was just not true," he said.
"If your metric is having more sex, men evidently do it later in life than women do. Single men were most likely to have sex four or more times a week in their 30s, and partnered men are most likely in their 40s. And for both single and partnered women, frequent sex is most likely in their 20s.
"I think the take-home message is there's no pattern here. The idea that men and women are completely out of sync — there's no truth to that at all."
Myth 7: Only men have wet dreams
"If you define wet dream as an intense dream that results in orgasm that wakes you up, well, then it turns out that women have them too, and at much higher numbers than you would think," Carroll said.
"Even back in the '50s, when you wouldn't expect lots of women to theoretically report this, the Kinsey Institute found that almost 40 percent of women had reported having a wet dream — as defined as I just did — at some point in the past."
"There have been more thorough studies published in journals," he added. "They did a study of students at a large Midwestern university and found that 30 percent of [women] had had a wet dream in the last year. So these are not uncommon."
Myth 8: Almost everything written about the G-spot
"If you talk to women, and you ask them what they think (and that's probably the most important thing), the collective wisdom seems to be that G-spots absolutely exist — that there are parts of the front of the vagina that are more sensitive," Carroll said.
"Unfortunately, we can't seem to prove it scientifically, in the sense that we can't find an area with more nerves. On individual scans we can't see any part of the vagina that appears different with respect to blood flow or amount of nerves or anything like that."
"Having said that," he added, "a lot of those studies are very, very small, and I wouldn't take them to be proven as conclusive. There's no scientific evidence proving it doesn't exist, and tons and tons of women are reporting that it does."
Myth 9: You can get an STD from a toilet seat
"What's amazing is how panicked people are about getting anything from a toilet seat. The toilet seat is like the cleanest thing in the bathroom, because it's the thing that they're obsessed about washing," Carroll said.
"Most sexually transmitted diseases don't live in the air, and they won't live outside the body. Almost all kinds of germs that cause sexually transmitted diseases will die very quickly, even if they were on a surface.
"There are almost no reports of sexually transmitted diseases, if any, being transmitted by toilet seats. And certainly not HIV. We could find really no evidence of real studies or anything where people could report that this had actually happened. Having said that, if it makes you feel better to wipe off toilet seat, go ahead."
This interview has been edited for length and clarity. It was originally published on July 2, 2014, and has been slightly updated since then.