5 Things You Didn't Know About Your Penis

Medical experts reveal interesting facts that men and women will find educational -- and surprising.

By Martin F. Downs
WebMD Feature

Reviewed by Louise Chang, MD

Here are some things you might have wondered about your penis, but were afraid to ask.

No. 1: Your Penis Does Have a Mind of Its Own

You've probably noticed that your penis often does its own thing. You may remember times when it was completely inappropriate to have an erection; and yet you couldn't wish it away.

It's true that you have less command over your penis than body parts like your arms and legs. That's because the penis answers to a part of the nervous system that's not always under your conscious control. This is called the autonomic nervous system, which also regulates heart rate and blood pressure.

Sexual arousal usually isn't voluntary. The conscious mind is complicit in it, but a lot of sexual arousal goes on in the sympathetic nervous system. In addition, impulses from the brain during the REM phase of sleep cause erections, whether you're dreaming about sex or about a test you forgot to study for. Heavy lifting or straining to have a bowel movement can also produce an erection.

Just as the penis grows without your consent, sometimes it shrinks. "The flaccid penis varies in size considerably within a given man," says Drogo Montague, MD, a urologist at the Cleveland Clinic. Exposure to cold water or air makes your penis shrink. That's a function of the sympathetic nervous system.

Psychological stress also involves the sympathetic nervous system, and stress has the same effect as a cold shower, Montague says. When you're relaxed and feeling well, your flaccid penis looks bigger than when you're stressed out.

The penis is "kind of a barometer of the sympathetic nervous system," Montague says. So the greeting, "How's it hanging?" is more apt than you might have realized.

No. 2: Your Penis May Be a 'Grower' or a 'Show-er'

Among men, there is no consistent relationship between the size of the flaccid penis and its full erect length.

In one study of 80 men, researchers found that increases from flaccid to erect lengths ranged widely, from less than a quarter inch to 3.5 inches longer.

Whatever the clinical significance of these data may be, the locker-room significance is considerable. You can't assume that a dude with a big limp penis gets much bigger with an erection. And the guy whose penis looks tiny could surprise you with a big erection.

An analysis of more than thousand measurements taken by sex researcher Alfred Kinsey shows that shorter flaccid penises tend to gain about twice as much length as longer flaccid penises.

A penis that doesn't gain much length with an erection has become known as a "show-er," and a penis that gains a lot is said to be a "grower." These are not medical terms, and there aren't scientifically established thresholds for what's a show-er or a grower.

Kinsey's data suggest that most penises aren't extreme show-ers or growers. About 12% of penises gained one-third or less of their total length with an erection, and about 7% doubled in length when erect.

No. 3: Your Penis Is Shaped Like a Boomerang

Your penis is shaped like a boomerang. Just like you don't see all of a big oak tree above ground, you don't see the root of your penis tucked up inside your pelvis and attached to your pubic bone.

In an MRI picture, the penis looks distinctly boomerang-like, as noted by a French researcher who studied men and women having sex inside an MRI scanner.

One method of surgical "penis enlargement" is to cut the ligament that holds the root of the penis up inside the pelvis. This operation may give some men a little extra length if more of the penis protrudes from the body, but there are side effects. This ligament, called the suspensory ligament, makes an erection sturdy. With that ligament cut, the erect penis loses its upward angle and it wobbles at the base. The lack of sturdiness can lead to injury.

No. 4: You Can Break Your Penis

There is no "penis bone," but you can break your penis all the same. It's called penile fracture, and it's not a subtle injury. When it happens, there's "an audible pop or snap," Montague says. Then the penis turns black and blue. And there's terrible pain.

Penile fracture is rare, and it typically happens to younger men because their erections tend to be quite rigid.

Here's how to avoid penile fracture: don't use your penis too roughly. A common way that penile fracture happens, Montague says, is when a man is thrusting too hard and fast during sex, and slams into his partner's pubic bone. Also, a woman who moves wildly while on top of a man during sex can break a man's penis.

Peyronie's syndrome is a related condition that tends to show up more in older men, Montague says. An older man's erection may not be as rigid, but still is hard enough for sex. Over time, if the penis bends too much a certain way during sex, small tears in the tissue can form scars, and the accumulated scar tissue gives the penis an abnormally curved shape.