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Memorialized in monuments, Grecian statues and bathroom graffiti everywhere, the penis may be the most famous and well-studied human organ on the planet.
But despite its outsized profile in the popular imagination, the male member still has a few secrets left to reveal. From penis spines to the ideal size and even penis shame, here are eight wild facts about the male sex organ.
Editor's note: This countdown was first published Aug. 21, 2013.
Average JoesSlide 2 of 17
The average erect penis is about 5.56 inches (14 cm) long, according to a 2013 study detailed in the Journal of Sexual Medicine that surveyed 1,661 men. But variety is the spice of life, and men in that study had members that ranged from 1.6 inches (4cm) long to 10.2 inches (26 cm) long.
Not all erections were created equal. Those who measured their penises after oral sex or intercourse sported larger penises than those who relied on fantasy alone, the study found.
And because it reduces blood flow to the penis, smoking can shorten the average penis by up to 0.4 inches (1 cm), other studies have found.Slide 3 of 17
Size mattersSlide 4 of 17
When it comes to penises, size does matter — at least for some women. Women who are more likely to have vaginal orgasms say it is easier to orgasm with men who have longer penises, according to a 2012 study published in the Journal of Sexual Medicine. Though it's not clear exactly why, a longer penis may be better able to stimulate the vagina and the cervix, study co-author Stuart Brody, a psychologist at the University of the West of Scotland, told LiveScience at the time. In a 2013 study detailed in the journal Proceedings of the National Academy of Sciences, researchers reported women said the ideal penis size varied with a man's height, with a larger organ looking better on taller men.Slide 5 of 17
Penile anomaliesSlide 6 of 17
Very rarely, a man may be born with two penises, a condition that affects every 5 million to 6 million males and is known as diphallus. Unfortunately, this condition doesn't mean double the fun: Both organs are rarely fully functional, and the condition often comes along with other anomalies in the genital area that require surgery to correct.
Men with another penis condition called priapism suffer from a persistent erection that won't go away after four hours. Usually the cause is a failure of blood to return from the penis to the rest of the body, although it is also occasionally found in those with sickle-cell anemia or leukemia. The situation is usually a medical emergency that requires a shot of pseudoepinephrine to constrict the smooth muscles in the penis.Slide 7 of 17
Ancient relicsSlide 8 of 17