While 65 percent of women have a left breast that is larger than the right, the same percentage of men have a left testis that hangs lower in the scrotum than the right. The left testicle also tends to be 7-10 percent smaller.

Why?

It’s a question that won an IgNobel prize for Chris McManus. Although scrotal asymmetry is faithfully reproduced in Greek sculpture, the British psychologist observed, the Greeks weren’t exactly anatomically correct. Greek sculptors faithfully made the left ball lower, but they mistakenly made it larger. Absurdly, they thought the testicles acted like weights to keep open the ducts when semen was released. The left one was erroneously believed to be heavier.

Hundreds of years have passed and we still don’t exactly why the testes aren’t side-by-side and symmetrical, but there are a few theories that touch on the mystery:



Temperature regulation. A French study found that the left and right testicles have different modes of thermal regulation. The left runs hotter than the right. They hang at different levels because they have separate arteries and veins that differ from each other in length, angle, and source. Sperm are temperature-sensitive, so having redundant semi-independent temperature regulation mechanisms may be beneficial. Researchers also speculate that twin testes, if snugly pressed against each other, would run too hot and thereby impair sperm production. More exposed surface area = cooler. Hanging at slightly different heights prevents them from crushing and overheating each other.

Handedness. Some studies have found that the left testicle hangs lower in right-handed men and the right testicle hangs lower in left-handed men. Handedness is linked to cerebral asymmetry, and there is some suggestion that testicle size and position are, too. Intriguingly, scientists are investigating whether cognitive performance can be predicted by the testes. According to a Canadian study, men with a larger right testicle performed better on spatial tasks than men with a larger left testicle. (Again, on most men, the left is smaller.)

Poor thing. The littler, lighter, lower-hanging left testicle has such a bad reputation that, according to the Urban Dictionary, it’s now slang for slacker.