First of all, handedness is definately genetic, not a learned trait. It's anyone's guess, however, why handedness would be related to which testicle hangs lower.

As to where I read this, it was in a sex advice column in a magazine, though I do not remember the magazine or the columnist's name. (It was not Dan Savage.) The letter writer had asked about this very subject of one testicle hanging lower and the columnist, who presumably had done some research, mentioned the handedness matter. He was careful to point out that there are exceptions. You, littledick and I are three exceptions.