It's fairly recent, too. As recently as the 1860s, circumcision was still primarily thought of as a "Jewish rite," something that for non-Jews would be done as a last resort in response to infections around the foreskin and other medical problems. But in the late 1800s and early 1900s, America wound up in a frenzy over the problem of masturbation that sounds suspiciously similar to the fears about recreational drugs a half century later -- masturbation was spoken of as a new, addictive fad among our children that doctors said could cause everything from psychosis to epilepsy.
While some might say hallucinating Megan Fox and getting the shakes can only be a positive.
And then, somehow, they decided that circumcision would prevent masturbation (because no Jewish man had ever been caught masturbating up to that point, we guess?). The benefits were claimed to be twofold:
First, it supposedly reduced the secretions that would get inflamed around the foreskin and thus get young boys in the habit of rubbing themselves. Seriously, books at the time treat genital itching as a gateway drug to masturbation -- a 1914 public school sex ed manual says keeping kids from scratching their junk is the only way to keep them out of the insane asylum, where all masturbators end up.
Masturbators and early 21st century rappers.