This article is the first in a new series called "Porn Anthropology," in which we explain the science behind some of pornography's most popular conventions.

It's hard to imagine a time when the "money shot" wasn't a signature of the smut industry. The shot -- where a male porn performer ejaculates, usually on a partner, and the camera captures the action in luxuriating detail -- is the defining aesthetic of contemporary pornography, both gay and straight. But it wasn't always that way.

The "money shot" can be traced back to the premiere of "Deep Throat" in 1972, according to Linda Williams, a film studies professor at UC Berkeley. That isn't to say that male performers didn't bust outside the body before then, but the legendary film “introduced narrativity in the genre and coined the cum shot as its defining figure," she writes in “Hard Core: Power, Pleasure and the Frenzy of the Visible.” Williams explains, "Where the earlier short, silent stag films occasionally included spectacles of external ejaculation (in some cases inadvertently), it was not until the early seventies, with the rise of the hard-core feature, that the money shot assumed the narrative function of signaling the climax of a genital event."

The question of why the money shot has since then, ahem, exploded in popularity is more complicated. The most obvious explanation is one of pure mechanics. “It has to do with the real physiology of orgasm and ejaculation,” says Lisa Jean Moore, author of “Sperm Counts: Overcome by Man’s Most Precious Fluid.” “The way that male orgasm is external, as opposed to female orgasm, which is internal, it sort of lends itself well to cin...