Alfred Kinsey identified people who didn't have sex as Group X.

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In his groundbreaking 1948 report, "Sexual Behavior in the Human Male," Alfred Kinsey wrote, "the world is not to be divided into sheep and goats" [source: Kinsey Institute]. By that, the controversial sexologist meant that it's a fallacy to neatly slice the human population into a sexual binary divided between heterosexual and homosexual camps. The Kinsey Scale he and his colleagues developed instead plotted people's sexual preferences along a sliding spectrum, leaving room for fleeting attractions and longer-term compulsions alike [source: Kinsey Institute].

For all that inclusiveness, one subgroup nevertheless didn't find a place on the Kinsey continuum. Adult men and women identified as Group X expressed "no socio-sexual contacts or relations" [source: Havlak]. That 1.5 percent of men and roughly 15 percent of women surveyed didn't appear to swing one way or the other; for them, sex was a moot point.

Kinsey's animal farm analogy was unwittingly apropos since it isn't uncommon for rams to fit into their own Group X. A study published in 2002 examined the mating preferences among adult rams and labeled 15 percent of its sample population as asexual, or not attempting to copulate with either male or female sheep [source: Roselli et al]. With his body of research, lead scientist Charles Roselli was using sheep brain imaging, hormone measurements and observational data to establish a biological basis for homosexuality, since a number of those same rams took up with other rams, rather than going for the females in heat [source: Saletan]. But just as Kinsey paid more attention to people's sexual proclivities, instead of Group X's lack thereof, Roselli largely looked over the asexual rams in the mix.

As of 2004, however, Group X and non-reproductive rams have cropped up in more classroom conversations. That was the year that asexuality -- considered by some to be the fourth sexual orientation -- came out of the closet.