With the biology of the birds and the bees firmly in place by the latter half of the 20th century, the compulsion to have sex with a monogamous partner was widely considered a matter of the metaphorical heart, rather than the physical body. Although sex and love intermingled in the bedroom, how they intermingled in the brain remained unclear until neuroendocrinologist C. Sue Carter began playing with a bunch of prairie voles in the early 1990s [source: Johnson].
The rodents were known for their monogamous, lifelong pair bonding -- a rarity in the animal kingdom, where promiscuity is the rule, rather than the exception. Carter correctly suspected the voles' fidelity had something to with how their brains processed oxytocin, a neurochemical in the brain associated mother-child bonding that's also released during sex [source: Carter]. Picking up Carter's cue in 1998, neurobiologist Thomas R. Insel analyzed prairie vole brains and noticed a striking distinction. Unlike their nonmonogamous montane vole cousins, the prairie voles oxytocin and dopamine receptors were clustered together, linking together the neurochemicals' effects of bonding and pleasure [source: Young, Wang and Insel]. That brain chemistry translates to a physiological incentive to not just find a sexual partner for a night, but rather one to grow old with. Further neurological research confirmed a similar oxytocin receptor arrangement in the human brain, thus explaining why the deep, passionate love between two people that Greek philosophers had long-pondered has persisted through the ages.