Chilean poet Pablo Neruda could be considered a literary love expert. In 1960, he penned "100 Love Sonnets," describing in fervid detail the multiple angles and manifestations of that deep, romantic attachment both hailed and hated by countless poets and writers like him. Honing in on the core of humans' most-prized emotion, Neruda sweetly refers to love in Sonnet No. 53 as "this endless simplicity of tenderness" [source: Clarke].
Not surprisingly, science doesn't handle love so gently. In fact, it doesn't even classify love as an emotion, but rather a goal-orientated motivational state that drives humans toward an array of strange behaviors reminiscent of those associated with drug addiction and psychosis [source: Fisher et al]. And at the risk of stripping love entirely of sentiment, science concludes that those crazy, sometimes stupid behaviors boil down to a cocktail of chemicals buzzing around our brains -- not fate or benevolent serendipity, as some believe. On the one hand, those cold, hard facts might kill the mood, while on the other, they offer a kind of comfort: When love overwhelms our actions, attention spans and impulses, we can understand how the following five neurochemical culprits are largely to blame.
Although it certainly isn't the most romantic hormone pumping through our veins, testosterone -- the compound responsible for facial hair and fast driving -- is necessary to warm up our internal love engines. And it isn't just for the guys, either; testosterone also exists in the female body to stoke physical attraction and sexual arousal.
A recent study from the University of Aberdeen identified how the hormone determines who stands out in the crowd. By measuring participants' blood levels of testosterone over a series of weeks and asking them to rate the attractiveness of photographed faces, the researchers charted a distinct pattern. Men with higher levels of testosterone exhibited a stronger preference for more feminine faces, whereas women with higher levels of the stuff were compelled toward more masculine faces [source: Durfee]. Just in case women's testosterone could use a boost, men pass it along in their saliva during passionate kissing [source: Kluger]. That helps explain why men and women spend time kissing as foreplay to sexual intercourse.
Serotonin is a somewhat counterintuitive hormone to make the list, since it actually promotes feelings of calm and contentedness. But it's possible to chart the lifespan of a romantic relationship by tracing the roller coaster ride of serotonin in the brain. During the early attachment phase of love, serotonin takes a back seat, residing at low levels, while other reward-regulating chemicals take over. As a result of that serotonin dampening, people become borderline obsessed with their beloved, unable to focus or eat whenever apart [source: Lite]. Eventually, once a relationship solidifies, the raphe nucleus in the brain stem begins to cook up more serotonin, eliciting those warm and fuzzy feelings of togetherness that typify longer-term attachment. The only downside of that serotonin upshot is the loss of excitement, also colloquially known as the end of the "honeymoon phase."
What happens in the brain during an orgasm? One word: oxytocin. Sexual intercourse promotes feelings of attachment over time in large part thanks to oxytocin, which is produced in the ventral tegmental area of the brain.
This chemical could aptly be nicknamed the "glue chemical" since it does such a swell job of binding people together. As oxytocin levels crest, it calms and combats the early phase intensity of romantic attachment, easing us into more stable relationships. Research confirms that in women especially, oxytocin fosters trust, happiness and bonding. To that end, this also is the same nonapeptide compound that bathes a new mother's brain, establishing the maternal link between her and child, and what spikes when we look at a picture of a loved one [source: Marazziti].
Prairie voles are often regarded as the animal kingdom's mascots of monogamy. As the only mammalian species genetically inclined toward pairing up 'til death do they part, the rodents have been studied extensively to find out what keeps them together in the wild. Along with improved chances for survival, the bonding oxytocin and vasopressin are the key neurological ingredients to the voles' faithfulness. Pair bond-promoting vasopressin in particular, which saturates the nucleus accumbens, a brain structure pivotal in sensing satisfaction, drives vole pairs to lifelong coupling. In one experiment at Emory University, scientists blocked vasopressin receptors in prairie vole brains, resulting in an outbreak of adultery [source: Yong]. Conversely, cranking up the levels of vasopressin in brains of meadow voles -- the prairie voles' promiscuous cousins -- sparked a monogamy movement.
Once testosterone gets the job done of attracting two people to each other and igniting their sexual energy, pleasure-inducing dopamine is released during sexual intercourse [source: Discovery Channel]. By powering the brain's limbic reward system, this chemical cultivates the addictive satisfaction that comes with euphoria derived from eating a delectable meal or making love or even using cocaine. That neurological treat dopamine doles out in the early stages of romantic attraction is partially responsible for the thrill we get when seeing the object of our affection and the craving to be with him or her in between. Indeed, studies have demonstrated that the brain, thanks to dopamine and its neurochemical cohorts, processes and manifests love much like an addiction, compelling people to settle down and reproduce [source: Fisher et al].
Though high levels of dopamine are characteristic of early stage romance and obsession, its presence is also a hallmark of satisfying, lasting relationships years later. Functional MRI scans of couples married at least 21 years who reported being madly in love showed above average activity in the brain's dopamine factory, the ventral tegmental area [source: Lite]. So while oxytocin and vasopressin are important for establishing monogamy, dopamine ensures that Pablo Neruda's "endless simplicity of tenderness" remains long after the honeymoon phase fades.