How Lust Works

Lust originates not in the contours of a shapely calf or a chiseled jaw line, but in the hypothalamus, a nugget of neurons in the brain that's function far outweighs its deceptively diminutive size [source: Fisher]. Situated behind the nose, the hypothalamus directs the pituitary gland to release a range of hormones, including gonadotropin-releasing hormone (GnRH), which has been implicated as a possible human pheromone. The hypothalamus also oversees the production of a class of hormones called androgens. Testosterone, the starring androgen, along with its chemical cousins dihydrotestosterone and androstenedione, sparks sexual arousal and stirs fundamental physical attraction. For men and women alike, higher levels of testosterone coursing through the body correlate to stronger sex drives and, accordingly, more active sex lives [source: HowStuffWorks.com]. No wonder that when lust pans out into kissing contact, testosterone is exchanged in lovers' saliva.

Lust also puts on an impressive show inside the brain. Repeated cognitive studies have found predictable patterns of arousal in the brain in response to titillating images. A functional MRI (fMRI) study conducted in 2002 at the University of Montreal surveyed male and female neurological commotion while viewing pornographic films. A constellation of brain regions lit up, including sites of visual processing, emotional regulation and reward [source: Karama et al]. The specific lust-related neurological hotspots include [source: Patek, Keenan and Shackelford]:

With all of that excitement in the brain, how do humans possibly stand a chance against lusty urges? Fortunately, the brain is also engineered with a safety valve of sorts. In the early 2000s, when University of Montreal neuroscientist Mario Beauregard asked male study participants to mentally resist arousal in the presence of prurient material, it engaged parts of the prefrontal cortex involved with self-awareness and behavioral regulation [source: Highfield]. The right superior frontal gyrus and right anterior cingulate gyrus, in particular, lent a helping hand to delineate between sexual fantasy and reality. That way, the brain serves as a neurological wingman to help the body stand up against lust.

But when all goes according to plan and initial lust leads to a romantic relationship, when does sexual desire develop into outright love?

Casually characterized as emotions, love and lust are, more accurately, motivational states. A Venn diagram of the two would certainly show overlap in terms of attraction and the neurological fireworks that burst in the brain when spying the apple of one's eye, but outside of that shared space, what distinguishes the two? The evolved emotional system of lust is a stepping stone and contributor to love, so how to do they function independently? Advice columns might offer signs to watch out for, such as the amount of time a couple spends in the bedroom together versus the amount of time they spend together elsewhere. And of course, there are those three pivotal words -- "I love you" -- that can clue a partner in to the other's intentions.

A team of psychologists at the University of Amsterdam published a trio of studies in 2009 and 2011 that illuminate how love and lust uniquely influence people's thinking patterns. Comparing how sentiments of love and lust foster creativity, lead author Jens Forster and fellow psychologists found that participants primed with feelings of love exhibited broader, long-term thinking processes, lending credence to the romanticized connection between love and artistic expression [source: Jacobs]. Lust, an immediate impulse for sexual satisfaction, inspired more analytic, short-term outlooks. That kind of love-induced global thinking versus lust-fueled local thinking can be applied to how people perceive their sexual partners [source: Forster, Ozelstel and Epstude]. In other words, when thoughts about another person stray from the immediate quandary of Friday night plans to how he or she might fair as a father or mother, a seedling of love may be sprouting.

The love-lust line is also reiterated in the scientifically established phases of long-term mating. By definition, sexual desire -- aka lust -- casts a wider net, seeking satisfaction based largely on physical attributes. The segue into genuine affection is marked by specificity, i.e., craving an emotional union with a special someone rather than just anyone [source: Fisher et al]. And for men especially, crossing that border from lust to shared love can come with a statistically higher risk of failure.

In 2009, a Jesuit scholar published a survey of Seven Deadly Sin confessions among Catholics. Most often, women were guilty of pride and envy [source: NPR]. Men, on the other hand, grappled with gluttony -- though not as much as lust. A single survey of religiously devout confessors isn't conclusive proof that males are the more lascivious sex, but it nevertheless offers minor evidence of a gender gap in lust.

Framed solely in terms of sex drive, men unfailingly out-lust women. A Case Western Reserve University meta analysis of sex drive-related studies from 1996 to 2000 supported that idea by revealing a clear-cut disparity in how often men fantasize about and want to engage in sexual intercourse, compared to women [source: Baumeister, Catansee and Vohs]. Moreover, the fMRI analyses mentioned earlier in this article also revealed more male brain activity in response to watching erotic films, versus the slightly more sedated female brain-on-porn [source: Patek, Keenan and Shackelford]. Women also exhibit robust sex drives, but men might have more piquant arousal patterns and attendant physical urges as a byproduct of sex drive-revving testosterone.

Being the lustier sex, statistically, comes with its downsides, though. Anyone who's experienced an unrequited crush empathizes with the discomfort that it can produce. The neural circuitry that drives attraction also stimulates literal cravings for a would-be sweetheart's attention and company, triggering a phase called limerance. First described in 1977 by psychologist Dorothy Tennov, limerance encompasses the aching longing, daydreaming and fear of rejection that goes along with sustained lust for a potential partner [source: Bering]. And while girls might be the ones dotting their i's with hearts and flipping through bridal magazines without a wedding date in sight, boys are far more likely to encounter limerance resulting from unrequited lust. Psychologist Roy Baumeister at Case Western Reserve University estimates that men are more likely than women to pine away for people who won't reciprocate by a three-to-two ratio [source: Goleman].

On the bright side, those lusty letdowns could be nature's way of preserving love as the intimate, profound motivational state that humans treasure so dearly. After all, if every lustful whim could be satisfied, people would probably never sit down and stay a while.