As an anthropologist, I have long been captivated by one of the most striking characteristics of our species: We form enduring pair bonds. The vast majority of other mammals—some 97 percent—do not.
In my previous work I proposed that humanity has evolved three distinct but overlapping brain systems that enable us to fall in love and form long-term emotional connections: the neural systems for the sex drive, romantic love, and attachment. We are all alike in having these three primary brain networks. In other ways, however, each of us is unique. We don't fall in love with just anyone. We have deep and idiosyncratic preferences. Why do we fall in love with one person rather than another?
There is much evidence that people generally fall in love with those of the same socioeconomic and ethnic background, of roughly the same age, with the same degree of intelligence and level of education, and with a similar sense of humor and grade of attractiveness.
But you can walk into a room of 40 people all from your background, with your level of education, degree of intelligence and good looks, and you don't fall in love with all of them. "The road of love is narrow," wrote Kabir, a 15th-century poet of India. "There is only room for one." How do we form this preference—one that is so crucial to our reproductive future?
Among the myriad forces that sculpt our romantic choices is what I call your "love map," an unconscious list of qualities you begin to build in childhood. Your mother's wit and way with words; your father's interest in politics and tennis; what your siblings like and hate; the values of your friends and teachers; what you see on television. All your childhood (and adult) experiences shape and reshape your template of the ideal romantic partner.
By the teenage years, each of us has constructed an idiosyncratic catalog of traits, values, aptitudes, and mannerisms that appeal to us. Then, when the timing is right and we meet a person who registers on our love map, a cascade of brain chemicals is triggered that tells us with euphoric certainty that we have found the one.
But I have come to believe that there's more to mate choice than your childhood, your background, your values, and your degree of good looks. These variables act in tandem with a silent partner: your biology. What sparked my thinking on this was a classic study now commonly known as the sweaty T-shirt experiment.
Women are unconsciously attracted to men with a different immune system; they do it by smell. If you are attracted to someone whose immune system is different from yours, why wouldn't you also be attracted to those with other genetic differences? Mates with distinctly different genetic profiles would produce more genetically varied young.
It is this line of logic and investigation that I embarked on two years ago. Psychologists have searched exhaustively to find personality factors that play a role in romantic attraction. Do opposites attract? Or is similarity the elixir of love? No consistent patterns emerge. Extroverts don't always fall for extroverts, for example—or for introverts. With some traits, people gravitate to those who are similar; in others, they prefer individuals who complement them. Psychologists report a temptation to throw in the towel on how personality influences partner selection.
Could nature have left this essential aspect of reproduction to the whims of upbringing and social background? I doubt it. Your choice of mate is crucial to your genetic future.
Moreover, it is now believed that 50 percent of variance in personality is due to "temperament"—our predisposition to think and act in certain ways. Cross-cultural surveys, brain imaging studies, population and molecular genetics, twin studies—all suggest that the traits of temperament are universal and tied to our genetic makeup. Could temperament play a role in mate choice?
Four Neural Systems
The most discussed traits of temperament are the "big five" personality factors: openness to new experience, conscientiousness, extroversion, agreeableness, and neuroticism (or anxiety). These traits are stable across the life cycle; if you're curious when young, you're likely to be curious when older. Twin studies, as well as the universality and stability of these personality styles, point to a genetic basis.
Since antiquity, poets, philosophers, and physicians have classified people into four styles of temperament. Plato called them the Artist, the Guardian, the Idealist, and the Rational. I theorize that these very broad basic styles of thinking and behaving are biologic trait clusters linked with specific constellations of genes, neurochemicals, and brain pathways, perhaps best described as genetic profiles. I hypothesize that we unconsciously gravitate to individuals with a somewhat different genetic profile—a strategy that evolved in tandem with human pair-bonding to enable our forebears to produce genetic variety in their young and raise infants with a wider array of parenting skills.
How genes interact with other genes, how genes build proteins, how proteins build brain and bodily pathways, how these pathways interact, and how the environment sculpts these systems at every level is wildly complex and still largely unknown. Nevertheless, current data suggest that four basic chemical systems—those for dopamine, serotonin, estrogen, and testosterone—are associated with suites of traits that echo Plato's four temperament styles.
