The Laws of Chemistry

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.

Applying a shorthand that synthesizes much information about biology and behavior, I refer to those whose temperament reflects the dominance of dopamine as Explorers. These men and women tend to be risk taking, novelty seeking, impulsive, creative, and curious—traits associated with specific genes or pathways in the dopamine system. Those in whom serotonin pathways may be dominant I call Builders; these men and women tend to be social, popular, cautious (but not fearful), rule following, conventional, and often religious or spiritual.

Those who express activity in estrogen pathways I dub Negotiators, women (and men) who tend to be verbally skilled, good at "reading" people's faces, posture, gestures, and tone of voice, contextual thinkers, compassionate, nurturing, imaginative, and agreeable. Those who express the effects of testosterone I call Directors, men (and women) who are direct, decisive, focused, outwardly competitive, analytical and logical, and skilled with machines and other rule-based systems. We're all a mix of all four types, of course; but most people express some of these behavioral syndromes more than others.

Serendipitously, I was asked by an online dating service, Chemistry.com, a subsidiary of Match.com, to apply what science knows about attraction to help build a state-of-the-art matching system for singles. The service provides an opportunity to test my theory in real life as well as the prospect of applying science to the public good.

First I developed a 56-item questionnaire to measure the degree to which men and women express each behavioral syndrome. A shortened version of my test appears at the end of this article.

Based on the test, Chemistry.com members are assigned to one of 12 categories, reflecting primary and secondary temperament type. I am, for example, an Explorer/Negotiator, a person with greater expression of dopamine and estrogen systems. One can be an Explorer/Director, Builder/Negotiator, and so forth.

So far, 1.6 million American men and women have taken the test, and I have analyzed data on the first 523,622. Builders predominate (42 percent of the population). There are far fewer Explorers (8 percent) than any other type—a frequency that correlates with the distribution of a specific genetic variant in the dopamine system associated with novelty seeking and risk taking. Perhaps Builders were essential in ancestral times to stabilize community life, while Explorers were crucial only under changing ecological circumstances. Explorers gravitate to big cities, perhaps drawn to the novelty, energy, excitement, extravagance, and risk. Builders stay home and guard the heartland.

To look at mate choice, I took a subset of 2,766 site members and examined their reactions to one another after a first meeting. The site asks members to report on their degree of attraction to the individual they just met.

First meetings are powerful. They often set the course for the entire relationship. The data so far suggest that just about everybody loves Negotiators; all types (except male Builders) gravitate to these agreeable, imaginative, verbal men and women. More important to my theory, men and women of all broad temperament types (except the Negotiator) express more initial attraction for individuals with different, rather than similar, behavioral and cognitive profiles.

Roots and Wings

Builders, bathed in serotonin, appear to be particularly attracted to the dopamine-rich Explorers, and vice versa. Perhaps the orderly, rule-following Builder needs the spontaneity and novelty of the Explorer, especially when rearing the young. And perhaps the Explorer gravitates to the Builder because he or she provides roots, rules, and loyalty—the gravitas an Explorer desires to help rear children.

Directors favor Negotiators. Perhaps they need the Negotiator's social and verbal skills, broad contextual perspective, flexibility, and nurturing. And although Negotiators are foremost drawn to their own kind, they are also drawn to the Director. Perhaps the direct, competitive, focused, decisive, logical, analytical, and mechanically talented Director balances out their flexible nature, another effective combination for bearing and rearing young.

Which combination makes the most effective match? I suspect each combination has its own advantages and liabilities.

Perhaps mate choice operates like a funnel. First you see a potential partner and size him or her up—physically. Too big, too little, too old, too young, too pink, too green, too messy, too neat. If they make the cut, then you talk. A bad accent, a dumb remark, a tasteless joke, a touch of arrogance, or some other idiosyncrasy may nip the budding romance. But if this stranger comes from your background, is of the right age, shares your level of intelligence and education, appears humorous and socially adept, has your values, fits within your love map, and the timing is right, you proceed. In these ways, similarity regularly reigns.

Then come your needs, psychological and physical. Many exchange good looks for money or rank. We are often attracted to those who mask our flaws and accentuate our better parts. Roles are important, too. As you size up him or her, you ponder whether you'd be comfortable as the wife of this college professor or the husband of that cafe singer. As you weigh the myriad social factors, I suspect that unconscious biological mechanisms are subtly pulling you toward those who complement you genetically.

My investigation is just beginning. But it has already altered my view of some of our behavior. Perhaps some of our ubiquitous marital friction stems from the need to choose partners suitable for producing and raising babies—but not altogether understandable as companions.

Some psychological thinking holds that women are drawn to men like their fathers and men to women like Mom, often to resolve childhood issues. Perhaps women do marry someone like Dad—but for biological reasons. As a child you may have experienced friction with him due to your different temperaments, but as an adult, with your biological reproductive mechanisms in full bloom, you find yourself attracted to the genetic qualities you lack—the same ones Dad has.

There is magic to love. I doubt we will ever understand (or harness) all the myriad forces that play a role in mate choice. But we can help Cupid as we eavesdrop on nature's plans.

Take the Quiz

Answer all questions by placing an X in the column that best represents you:

1. I find unpredictable situations exhilarating.

   __Strongly Disagree

   __Disagree

   __Agree

   __Strongly Agree

2. I do things on the spur of the moment.

   __Strongly Disagree

   __Disagree

   __Agree

   __Strongly Agree

3. I am always coming up with new ideas.

   __Strongly Disagree

   __Disagree

   __Agree

   __Strongly Agree

4. I have a very wide range of interests.

   __Strongly Disagree

   __Disagree

   __Agree

   __Strongly Agree

5. I make plans for things that will happen way down the road.

   __Strongly Disagree

   __Disagree

   __Agree

   __Strongly Agree

6. I place a high degree of importance on how other people view me.

   __Strongly Disagree

   __Disagree

   __Agree

   __Strongly Agree

7. In general, I think it is important to follow the rules.

   __Strongly Disagree

   __Disagree

   __Agree

   __Strongly Agree

8. I tend to be cautious, but not fearful.

   __Strongly Disagree

   __Disagree

   __Agree

   __Strongly Agree

9. I can see many different ways to solve a particular problem.

   __Strongly Disagree

   __Disagree

   __Agree

   __Strongly Agree

10. I am more intuitive than most people.

    __Strongly Disagree

    __Disagree

    __Agree

    __Strongly Agree

11. I change my mind easily.

    __Strongly Disagree

    __Disagree

    __Agree

    __Strongly Agree

12. I get uncomfortable when I see someone standing alone at a party.

    __Strongly Disagree

    __Disagree

    __Agree

    __Strongly Agree

13. I am very focused.

    __Strongly Disagree

    __Disagree

    __Agree

    __Strongly Agree

14. I think it is more important to do a good job than to have people like me.

    __Strongly Disagree

    __Disagree

    __Agree

    __Strongly Agree

15. It takes a lot of evidence to make me change my mind on most issues.

    __Strongly Disagree

    __Disagree

    __Agree

    __Strongly Agree

16. I like to cut through the uncertainties to get to the point.

    __Strongly Disagree

    __Disagree

    __Agree

    __Strongly Agree

To Score

Strongly Disagree = 1 (point)

Disagree = 2

Agree = 3

Strongly Agree = 4

Add up the number of points in each of the four sections: 1-4; 5-8; 9-12; 13-16.

Questions 1-4 measure the degree to which you are an EXPLORER.

Questions 5-8 measure the degree to which you are a BUILDER.

Questions 9-12 measure the degree to which you are a NEGOTIATOR.

Questions 13-16 measure the degree to which you are a DIRECTOR.

The two highest numbers are your primary and secondary types.