To figure out how we pick mates, scientists have measured every shape and angle of the human face, studied the symmetry of dancers, crafted formulas from the measurements of Playboy models, and had both men and women rank attractiveness based on smelling armpit sweat.
After all this and more, the rules of attraction for the human species are still not clearly understood. How it all factors into true love is even more mysterious.
But a short list of scientific rules for the game of love is emerging. Some are as clearly defined as the prominent, feminine eyes of a supermodel or the desirable hips of a well-built man. Other rules work at the subconscious level, motivating us to action for evolutionary reasons that are tucked inside clouds of infatuation.
In the end, lasting love depends at least as much on behavior as biology. But the first moves are made before you're even born.
Symmetry equals sex
Starting at conception, the human body develops by neatly splitting cells. If every division were to go perfectly, the result would be a baby whose left and right sides are mirror images. But nature doesn't work that way. Genetic mutations and environmental pressures skew symmetry, a process with lifelong implications.
Good symmetry shows that an individual has the genetic goods to survive development, is healthy, and is a good and fertile choice for mating.
"It makes sense to use symmetry variation in mate choice," said evolutionary biologist Randy Thornhill of the University of New Mexico. "If you choose a perfectly symmetrical partner and reproduce with them, your offspring will have a better chance of being symmetric and able to deal with perturbations."
Thornhill has been studying facial symmetry for 15 years. He scanned faces and bodies into computers to determine symmetry ratios. Both men and women rated symmetrical members of the opposite sex more attractive, and they also inferred them as being in good health. The differences can be just a few percent — perceivable, though not necessarily consciously noticeable.
By questioning the study participants, Thornhill also found that men with higher degrees of symmetry enjoy more sexual partners than men of lower symmetry.
"Women's sex-partner numbers are dependent on things other than attractiveness," Thornhill told LiveScience. "Because of the way that the sexual system in humans works, women are choosy. They are being sexually competed for. They have to be wooed and all that."
Body shape is of course important, too. And scientists have some numbers to prove it. Psychologist Devendra Singh of the University of Texas studied people's waist-to-hip ratio, or WHR.
Women with a WHR of 0.7 — indicating waist significantly narrower than the hips — are most desirable to men.
And an analysis of hourglass figures of Playboy models and Miss America contestants showed that most of these women boast a WHR of 0.7 or lower.
In general, a range of 0.67 to 1.18 in females is attractive to men, Singh concluded in a 2004 study, while a 0.8 to 1.0 WHR in men is attractive to women, although having broad shoulders is more of a turn-on.
What exactly is encoded in the hip ratio? A big fat clue to whether the person will have enough energy to care for offspring.
Where fat is deposited on the body is determined by sex hormones; testosterone in men, and estrogen in women. If a woman produces the proper amount and mixture of estrogen, then her WHR will naturally fall into the desired range. The same goes for a male testosterone.
People in the ideal hip-ratio range, regardless of weight, are less susceptible to disease such as cardiovascular disorders, cancer and diabetes, studies have shown. Women in this range also have less difficulty conceiving.
"The idea is that beauty is conveying information about health and fertility, and we admire that," Singh said in a telephone interview.
The structure of a person's face also gives insight to fertility.
Estrogen caps bone growth in a woman's lower face and chin, making them relatively small and short, as well as the brow, allowing for her eyes to appear prominent, Thornhill explained. Men's faces are shaped by testosterone, which helps develop a larger lower face and jaw and a prominent brow.
Men and women possessing these traits are seen as attractive, Thornhill said, because they advertise reproductive health.
Thornhill also points to the booming nip-'n'-tuck business — which is very much about improving a person's symmetry — as evidence that people find the quality attractive.
Another recent study revealed that symmetrical dancers are seen as more attractive. And research reported last month found women both smell and look more attractive to men at certain times of the month.
Symmetrical men also smell better.
Borrowing sweaty undershirts from a variety of men, Thornhill offered the shirts to the noses of women, asking for their impressions of the scents. Hands down, the women found the scent of a symmetrical man to be more attractive and desirable, especially if the woman was menstruating.
By now you might be wondering how much of this we're consciously aware of. The rules of attraction, it turns out, seem sometimes to play out in our subconscious.
In some cases, women in Thornhill's study reported not smelling anything on a shirt, yet still said they were attracted to it.
"We think the detection of these types of scent is way outside consciousness," Thornhill said.
A 2002 study found that women prefer the scent of men with genes somewhat similar to their own over the scent of nearly genetically identical or totally dissimilar men.
These subconscious scents might be related to pheromones, chemical signals produced by the body to communicate reproductive quality. The human genome contains more than 1,000 olfactory genes — compared with approximately 300 genes for photoreceptors — so pheromones have received a lot of attention from basic research scientists as well as perfume manufacturers.
But the role of pheromones in the human realm remains controversial.
Milos Novotny of the Institute of Pheromone Research at Indiana University has shown that special molecules produced by male mice can simultaneously attract females and repel, and even anger, rival males. Other studies have found similar responses throughout the animal kingdom.
Yet many researchers are not sold on the idea that these odorless compounds play a role in human attraction. Count evolutionary biologist Jianzhi Zhang of the University of Michigan among the skeptical.
In 2003, Zhang showed that a gene mutated 23 million years ago among primates in Africa and Asia that are considered to be human ancestors, allowing them to see color. This let the males notice that a female's bottom turned bright red when she was ready to mate.
"With the development of a sexual color scheme, you don't need the pheromone sensitivity to sense whether a female monkey is ready to mate," Zhang said. "It's advantageous to use visual cues rather than pheromones because they can be seen from a distance."
A study last year, however, suggested that human pheromones affect the sexual area of the brains of women and gay men in a similar manner.
Sex goes visual
Pheromones, like other scents, hitch a ride through the air on other particles, such as water droplets. They generally hover just 10 inches (30 centimeters) off the ground, however. So odds are slim they'll waft up to a human nose and fuel sudden passion at a nightclub.
Watch any construction worker whistling at a passing woman from half a block away, and you can see how visual cues can be more powerful.
And while they enter the nose like other scents, that's where the comparison stops. A pheromone's destination is a special organ called the vomeronasal organ, which humans now lack. From here the sexy scent travels along a neural pathway to the brain separate from other scents.
Evolution played a role in this, too.
After our ancestors began to see color, a gene important in the pheromone-signaling pathway suffered a deleterious mutation, making it impossible for the scent signals to reach the brain, Zhang said. Imagine a train leaving Los Angeles for New York, then discovering that the tracks in St. Louis are destroyed.
Although the classical pheromone pathway in both Old World primates and humans is dysfunctional, the mechanism for producing pheromones still works. Some scientists believe human pheromones might be influencing our decisions along the normal olfactory pathway.
The rules of attraction might drive our initial decisions, for better or worse. But lasting relationships are about much more than what we see and smell.
Behavior plays a key role, with biology an intriguing contributing factor.
One of the oldest theories about attraction is that like begets like. It explains that eerie perception that married couples sometimes look awfully similar.
Last year, J. Philippe Rushton, a psychologist at the University of Western Ontario, looked into the relationships of people's genes. Based on a set of heritable personality traits, having similar genetics plays 34 percent of the role in friendship and mate selection, he found.
"The main theory is that some genes work well in combination with each other," Rushton told LiveScience. "If these genes evolved to work in combination, then you don't want to break that up too much for your offspring. Finding a mate with similar genes will help you ensure this."
If your spouse is genetically similar, you're more likely to have a happy marriage, for example. Child abuse rates are lower when similarity is high, and you'll also be more altruistic and willing to sacrifice more for someone who is more genetically like you, research shows.
It probably comes as little surprise people are drawn to individuals with similar attitudes and values, as psychologist Eva Klohnen at the University of Iowa found last year in a study of newlywed couples. These characteristics are highly visible and accessible to others and can play a role in initial attraction.
When it comes to sticking together for the long haul, researchers have shown that likeness of personality, which can take more time to realize, means more.
Comedy can also help a relationship. But the importance of humor is different for men and women, says Eric Bressler of McMaster University.
A women is attracted to a man who makes her laugh, Bressler found in a study last year. A man likes a woman who laughs at his jokes.
Somewhere amid attraction and sex, we all hope, are strong feelings of love. But which of all the motivations really drives us?
Interestingly, brain scans in people who'd recently fallen in love reveal more activity related to love than sex. "Romantic love is one of the most powerful of all human experiences," says Helen Fisher, an anthropologist at Rutgers University. "It is definitely more powerful than the sex drive."
The rules of attraction make up a pretty long list. No scientist knows the order of the list. But near the top is perhaps one of the toughest characteristics to gauge in advance in the search for the perfect partner.
Despite all their differences, men and women place high value on one trait: fidelity.
Cornell University's Stephen Emlen and colleagues asked nearly 1,000 people age 18 to 24 to rank several attributes, including physical attractiveness, health, social status, ambition, and faithfulness, on a desirability scale.
People who rated themselves favorably as long-term partners were more particular about the attributes of potential mates. After fidelity, the most important attributes were physical appearance, family commitment, and wealth and status.
"Good parenting, devotion, and sexual fidelity — that's what people say they're looking for in a long-term relationship," Emlen says.