The scientific rules for the game of love

Animal attraction
Pheromones clearly act as sexual attractants in the animal world. Older male elephants, for example, exude sexual prowess with a mix of chemicals the younger bulls can't muster.

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.

Lasting relationships
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.

True love
Somewhere amid attraction and sex, we all hope, are strong feelings of love. But which of all the motivations really drives us?

American Physiological Society Brain scans indicate that feelings of passionate love are linked to activation of the anteromedial caudate body, marked by a "C" and an arrow in this picture.

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.

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