Story highlights We're apt to fall in love with people who seem mysterious or challenging to us

When people who are in love look at each other, their critical judgment is dulled

Humans are naturally hardwired to stick together

Ten things to know about love and romance, from why we fall hard to why we cheat.

What Rules Attraction?

In general, you gravitate toward people like you. Good-looking people tend to go for similarly good-looking types, and those from a particular socio-economic background favor their own. Experts believe this happens because perceived equality contributes to a stable union. Well-known actresses pair up with rock stars, for example, because such men tend to be as rich and famous as they are. But once you get past the bone structure and bank account and into personality attributes, opposites often attract.

"We're apt to fall in love with those who are mysterious and challenging to us," says Helen Fisher, a professor of anthropology at Rutgers University, in New Brunswick, New Jersey, and the author of "Why We Love: The Nature and Chemistry of Romantic Love." "This pull to another biological type could also be adaptive," says Fisher. "If two very different people pool their DNA, they'll create more genetic variety, and their young will come to the job of parenting with a wider array of skills."

How Much Do Looks Count?

Physical features are important to both sexes, but a bit more so to men. "During attraction, the parts of a man's brain associated with processing visual information are more active," says Louann Brizendine, a clinical professor of psychiatry at the University of California, San Francisco, and the author of "The Female Brain." "That's true for women too, but they also show activity in the brain regions that integrate decision making, which suggests they're thinking about a little bit more than just how he looks."

Is Love Blind?

Not exactly, but once you're hooked, your vision gets cloudy. "When you're in a relationship, you're aware of the other person's flaws, but your brain is telling you it's OK to ignore them," says Lucy Brown, Ph.D., a professor of neuroscience at the Albert Einstein College of Medicine, in New York City, who specializes in the brain's response to love.

Studies at the Wellcome Department of Neuroimaging at University College in London found that when romantic partners look at each other, the part of the brain associated with social assessment and negative emotion is relatively dormant and critical judgment is dulled. According to Fisher, this mechanism may have evolved to help people stick together through early, sometimes stressful child-rearing stages.

Can Love Be Addictive?

Love plays havoc with your body chemistry, causing you to act like an addict bent on scoring her next fix. Studies have found, for instance, that serotonin levels decrease by up to 40 percent in the newly smitten, causing some to show signs of obsessive-compulsive disorder, a condition associated with low serotonin -- which is why you can't seem to get the other person out of your head. Additionally, cortisol, a stress hormone linked with the fight-or-flight response, is released, so you're constantly on high alert. Sound familiar?

Research published by a team that included Brown and Fisher found that people who had recently fallen in love showed strong activity in the area of the brain that produces and receives dopamine, a neurotransmitter associated with addictive behavior whose activity increases when you expect to receive a reward. Gamblers and drug addicts experience similar dopamine activity. "You're not supposed to be satisfied," explains Fisher. "You're supposed to be driven so that you can win the person and eventually stabilize your internal chemistry."

When a relationship ends, you experience symptoms that are similar to an addict's withdrawal. Your dopamine levels go down, so your mood suffers. Your serotonin levels remain low, so your obsessive-compulsive disorder symptoms may not go away. In response to these imbalances, some scientists believe, risk-taking tendencies go up. "When you can't have someone but you're not willing to accept that, you try harder and become more extreme about it," says Fisher. Paradoxically, she says, this compulsive behavior may help you move on faster: "Either you win the person back or you drive him away."

What Makes People Commit?

Humans are hardwired to stick together. Intimate relationships trigger the production of oxytocin and vasopressin, chemicals that scientists have nicknamed "cuddle hormones." A mere touch from a loved one can elevate their levels, and after sex they flood the system. "We think of these hormones as playing an important role later on in the relationship, when you really know the person's flaws," says Brown.

Why Are Some More Reluctant to Commit Than Others?

Gene variation may be partly to blame. Scientists at Emory University, in Atlanta, looked at the effect of vasopressin in two closely related kinds of rodents -- the prairie vole and the meadow vole. Like humans, the prairie vole is one of the 3 percent of mammalian species that form monogamous pair bonds. The meadow vole doesn't. But when male meadow voles were injected with a gene responsible for releasing vasopressin receptors, they immediately lost their wanderlust, paired up, and settled down.

The study's researchers think the number of vasopressin receptors an individual has could lay the foundation for his propensity to commit. "There's something at work with a couple that stays together for 50 years, bad years included," says Melvin Konner, M.D., a professor of anthropology and behavioral biology at Emory, who wrote a commentary on the experiment. "It's hard to imagine that it's just a question of compatible personalities or strict beliefs."

Does Love Make You More Trusting?

Lovers do tend to see the world through rose-colored glasses. In one experiment, researchers devised a game in which subjects were given a sum of money to invest with a trustee, either in a lump sum or piecemeal. Anything given to the trustee would triple in value, but it was up to the subject to decide how much to turn over. Half the participants used a nose spray before the experiment that was a placebo; the other half used one with oxytocin.

Subjects who took the oxytocin were nearly twice as likely to turn all their money over to a trustee. A subsequent experiment at the National Institute of Mental Health (NIMH), in Washington, D.C., found that subjects who inhaled oxytocin before looking at pictures of threatening faces had markedly lower activity in their brains' fear centers. "These results suggest that oxytocin increases trust," says Thomas Insel, M.D., director of the NIMH.

Why Do People Cheat?

Attraction, romantic love, and attachment involve three overlapping but separate brain systems. "It's not hard for somebody to sexually desire one person, be infatuated with another, and still want to spend the rest of his or her life with a third," says Fisher. Because each kind of love serves a unique need and exists in a different context, cheaters are able to divide their emotional resources.

What makes one person more likely to cheat compared with another? The answers are both inconsistent and varied. Fisher suspects the propensity to stray may be stronger in people who have novelty-seeking, dopamine-sensitive personalities. But factors unique to the relationship -- a need for attention, a desire to get out of the situation -- are just as likely to fuel infidelity.

Can Love Affect Your Health?

Research has found that couples in good relationships tend to be healthier and happier. "Happily married couples report lower stress than single people, in part because they provide each other with emotional support in difficult times," says Janice Kiecolt-Glaser, a professor of psychiatry at Ohio State University, in Columbus. "Lower stress translates into better health and immune function." For example, people who are in conflict-ridden relationships might see cuts and bruises heal more slowly -- by as much as 40 percent, according to a 2005 experiment at the Ohio State University College of Medicine.

And breakups have been shown to cause physical pain. A 2003 study looked at people playing a virtual ball-tossing game. Those people rejected during the game showed activity in the pain area of their brains. "In evolutionary terms, exclusion can be as bad for survival as a real injury, and our bodies automatically know this," explains the study's author, Naomi Eisenberger, a postdoctoral scholar at the University of California, Los Angeles.

What Keeps People Together?

Hormones and hard work. Restlessness sets in one to two years into a relationship, according to new research from the Universities of Pavia and Pisa, in Italy. That's the period in which the chemical activity associated with new love (high dopamine, for example) dies down.

Fortunately, there are ways to keep the spark alive. Sexual contact drives up dopamine levels. Novelty does, too, which is why you tend to feel so good about somebody after taking a trip or going through an unusual experience together. Frequent physical contact is most likely to maintain elevated oxytocin levels, which is why holding hands, stroking your partner, or any other kind of touch can create feelings of attachment.