Physical attraction may be as old as time, but new studies are beginning to uncover the science behind sex appeal. Unexpected factors—like biochemical odors, face shape and voice pitch—just might have more to do with your choice of mate than anyone ever expected.

Karl Grammer and Elizabeth Oberzaucher are leading the research on the human scent's influence on sexual attraction. They've found that when women are ovulating, they produce copulins, a scent that attracts men. The researchers believe when a man gets a whiff of copulins, his testosterone levels rise. As a result, he secretes androstenone, an odor that repels women who aren't ovulating.

Sex therapist Dr. Laura Berman says attraction has a lot more to do with science and evolution than people might think. "We are innately all puppies in heat," she says. "We are capable of discerning 10,000 different scents consciously. But then there's a whole realm of unconscious scents that we're not even aware that we're smelling." These odors let people know when a woman is fertile and when a man's testosterone levels are high, Dr. Berman says.

One common complaint Dr. Berman hears among women with relationship problems is that they love their partner, but they're not in love with him. "There is a difference between love and chemistry," Dr. Berman says.

Another unexpected factor that has been found to directly affect attraction is voice pitch. Dr. Gordon Gallup, a professor at the State University of New York at Albany, says that women with higher estrogen levels have higher voices, which makes them more desirable to men. "When females are midcycle, when they're the most fertile, the most likely to conceive, their voices are rated as being significantly more attractive," he says.

It's hard to pinpoint what distinguishes a gorgeous face from an average one, but some researchers are getting pretty close. Psychologist Dr. Lisa DeBruine of the University of Aberdeen in Scotland says she's found that women's faces get more attractive to men when they are ovulating. "We're not entirely clear why there's this difference, but we think that the women might look healthier, have a bit of a healthier glow when they are ovulating."

Symmetry is another factor that determines a face's attractiveness. Dr. Kendra Schmid, an assistant professor of biostatistics at the University of Nebraska Medical Center, says there is a formula for the "perfect" face. She uses 29 different measurements to determine someone's appeal on a scale of 1 to 10.

Not everything that goes into finding a partner is biological. Researchers have also found that if a woman looks at the face of a man whom she knows nothing about, she will give it a rating on a scale of 1 to 10 that's different than if she is shown the same face and a corresponding income. When a man makes a lot of money, a woman will rate him higher on an attractiveness scale than she would that same man with a smaller income.

Dr. Berman says this isn't a case of women being gold diggers. "It goes back again to evolution. When we were having babies who were very dependent on us, we couldn't hunt and take care of ourselves, so we were looking for the man who had the most social status, who was the best hunter, who was going to bring home the biggest chunk of meat for our babies," she says. "It's the same thing today."

Smell, voice pitch and face symmetry might be unconscious detectors of attractiveness, but if you're trying to consciously assess the future of a relationship, Dr. Berman says there's one surefire way. "You really learn a lot from a kiss. ... Sixty-six percent of women say they would dump a guy after a bad first kiss," she says. "It's not only because you're getting that good whiff of him so you're seeing if you like the way he smells. All your senses are engaged, and you get a sense of his sensuality, his connection, whether he's a take-charge kind of guy or more of a stepping back kind."