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. "Bars and nightclubs across the world are the battlefield for this invisible biochemical war," Oprah says.

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