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.