When it comes to sexual attraction, it seems the old adage is true: Opposites do attract. Women in one study were given sweat-soaked T-shirts worn by men (a scientific wet T-shirt contest of sorts) and had to rate which one smelled best. Amazingly, they picked the duds worn by men who had different genes than them—proof that we instinctively vary our gene pool to keep our babies free of birth defects, and that marrying your cousin is still a really bad idea.

QUIZ: Do You Have Good Relationship Instincts?