鈥淣ature teaches beasts to know their friends,鈥 wrote Shakespeare. In humans, nature may be less than half of the story, a team led by University of Colorado Boulder researchers has found. In the first study of its kind, the team found that genetic similarities may help to explain why human birds of a feather flock together, but the full story of why people become friends 鈥渋s contingent upon the social environment in which individuals interact with one another,鈥 the researchers write.