“A study at the University of Utah found that if you ask someone why he is friendly with someone else, he’ll say it is because he and his friend share similar attitudes. But if you actually quiz the two of them on their attitudes, you’ll find out that what they actually share is similar activities. We’re friends with the people we do things with, as much as we are with the people we resemble. We don’t seek out friends, in other words. We associate with the people who occupy the same small, physical spaces that we do.”