Why 'Six Degrees of Separation' Breaks Down Inside Cities

You've probably heard of Stanley Milgram's famous "six degrees of separation" experiment: Subjects in the Midwest were each asked to send a package through the mail to a stranger in Boston, whom they could try to reach only by forwarding the package to other people they knew on a first-name basis. Milgram reported it took only five people for the package to pass from one stranger to another, thousands of miles apart.

The study inspired one of the most lasting theorems in the social sciences, but it's also been beset with criticism since its 1967 publication in Psychology Today: In an earlier, unpublished study, only five percent of packages made it to their targets, as psychologist Judith Kleinfeld reports. In published follow-ups, fewer than 30 percent of the packages reached their destinations.