In May 1967, the psychologist Stanley Milgram described a study in Psychology Today. He sent letters to people in Omaha, Nebraska, and Wichita, Kansas. Each letter had to reach a stranger in the Boston area. A sender could pass it only to someone they knew personally. That person would pass it on in the same way.
In a 1969 follow-up, Jeffrey Travers and Milgram started letters with 296 people in Nebraska and Boston. Knowing someone personally meant knowing them “on a first name basis.” Sixty-four letters reached the target. On average, each passed through 5.2 people on the way.