But before this "obedience to authority" research, Milgrim worked on a less well known project where he had volunteers, primarily in the Midwest, attempt to connect with someone they did not know, a lawyer who lived in Boston.
The participants were told to send a letter in the mail to someone they knew personally who they thought might know, or might know someone who knew, this lawyer.
Of the chains that were completed, the average number of links was six. This became known in the general population as six degrees of separation, later the basis of a play and movie of the same name.
The number six is really arbitrary, according to Professor Duncan Watts of Columbia University. "It doesn't really matter whether it is six or seven or eight. What matters is that it's not hundreds or thousands."
But as widespread as the notion has become, there has been very little effort to actually prove the theory in experiments.
Watts has led perhaps the most significant research to date on the subject at the Small World Project at Columbia University in New York.
That experiment is carried out online, and some 60,000 people from 170 countries have taken part.
Each participant is assigned a random "target," one of 18 people located around the world. A participant's job is to link to this person via e-mail. But he or she can't just send an e-mail directly to the target; they must connect by creating a human chain.
First they e-mail someone they know personally, and they ask that person to continue the links by e-mailing someone else they know hoping to eventually get an e-mail to someone who knows the target personally, completing the chain.
Of the hundreds of chains that have been completed, Watts said the average number of links has in fact been six.