In 1967, American sociologist Stanley Milgram devised a new way to test the Six Degrees of Separation theory, which he called 'the small-world problem.' He randomly selected people in the mid-West to send packages to a stranger located in Massachusetts. The senders knew the recipient's name, occupation, and general location. They were instructed to send the package to a person they knew on a first-name basis who they thought was most likely, out of all their friends, to know the target personally. That person would do the same, and so on, until the package was personally delivered to its target recipient.
Although the participants expected the chain to include at least a hundred intermediaries, it only took (on average) between five and seven intermediaries to get each package delivered. Milgram's findings were published in Psychology Today and inspired the phrase 'six degrees of separation.'
[Extracted from: http://www.history.com/topics/water-and-air-pollution]