Psychology
Six degrees of separation started as a chain-letter experiment
In 1967, Stanley Milgram mailed folders to strangers in Nebraska and asked them to forward each one toward a specific stockbroker in Boston, passing it only to someone they knew personally. The chains that arrived averaged about six links, seeding the idea that everyone on Earth is connected by 'six degrees of separation'. Later analysis found the original math was shakier than the legend, but the phrase stuck.