Small‐World Networks Can Provide a New Tool to Study Diverse Systems

The concept of six degrees of separation, as studied over three decades ago by the social psychologist Stanley Milgram, grew from an experiment in which two target persons were identified in the Boston area, and then individuals in Kansas and Nebraska were each sent a letter describing some parameters of the two targets. Each individual was given a folder with the rule for reaching the target: If the individual didn't know the target, he was asked to send the folder to a friend he knew on a first‐name basis who was likely to know the target person. Mil‐gram's surprising result was that the number of links needed to reach the target person had a median value of six. This concept is also known as the small‐world phenomenon.