Origins and Milgram's small-world experiment
The idea first appeared in the Hungarian writer Frigyes Karinthy's short story "Chains" (1929). Ithiel de Sola Pool and Manfred Kochen began modeling acquaintance networks mathematically in the 1950s (published 1978), and the psychologist Stanley Milgram made the idea famous with his 1967 experiment. Milgram asked residents of Nebraska and Kansas to get a letter to a specific person in Boston, passing it only to an acquaintance they thought more likely to know the target. In Travers and Milgram's (1969) report, letters that arrived passed through about 5.2 intermediaries on average, which gave rise to the phrase "six degrees." The phrase itself was popularized by John Guare's play Six Degrees of Separation (1990) and the "Kevin Bacon game."
What network science explains
Watts and Strogatz (1998) showed that adding just a few random links to a regular, highly clustered network sharply shortens average path length while preserving high clustering, the "small-world network" model. Granovetter's weak ties play exactly this shortcut role. Jon Kleinberg (2000) showed that for people to find short paths using only local information, without a global map, long-range links must be distributed with distance in a particular proportion: the condition of navigability. Barabási and Albert's (1999) scale-free model explains how highly connected hubs shorten paths further, a cumulative-advantage process resembling the Matthew effect.
Large-scale tests and critiques
In an email experiment with more than 60,000 participants in 13 countries, Dodds, Muhamad and Watts (2003) found successful chains were about five to seven steps long, though most chains broke off. Leskovec and Horvitz (2008) measured an average distance of 6.6 in a Microsoft Messenger network of about 180 million users, and Backstrom et al. (2012) calculated an average distance of about 4.74 (about 3.74 intermediaries) among roughly 700 million active Facebook users. There are critiques. The psychologist Judith Kleinfeld (2002) pointed out that only a minority of letters in Milgram's original study arrived and that the choice of starters was biased. Researchers also stress that the existence of a short path is not the same as people being able to find it, and that distances may differ by class and region.
FAQ
- Is everyone really connected by six steps?
- There is strong evidence that short paths exist on average, but it does not mean everyone is exactly six steps apart. Some people and groups are isolated, and averages vary with the network measured.
- How many degrees are there on Facebook?
- Backstrom et al. (2012), using 2011 data, computed an average distance of about 4.74 among active users; a 2016 Facebook analysis cited about 3.5 intermediaries.
Sources reviewed: 2026-10-26
References
- Karinthy, F. (1929). Láncszemek [Chains]. In Minden másképpen van. Budapest: Atheneum.
- de Sola Pool, I., & Kochen, M. (1978). Contacts and influence. Social Networks, 1(1), 5–51.
- Milgram, S. (1967). The small-world problem. Psychology Today, 1(1), 61–67.
- Travers, J., & Milgram, S. (1969). An experimental study of the small world problem. Sociometry, 32(4), 425–443.
- Watts, D. J., & Strogatz, S. H. (1998). Collective dynamics of 'small-world' networks. Nature, 393(6684), 440–442.
- Kleinberg, J. M. (2000). Navigation in a small world. Nature, 406(6798), 845.
- Barabási, A.-L., & Albert, R. (1999). Emergence of scaling in random networks. Science, 286(5439), 509–512.
- Dodds, P. S., Muhamad, R., & Watts, D. J. (2003). An experimental study of search in global social networks. Science, 301(5634), 827–829.
- Kleinfeld, J. S. (2002). The small world problem. Society, 39(2), 61–66.
- Leskovec, J., & Horvitz, E. (2008). Planetary-scale views on a large instant-messaging network. In Proceedings of WWW 2008 (pp. 915–924).
- Backstrom, L., Boldi, P., Rosa, M., Ugander, J., & Vigna, S. (2012). Four degrees of separation. In Proceedings of the 4th ACM Web Science Conference (pp. 33–42).
- Guare, J. (1990). Six Degrees of Separation. New York: Vintage.