Three degrees of influence

Three degrees of influence is a theory in the realm of social networks,^[1] proposed by Nicholas A. Christakis and James H. Fowler in 2007. This argument is basically that peer effects need not stop at one degree of separation. Rather, across a broad set of empirical settings, using both observational and experimental methods, it has been observed that the effect seems, in many cases, to no longer be meaningful at a social horizon of three degrees.

The theory has since been explored by scientists in numerous disciplines using diverse statistical, mathematical, psychological, sociological, and biological approaches. Numerous large-scale in-person and online experiments have documented this phenomenon in the intervening years.

Beginning in the early 2000's, Christakis and Fowler explored the impact of social connections on behavior, describing how social influence and social contagion do not end with the people to whom a person is directly connected. People influence their friends, who in turn influence their friends, and so on. Hence, a person's beliefs and actions can influence people they have never met, to whom they are only indirectly tied.

Using both observational and experimental methods, Christakis and Fowler examined diverse phenomena, such as obesity, happiness, cooperation, voting, and other behaviors and beliefs. Investigations by other groups subsequently explored many other phenomena in this way (such as crime, social learning, etc.).

In short, Christakis and Fowler posited that diverse phenomena "ripple through our network, having an impact on our friends (one degree), our friends' friends (two degrees), and even our friends' friends' friends (three degrees). Our influence gradually dissipates and ceases to have a noticeable effect on people beyond the social frontier that lies at three degrees of separation."^[2] They posited a number of reasons for this decay, and they offered informational, psychological, and biological rationales.