Using both observational and experimental methods, Christakis and Fowler examined diverse phenomena, such as obesity, happiness, cooperation, voting, and other behaviors and beliefs.
[13][14] But other scholarship using sensitivity analysis found that the basic estimates regarding the transmissibility of obesity and smoking cessation, for example, are quite robust,[15][16] or otherwise replicated or supported the findings,[17][18] e.g., in the case of alcohol consumption.
From a theoretical perspective, it has been shown [26] that the three-degrees-of-influence property naturally emerges as the outcome of the interplay between social influence, or learning dynamics, and complex networks.
[30] Christakis and Fowler reviewed critical and supportive findings regarding the three degrees of influence phenomenon and the analytic approaches used to discern it with observational data in 2013.
These studies have found strong causal evidence of contagion processes that spread beyond dyads (including out to two, three, or four degrees of separation) using randomized controlled experiments.
[38] An RCT of 24,702 people in 176 villages in Honduras (published by Edo Airoldi and Christakis in 2024) documented the spread of exogenously introduced maternal and child health knowledge and practices to two degrees of separation (among other findings).
[41] Diverse lines of work have also explored the specific biopsychosocial mechanisms for the boundedness of contagion effects, some of which had been theorized by Christakis and Fowler.
[42] Another set of experiments documented the impact of information distortion, noting that "despite strong social influence within pairs of individuals, the reach of judgment propagation across a chain rarely exceeded a social distance of three to four degrees of separation.... We show that information distortion and the overweighting of other people's errors are two individual-level mechanisms hindering judgment propagation at the scale of the chain.
"[43] And experiments with fMRI scans in a sociocentrically mapped network of graduate students, published in 2018, showed that neural responses to conceptual stimuli were similar between friends, with a nadir at three degrees of separation, providing further biological evidence for this theory.