James H. Fowler and Nicholas A. Christakis, “Dynamic Spread of Happiness in a Large Social Network: Longitudinal Analysis over 20 Years in the Framingham Heart Study,” British Medical Journal 337 (2008): a2338, https://www.bmj.com/content/337/bmj.a2338. Smoking: Nicholas A. Christakis and James H. Fowler, “The Collective Dynamics of Smoking in a Large Social Network,” New England Journal of Medicine 358 (2008): 2249–58, https://www.nejm.org/doi/full/10.1056/nejmsa0706154. Sexual diseases: Elizabeth Landau, “Obesity, STDs Flow in Social Networks,” CNN, October 24, 2009,
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