.John T. Cacioppo, James H. Fowler, and Nicholas A. Christakis, “Alone in the Crowd: The Structure and Spread of Loneliness in a Large Social Network,” Journal of Personality and Social Psychology 97, no. 6 (December 2009): 997–91, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2792572/.