In 2014, Facebook published a study in the Proceedings of the National Academy of Sciences that ignited an immediate firestorm. Researchers had secretly manipulated the emotional content of approximately 700,000 users’ news feeds over one week, reducing either positive or negative posts to test whether emotional states could be transmitted through text-based social media without direct human contact. The results confirmed what the researchers suspected: users who saw fewer positive posts produced fewer positive posts themselves, and users who saw fewer negative posts produced fewer negative posts. Emotions had spread through the network the way viruses spread through populations, not...