When Jonah Berger and Katherine Milkman published their landmark 2012 study on what makes content go viral, they analyzed nearly 7,000 New York Times articles and found that the most powerful predictor of whether an article would make the paper’s most-emailed list was not its news value, not its writing quality, and not even its emotional content in general. The most powerful predictor was whether the article activated emotions of high arousal that connected to the reader’s sense of self. Content that made readers feel something about who they were, what they stood for, and what kind of person they...