Why are certain pieces of online content more viral than others? This article takes a psychological approach to understanding diffusion. Using a unique dataset of all the New York Times articles published over a three month period, the authors examine the link between integral affect (i.e., the emotion evoked) and whether content is highly shared. Results suggest a strong relationship between emotion and virality, but indicate that this link is more complex than mere valence alone. Positive content is more viral (than negative content), as is content that inspires awe. But while sad content is less viral, anger or anxiety inducing articles are both more likely to make the paper’s most emailed list. These results hold controlling for how surprising, interesting, or practically useful content is (all of which are positively linked to virality), as well as external drivers of attention (e.g., how prominently articles were featured). The findings shed light on why people share online content, provide insight into how to design effective viral marketing campaigns, and underscore the importance of individual-level psychological processes in shaping collective outcomes.