The Enlightenment famously celebrated the rational individual. But the period also saw the darkness of the Terror, in which these same individuals seemed to become an unreasoning, destructive mass. Nineteenth-century thinkers worried about how emotional aberrations could spread in groups; as literacy spread, they considered the potential of texts to spread such dangers. This essay will outline four phases of the period’s conversation about contagious suicide, in the context of the tension between the self-contained, sympathetic individual and the animalistic, “imitative” mass human. From an initial Romantic focus on suicidal male geniuses, mid-Victorians turned to considering suicides as showing a feminised susceptibility to emotional contagion. Scientific developments at the end of the 1850s threw the concept of free will into doubt, again shifting attitudes about responsibility and the nature of imitation. By the 1880s, suicide began to seem again a potentially reasonable decision. Drawing on literary, psychological, and philosophical texts, this essay outlines changing ideas about the nature of genius, free will as foundational for the modern polity, and the role of science and statistics in human behaviour.