Folly is an under-explored vice, despite its common occurrence and close relationship to core aspects of practical rationality and the good life. This paper develops an account of folly as a subspecies of imprudence and distinctive source of wrongdoing, with a special focus on its relational, social or inter-personal aspect. Drawing on Rotenstreich’s historically-based account, folly is defined as a form of practical irrationality resulting from closedness to the world. I expand Rotenstreich’s view and depart from him on two key (...) points. First, I argue that folly should be cleanly differentiated from stupidity. Second, I show that the wrong in folly—i.e. the harms it involves intrinsically, as opposed to as a matter of causal consequence—is partly obscured by Rotenstreich’s exclusive focus on the fool as an isolated individual agent. When, instead, we consider folly as a pathology of interpersonal relations, it reveals itself to be a vice which infects and undermines human relationships of love and care, and thus to be a serious threat to living well or human flourishing in almost all of their forms. (shrink)