Individual human psychology is ill-equipped to responsibly manage extremely powerful technology, such as nuclear weapons, synthetic biology, or advanced artificial intelligence. Our ongoing research explores the psychological tendencies that could lead humans to (accidentally) harm millions of individuals. In particular, we will explore the so-called unilateralist’s curse: situations in which actors can single-handedly impose a (dangerous) outcome on everyone. Many high-stakes decisions share this structure; it only takes one smallpox researcher to publish the virus’ genome, one government to allow radical climate geoengineering, and one policymaker to veto a unanimous motion. In these cases, are people too inclined to impose outcomes on others unilaterally, and if so, why? And how can we design psychologically-informed policies to minimize catastrophic risks from reckless human behavior?