Both causation and moral responsibility seem to come in degrees, but explaining the metaphysical relationship between them is more complex than theorists have realized. This paper poses an original puzzle about this relationship and uses it to reach three important conclusions. First, certain natural resolutions of the puzzle reveal the existence of a new sort of moral luck called proportionality luck. Second, there is indeterminacy in the type of causal relation deployed in assessments of moral responsibility. Finally—and most importantly—leading theories of causation do not have the ability to capture the sorts of causal differences that matter for moral evaluation of agents’ causal contributions to outcomes.