Sometimes, a group of people can produce a morally bad outcome despite each person’s individual act making no difference to whether the outcome is produced. Since each person’s act makes no difference, it seems the effects of the act cannot provide a reason not to perform it. This is problematic, because if each person acts in accordance with their reasons, each will presumably perform the act—and thus, the bad outcome will be brought about. I suggest that the key to solving (...) this problem is to make it true of each person that their act would in fact make a difference to the relevant outcome. Fortunately, I contend, this can be accomplished by each person simply forming a particular type of attitude. I argue that each person has an obligation to form the relevant attitude in collective action cases, on pain of being immoral or irrational. (shrink)