In this paper, I describe a hitherto overlooked kind of practical irrationality, which I call irrational option exclusion. An agent who suffers from this problem does not merely fail to act on her best judgementshe fails to realize that the superior action is even an option for her. I furthermore argue that this kind of irrationality is serious enough to undermine moral responsibility. I show that an agent suffering from this problem has compromised reasonsresponsiveness, does not really express her will through action, and has a hard time doing otherwise; thus, from the standpoint of several popular moral responsibility theories, we ought to conclude that her responsibility is at the very least diminished. Keywords Moral responsibility. Reasons-responsiveness. Ability to do otherwise. Quality of will. Practical irrationality In this paper, I will describe a hitherto overlooked kind of practical irrationality, which I call irrational option exclusion. An agent is practically irrational when she fails to do what she judges best. This is not always responsibility undermining; it is widely agreed that agents can be responsible for akratic acts. But an agent who suffers from irrational option exclusion does not merely fail to act on her best judgementshe fails to realize that the superior action is even an option for her. I will argue that when an agent's rational thinking is compromised in this way, her moral responsibility is compromised as welland this is so, regardless of which moral responsibility theory we adopt. 1 The Milgram Obedience Experiments: The Case of Mr. Braverman For a first illustration of the phenomenon, let us take a look at the much discussed Milgram obedience experiments (Milgram 1974/2010). Prima facie, experiments such as Milgram's seem