This article investigates the misuse of "rationality" in academic and political discourses, focusing on the Iranian nuclear project. The concept of rationality is ubiquitous; scholars, pundits, and practitioners turn to it, sometimes unwittingly, to describe, explain, and predict. When concerning concrete security and foreign policies, however, this praxis borders on malpractice: rationality-based descriptions are largely either false or unfalsifiable; many observers fail to explicate the meaning of "rationality" they employ; and the concept is frequently used politically to distinguish between "us and them." Empirically, I show that rationality has played an opaque and excessive role in the Western accounts of Iranian nuclear policy. Both "optimists" and "pessimists" have frequently, but faultily, turned to rationality/irrationality to explain Iran's moderate/belligerent nuclear policy and its susceptibility/resistance to nuclear deterrence. The malpractice of "rationality" in discussing such matters has become a bad habit, which is best uprooted.