Since the Nuclear Nonproliferation Treaty (NPT) came into force almost 40years ago, only four states have acquired nuclear weapons. What accounts for such near-universal compliance? This paper argues that social psychology can help us understand the puzzle of nuclear restraint in two ways. First, nuclear forbearance should be unpacked into three outcomes: persuasion (behavior resulting from genuine transformation of preferences), social conformity (behavior resulting from the desire to maximize social benefits and/or minimize social costs, without a change in underlying preferences), and identification (behavior resulting from the desire or habit of following the actions of an important other). Second, through social psychology, we can specify the mechanisms by which the norm of nonproliferation has influenced policymakers. Indeed, the case of Japan shows that both these contributions help us better understand nuclear decision-making and offer larger insights into regime compliance more generally.