This paper proposes to approach the issue of ‘transnational nuclear perspectives’ by linking the study of British nuclear history to behavioural psychology, and specifically the influential work of Nobel Laureate Daniel Kahneman and Amos Tversky, among others, on the topic of judgement and decision-making under uncertainty. It will use the story of the Polaris upgrade programme, whose raison d’etre was the need to purported missile defences around the Moscow to explore the issues of how ‘facts’ and other knowledge-claims travel between knowledge-making enterprises and other groups: in this case British nuclear weapons scientists (and the civil servants closely involved with them), and those politicians and civil servants they were advising. In doing so it will argue that the claims for nuclear programmes that were presented at the time as deliberative and logical (and have continued to be so in the historiography) can only be understood through an examination of the heuristics and biases – subjective assessments made using adjustment, anchoring, representativeness and availability as well as group polarisation.