Common law judges have traditionally been concerned about bias and the appearance of bias. Bias is believed to threaten the administration of justice and the legitimacy of legal decision-making, particularly public confidence in the courts. This article contrasts legal approaches to bias with a range of biases, particularly cognitive biases, familiar to scientists who study human cognition and decision-making. Research reveals that judges have narrowly conceived the biases that threaten legal decision-making, insisting that some potential sources of bias are not open to review and that they are peculiarly resistant to bias through legal training and judicial experience. This article explains how, notwithstanding express concern with bias, there has been limited legal engagement with many risks known to actually bias decision-making. Through examples, and drawing upon scientific research, it questions legal approaches and discusses the implications of more empirically-based approaches to bias for decision making and institutional legitimacy.