Biased assimilation describes the cognitive tendency of decision‑makers to interpret, weigh or recall evidence in a way that favours their existing beliefs, assumptions or hypotheses. In legal practice, it is often discussed in relation to judicial reasoning, fact‑finding by judges and juries, police investigations, credibility assessments in asylum and immigration claims, and decision‑making by public authorities and regulators. The term is drawn from psychology and behavioural science and is not a defined legal term in UK or Irish legislation or case law, but it is increasingly referenced in legal scholarship, judicial training and arguments about fairness, impartiality and due process. Biased assimilation is relevant to issues of apparent bias, procedural fairness, legitimate expectation, rationality and the duty to give adequate reasons, particularly where a decision appears to discount or minimise contrary evidence. It may underpin challenges by way of judicial review, statutory appeal or rehearing, as well as submissions on the proper approach to expert evidence and witness testimony. Usage and understanding of the concept are broadly consistent across England and Wales, Scotland, Northern Ireland and Ireland, although its impact is mediated through each jurisdiction’s distinct rules on evidence, appeals and standards of review.