The expert witness practice of psychiatrists is under constant scrutiny by the courts and, in the UK, the General Medical Council, as well as within appraisal and revalidation as part of a doctor's overall practice. Regulation and appraisal of expert witness practice must address not only technical competence, including demonstration of a real understanding of the interface between medicine and law, but also ethical probity, including in respect of bias, which is the most challenging appraisal focus. In psychiatry, there is much room for ‘values expression’, and therefore bias, in the offering of expert opinion. This article first describes various legal and psychological definitions of bias; then addresses the sources and routes to expression of bias within expert witness practice, viewed legally, psychologically and neuroscientifically. Finally, it proposes ways in which inevitable bias can be minimised by the individual practitioner.