Definition
An expert witness is a person whom a court accepts as qualified to offer opinion — not merely fact — on a technical question that lies outside ordinary experience. Forensic psychologists appear as expert witnesses to address competency to stand trial, mental state at the time of an offence, reliability of eyewitness memory, risk of future violence, and the validity of investigative techniques such as profiling or polygraph.
Unlike fact witnesses, who report only what they personally observed, experts may interpret evidence, apply scientific methods to facts in the case, and explain probabilities. In exchange, they are subject to gatekeeping rules that screen out unreliable testimony and to cross-examination that probes their methods, assumptions, and potential bias.
Why it matters
How it works
A psychologist instructed as an expert reviews case materials, conducts interviews or assessments, applies validated tools, and writes a report. At trial they testify to that report, are cross-examined, and may be challenged on qualifications, methodology, or bias. The judge decides whether the testimony reaches the threshold to be heard by the jury at all.