Definition
Adversarial bias (sometimes called 'allegiance effect') is the systematic distortion in expert evidence that arises from the structure of adversarial trials, where each side hires and instructs its own experts. Experiments and field studies show that experts working from identical materials reach different conclusions depending on which side instructs them — even when they sincerely believe they are being objective.
The bias is not the same as dishonesty. It is the cumulative effect of subtle case-framing, selective disclosure, anchoring on the instructing solicitor's narrative, and the economic reality that experts who consistently favour one side build a practice while those who do not lose work. The result is a measurable tilt that is hard to detect in any single case.
Why it matters
Where it shows up
Adversarial bias is most visible — and most consequential — in clinical opinion evidence: mental-state defences, competency evaluations, custody disputes, and risk assessments. Wherever the underlying judgement requires interpretation, the bias has room to operate.