Observations on Bias, Interest and Corruption in Evidence Law

As part of my Master of International Law studies, I’ve been exploring the ways in which courts handle challenges to witness credibility. One of the most fascinating areas is how the law distinguishes between bias, interest, and corruption. These categories, carefully analysed by Wigmore and later developed through case law, provide insight into how evidence law balances fairness to the parties with the integrity of the fact-finding process.

Observations on Bias, Interest and Corruption in Evidence Law

Defining the Categories

• Bias refers broadly to any form of hostility or favouritism a witness may hold toward a party. It encompasses prejudice, revenge, fear, or even friendship. Examples range from a disgruntled former employee seeking revenge on an employer to a family member reluctant to testify due to fear of reprisals.

• Interest is narrower, referring to a personal stake in the outcome of the litigation. This might include financial gain or loss, legal liability, or other direct consequences flowing from the verdict.

• Corruption goes further still. It involves conscious dishonesty, such as giving or taking a bribe, offering to fabricate testimony, or bargaining with evidence in exchange for leniency or advantage. It signals a fundamental unscrupulousness toward the case at hand.

These distinctions overlap but help courts structure the limits of cross-examination and the admissibility of evidence.

Why They Matter

Ordinary witnesses often come with clear biases — relatives, close associates, or rivals. Courts tend to accept this as background and allow it to be weighed without elaborate proof. But when bias or interest is not obvious, a witness may be asked about their relationship to the parties. If denied, the answer can be contradicted. This marks an important difference from other “credit-only” questions, where answers are usually final.

Corruption is treated most seriously. Offers to bribe, threats to lie under oath, or attempts to procure false testimony are not seen as mere collateral attacks. They go directly to whether the justice system can rely on the witness at all.

Illustrations from Case Law

Examples stretch across centuries:

• A witness threatening revenge on a former master (Attorney-General v Hitchcock (1847) 1 Exch 91).

• A mistress testifying for her patron (Thomas v David (1836) 7 C & P 350).

• A prosecution witness offering favourable evidence if disciplinary proceedings against him were dropped.

• An accused’s associate prepared to fabricate evidence in exchange for leniency (Nicholls v R (2005) 219 CLR 196).

Each illustrates how bias, interest, or corruption can infect testimony and why courts are alert to these dynamics.

My Reflections

What stands out to me is how these doctrines attempt to strike a balance. On the one hand, courts must allow legitimate probing of witness motives — otherwise, juries and judges risk being misled by apparently neutral testimony. On the other, there is a danger of trials degenerating into sprawling collateral disputes over every past quarrel or impropriety.

From a comparative perspective, international courts and tribunals often confront similar issues. Witnesses in human rights or war crimes trials, for example, may have deep-seated fears, loyalties, or even direct benefits at stake. Understanding the categories of bias, interest, and corruption helps to frame credibility assessments in a way that is principled, not arbitrary.

In my view, the strength of the common law approach lies in its flexibility. Bias and interest can be shown through contradiction when necessary, and corruption is always relevant. At the same time, judicial discretion prevents disproportionate attacks that would distract from the real issues in the case.