Confirmation Bias and the Law

“The human understanding when it has once adopted an opinion […] draws all things else to support and agree with it” — Frances Bacon, 1620

Once we hold a particular view or hypothesis, we are more likely to search for, acknowledge, give credence to, and remember information that confirms it, regardless of whether that information is true. This phenomenon, known as confirmation bias, may be one of the most dangerous cognitive biases affecting decision-making in a judicial context.

Consider psychologist Peter Watson’s so-called 2-4-6 test. Participants were told that the experimenter had a rule in mind that classified three sets of numbers, and that “2-4-6” conformed to that rule. The subjects then proposed their own sets of numbers, were told whether it conformed to the rule or not, and were allowed to continue until they felt sure they knew the rule.

The rule was actually “any set of three increasing numbers”, but participants typically had a difficult time discovering this. They often believed the rule to be something such as “even numbers increasing” or “numbers increasing in equal intervals”, and the positive feedback received for sets following these incorrect rules strengthened their convictions. The fact that participants failed to even consider generating sequences seriously at odds with their focal hypothesis (such as 100-40-17) demonstrates the strength of confirmation bias.

As a form of motivated cognition, it affects all levels of belief formation and behaviour. If we dislike somebody, we give disproportionate weight to any evidence that they may be unpleasant, ignore evidence to the contrary, and interpret ambiguous evidence in our favour. At a wider level, we choose the friends, read the websites, and support the political parties we expect to confirm our existing views of the world.

The implications of this bias for the judicial system are multiple. Trials, for example, are frequently long and complex, and studies have shown that members of the jury often form their decisions early and interpret subsequent evidence in a way that supports their premature conclusions. This process has leads to a polarisation of attitudes among jurors, as each member of the jury becomes more and more entrenched in their position as the trial develops.

In the 2013 murder trial of David Camm,  the defense lawyer argued that Camm was charged with the murder of his family solely due to the effect of confirmation bias in the investigation. Every piece of evidence against him transpired to be inaccurate or unreliable, yet the charges against him were not dropped (though Camm was eventually acquitted).

The Central Park jogger case is another example. In 1989, five teenagers confessed to raping and assaulting a woman as she jogged through Central Park. They quickly retracted their statements, alleging that police had coerced their confessions. No physical or eyewitness evidence linked the suspects to the attack; in fact, semen recovered from the victim appeared to come from a single donor and did not match any of the five suspects. Nevertheless, a jury convicted all five of them.

 The detective’s and prosecutor’s statements made at the time demonstrated the intensity of their commitment to their theory of the case. They found all evidence that severely undermined their theory incredible, or modified their version of events to accommodate the new evidence within their original hypothesis. (In 2002, another man confessed to the crime. His DNA matched the victim, and a judge  overturned the original defendants’ convictions).

Of course, all lawyers arguably ‘exploit’ confirmation bias to some extent, as to build a case is to argue that the evidence at hand supports the conclusion dictated by the client. However, there is a clear difference between evaluating all evidence impartially in order to build a case consciously, and using selected evidence to justify a conclusion already drawn. Being able to objectively assess a situation is critically important to the practice of law. Engaging in case-building via confirmation bias without being aware of doing so may lead to overconfidence in the strength of the resulting case.

So, what can we do? Unfortunately, simply being aware of confirmation bias is not enough to mitigate its effects, and neither is it a matter of lacking intelligence. As studies have shown , while some cognitive biases do correlate with IQ, confirmation bias does not.

The most effective strategy is to develop a mind-habit of always ‘thinking the opposite’. Always consider the possibility that you might be (drastically) wrong, actively search for evidence that this might be the case,  consider how new evidence could  challenge as well as strengthen your beliefs, and be discerning about the information you choose to process.


Devine, Patricia G.; Hirt, Edward R.; Gehrke, Elizabeth M. (1990), “Diagnostic and confirmation strategies in trait hypothesis testing”, Journal of Personality and Social Psychology (American Psychological Association)

Myers, D.G.; Lamm, H. (1976), “The group polarization phenomenon”, Psychological Bulletin 83 (4): 602–627

Nickerson, R. S. (1998). Confirmation bias: A ubiquitous phenomenon in many guises. Review of General Psychology, 2(2), 175-220

Roach, Kent (2010), “Wrongful Convictions: Adversarial and Inquisitorial Themes”, North Carolina Journal of International Law and Commercial Regulation

Russo, J. Edward, and Margaret G. Meloy. “Hypothesis generation and testing in Wason’s 2–4–6 task.” Unpublished manuscript (2002).

Schanberg, S. H, (2002, November 26). A journey through the tangled case of the central park jogger. Village Voice, p. 36.

Stanovich, Keith E., Richard F. West, and Maggie E. Toplak. “Myside bias, rational thinking, and intelligence.” Current Directions in Psychological Science 22.4 (2013): 259-264.


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