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  • Writer's pictureTerry Ang

Understanding Cognitive Biases


From litigation to contract negotiations, the legal process is very entwined with the human condition.

“Justice systems around the world are fundamentally human endeavours. As such, they rely on human cognitive systems that are subject to bias.” - (Charman and Douglass et al, 2019, p.30).

As much as lawyers purport to be rational, open-minded individuals armed with critical legal reasoning skills, it is impossible for us to be devoid of cognitive biases. Cognitive bias is an irrational systematic pattern of errors that affect a person’s decision-making process.

David Bonazzi Bias
Illustration by Davide Bonazzi @

Biases play a significant role in law. Current studies on bias reveal that the integrity and validity of merit we assign to evidence are questionable, at best. Within the law of evidence, there is a difference between evidence evaluation and evidence integration.

Evidence evaluation is how we determine the credibility, reliability and accuracy of a piece of evidence.

Evidence integration is how we integrate that piece of evidence into the overall sense of the situation.

An example of evidence evaluation is to scrutinise the process of how a DNA sample was retrieved from a crime scene. On the other hand, an example of evidence integration is how the prosecution intends to incorporate that piece of DNA to determine who the suspect.

The understanding of bias in the process of evidence integration does not receive as much attention and scrutiny. This can be illustrated by how police officers or detectives approach ambiguous pieces of evidence. A study in the Journal of Applied Research in Memory and Cognition shows that police officers and prosecutors who instinctively believed that the suspect is guilty will evaluate a piece of evidence more harshly. They will subconsciously undermine the strength of alibis. They might subconsciously inflate their thoughts and think that two different handwritings are the same. It gets to the point that their final beliefs of guilt would be the same as their initial beliefs of guilt regardless of the evidence evaluation process.

Illustration by

We can go on and on about the psychological underpinnings in legal reasoning and analysis. However, it might be more interesting to look at a couple of common cognitive biases in law and understand how and why they happen.

Confirmation Bias

Confirmation Bias Also known as myside bias is when a person only values elements within a piece of information or evidence that affiliates with their own personal opinion or ideal version of events. There are 3 types of sub-categories in this bias.

  1. Narrow search for information: People search for evidence or information that best supports their beliefs or hypothesis.

  2. Biased interpretation of information: People interpret statistically and object evidence different, based on their beliefs and attitudes towards a subject.

  3. Biased memory recall of information: People neutrally gather evidence and information. But when they recall said evidence, they subconsciously put more weight on the evidence that supports their beliefs.

Psychologists believe that there are explanatory and confirmatory thinking processes. The former looks at many different ideas and objectively maps out each other’s pros and cons, whereas the latter intensifies efforts on justifying a single point of view. There are many theorised reasons behind confirmation bias. Some psychologists believe that it stems from efforts to boost self-esteem. Practically speaking, it is also a more efficient way to process information. A way to explain this reasoning is through a cost-benefit analysis. For example, a law firm could ask one-sided questions focused on a candidate's academic skills. Having the best grades in law school does not make a person the best lawyer, but it is the most cost-effective way for the firm to filter and ensure that the most costly errors are avoided.

The police example given above is one such instance of confirmation bias. A famous case in New Zealand is the wrongful imprisonment of Teina Pora (Pora vs The Queen[2015] UKPC 9). The prosecution, in this case, built an entire issue of Mr Pora’s questionable confession and his background as a gang prospect. No evidence was submitted at all during the trial. What is more shocking is that information disclosed through court documents show that police have awarded up to $20,000 and indemnity offers for his relatives to testify against him. The defendant spent 20 years in jail before the Privy Council quashed his sentence and was awarded $3.5 million by the New Zealand government as compensation for the prosecution’s blunder.

Anchoring Bias

Also known as focal, this is the bias where a person will rely heavily on the first piece of evidence or information s/he consumed. A University of Chicago Law school paper has empirical data that showed seasoned and experienced judges are less likely to be heavily influenced by preliminary evidence, plaintiff’s claim or the prosecution’s sentence demand, all of which are events that happen at the start of legal processes. Reasons behind this bias have been described by pioneering Israeli psychologists Tversky and Kahneman with the theory called anchoring-as-adjustments, meaning that a person will adjust their ideas based on the initial information/evidence they received.

Illustration by The New Yorker.

Anchoring has shown to be a helpful tactic in negotiations. Tversky and Kahneman stated that while many factors can influence a negotiation decision, people tend to hone in and focus only on one key aspect they believe is the most crucial. They also found that initial offers have way more residual influence through the entirety of a negotiation process than subsequent counter-offers.

A house-appraising experiment has been done to understand the effects on anchoring. Two groups of people were given different initial prices of a house, one being a more general sum ($1,000,000) and the other being more specific ($975,800). They were asked to appraise the actual value of the house. People in the party given the more general one appraised the place higher on average than the latter. Putting an analogy into a legal perspective, the sentence demand from the prosecution will likely have a lingering effect on the final sentence given in a case, depending on how high or low it is.


An article published by International Law Firm Hogan Lovell's have adapted the serenity prayer to which reflects the need for hard evidence, despite our proclivity for inherent biases.

“grant me the serenity to accept the evidence that falsifies my hypothesis, the courage to use the evidence that justifies my hypothesis, and the wisdom to know the difference.”

To err is to be human. In my next article, I will be writing and discussing ways of eliminating bias in law.


Journal articles, books, gov. department websites and essays

  • James Shanteau (2003), Sandra L. Schneider (ed.), Emerging perspectives on judgment and decision research, Cambridge.

  • Jonathan Haidt (2013), The righteous mind: Why good people are divided by politics and religion, London.

  • Charles G. Lord, Lee Ross, and Mark R. Lepper (1979) Biased assimilation and attitude polarization: The effects of prior theories on subsequently considered evidence, Journal of Personality and Social Psychology 37 (11): 2098–109, Stanford.

  • American Psychological Association (2018). Why we're susceptible to fake news – and how to defend against it Skeptical Inquirer. 42 (6): 8–9, New York.

  • Yun-Chien Chang, Kong-Pin Chen and Chang-Chin Lin (2016) Anchoring Effect in Real Litigation: An Empirical Study Coase-Sandor Working Paper Series in Law and Economics, No. 744, Chicago.

  • Raymond S. Nickerson (1998) Confirmation Bias: A Ubiquitous Phenomenon in Many Guises Review of General Psychology 2 (2): 175-220, Massachusettes.

  • Charman, Steve & Kavetski, Melissa & Hirn Mueller, Dana. (2017). Cognitive Bias in the Legal System: Police Officers Evaluate Ambiguous Evidence in a Belief-Consistent Manner Journal of Applied Research in Memory and Cognition.


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