Decision Biases Among Lawyers: Conjunction Fallacy

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Dec 01, 2020

The conjunction fallacy suggests that decision-makers judge the probability of an event occurring based, in part, on the level of detail in which these events are described.[1] Stated differently, “[the] conjunction fallacy occurs because we make the mental shortcut from our perceived plausibility of a scenario to its probability.”[2]

The conjunction fallacy is quite intuitive, and research shows that both expert attorneys and judges engage in this type of thinking.[3] This conflation explains why, for example, it may seem more likely “that New York City might face a terrorist act committed by Muslim extremists than that New York City might face a terrorist attack.”[4] The former, more detailed version of the story may come across as more plausible (in part due to the operation of negative stereotypes), despite being less probable than the latter, statistically speaking.

Stated more precisely, the conjunction fallacy occurs when a decision-maker assigns a higher probability to an event of higher specificity, in violation of the laws of probability.[5] Consider the following two scenarios that were presented to study participants, each of whom were asked to evaluate the probability of one of two scenarios occurring:

  1. A massive flood somewhere in North America next year, in which more than 1,000 people drown.
  2. An earthquake in California sometime next year, causing a flood in which more than 1,000 people drown.[6]

The laws of probability dictate that scenario (1) must be more probable than scenario (2). This is because scenario (1) is more inclusive—that is, if scenario (2) occurs, scenario (1) must also occur (but not the other way around). However, the authors of the study found that the “probability judgements [of the study participants] were higher for the richer and more detailed scenario.”[7] The two scenarios triggered a form of intuitive thinking that caused study participants rely to on heuristics, or mental shortcuts.[8]This heuristic-based reasoning resulted in the “uncritical substitution of plausibility for probability.”[9] The more detailed California earthquake story was more plausible, or more coherent, than the North America story, despite being less probable.[10]

This effect is found in expert attorneys’ decision-making as well as with judges.[11] Fox and Birke conducted a study with over 200 practicing attorneys where the median reported experience of the group of attorneys was seventeen years.[12] The attorneys were asked about the famous Jones v. Clinton case, which was ongoing at the time.[13] At the time of the study, the case could have been resolved either through a judicial verdict or non-judicial outcome. Groups of attorneys were presented with scenarios and were asked to rate the probability of that scenario occurring. Two of these scenarios were:

  1. The case is resolved by an outcome other than a judicial verdict.
  2. The case is resolved through settlement.[14]

Again, because scenario (1) is more inclusive, it must be more probable. In other words, if the case is resolved through settlement, it must also be true that the case is resolved by an outcome other than a judicial verdict. However, if the case is resolved by an outcome other than a judicial verdict, it does not necessarily follow that there must have been a settlement. For instance, there could have been dismissal as a result of judicial action, legislative grant of immunity to Clinton, or withdrawal of the claim by Jones.[15] Yet, despite the attorneys’ experience, they judged the prospect of a settlement as more probable.[16] Studies have also been done with judges showing similar results.[17]

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[1] See, e.g., Guthrie et al., The Hidden Judiciary, at 1509.

[2] Mental Model: Bias from Conjunction Fallacy, Farnam Street, (last accessed Mar. 24, 2020).

[3] See generally Craig R. Fox & Richard Birke, Forecasting Trial Outcomes: Lawyers Assign Higher Probability to Possibilities That Are Described in Greater Detail, 26 L. & Hum. Behavior 159, 162–63 (2002).

[4] See, e.g., Guthrie et al., The Hidden Judiciary, at 1509.

[5] Mental Model: Bias from Conjunction Fallacy, Farnam Street, (last accessed Feb. 18, 2020).

[6] Daniel Kahneman, Thinking Fast and Slow 159–60 (Farrar, Straus and Giroux eds., 2011).

[7] Id.

[8] Id.

[9] Id. at 159.

[10] Id. at 159–60.

[11] See generally Fox & Birke, at 162–63 (attorneys). Studies have been done on judges showing similar results. See Guthrie et al., The Hidden Judiciary, at 1509–12.

[12] See Fox & Birke, at 162–63.

[13] Id.

[14] Id.

[15] Id.

[16] Id. 

[17] See Guthrie et al., The Hidden Judiciary, at 1509–12.

About the Author

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Tom Spiegler


Tom is a Co-Founder and Managing Director at The Decision Lab. He is interested in the intersection of decision-science and the law, with a focus on leveraging behavioral research to shape more effective public and legal policy. Tom graduated from Georgetown Law with honors. Prior to law school, Tom attended McGill University where he graduated with First Class Honors with majors in Philosophy and Psychology.

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