Conjunction Error in Motive-Action Schemas
People often judge a conjunction of events (e.g., a crime with a plausible motive) as more likely than the simpler event alone, violating the probability principle that a conjunction cannot be more probable than its constituents. This bias is demonstrated in legal contexts where adding a nonobvious but plausible motive increases the perceived likelihood of an action.
THE IDEA
Motives and Crimes
A conjunction error in a motive–action schema is illustrated by the following problem – one of several of the same general type administered to a group of 171 students at UBC:
John P. is a meek man, 42 years old, married, with two children. His neighbors describe him as mild-mannered, but somewhat secretive. He owns an import–export company based in New York City, and he travels frequently to Europe and the Far East. Mr. P. was convicted once for smuggling precious stones and metals (including uranium) and received a suspended sentence of 6 months in jail and a large fine.
Mr. P. is currently under police investigation.
Please rank the following statements by the probability that they will be among the conclusions of the investigation. Remember that other possibilities exist and that more than one statement may be true. Use 1 for the most probable statement, 2 for the second, etc.
- Mr. P. is a child molester.
- Mr. P. is involved in espionage and the sale of secret documents.
- Mr. P. is a drug addict.
- Mr. P. killed one of his employees.
One-half of the subjects (n = 86) ranked the events above. Other subjects (n = 85) ranked a modified list of possibilities in which the last event was replaced by
- Mr. P. killed one of his employees to prevent him from talking to the police.
Although the addition of a possible motive clearly reduces the extension of the event (Mr. P. might have killed his employee for other reasons, such as revenge or self-defense), we hypothesized that the mention of a plausible but nonobvious motive would increase the perceived likelihood of the event. The data confirmed this expectation. The mean rank of the conjunction was 2.90, whereas the mean rank of the inclusive statement was 3.17 (p < .05, by t test). Furthermore, 50% of the respondents ranked the conjunction as more likely than the event that Mr. P. was a drug addict, but only 23% ranked the more inclusive target event as more likely than drug addiction. In other problems of the same type, we found that the mention of a cause or motive tends to increase the judged probability of an action when the suggested motive (a) offers a reasonable explanation of the target event, (b) appears fairly likely on its own, and (c) is nonobvious, in the sense that it does not immediately come to mind when the outcome is mentioned.
We have observed conjunction errors in other judgments involving criminal acts in both the A → B and the M → A paradigms. For example, the hypothesis that a policeman described as violence-prone was involved in the heroin trade was ranked less likely (relative to a standard comparison set) than a conjunction of allegations – that he is involved in the heroin trade and that he recently assaulted a suspect. In that example, the assault was not causally linked to the involvement in drugs, but it made the combined allegation more representative of the suspect’s disposition. The implications of the psychology of judgment to the evaluation of legal evidence deserve careful study because the outcomes of many trials depend on the ability of a judge or a jury to make intuitive judgments on the basis of partial and fallible data (see Rubinstein, 1979; Saks & Kidd, 1981).
When to use it
Evaluating the strength of circumstantial evidence in criminal trials
Assessing risks in business decisions where adding plausible scenarios increases perceived likelihood
Training legal professionals to recognize and mitigate cognitive biases in evidence interpretation
Designing decision support systems that highlight conjunction fallacies in probability estimates
FROM THEORY TO EVERYDAY LIFE
In practice
Mr. P. Investigation
One group ranked the inclusive statement that Mr. P. killed an employee; another ranked the narrower statement that he killed an employee to prevent disclosure to the police. The narrower event received a mean rank of 2.90, compared with 3.17 for the inclusive event. In the respective groups, 50% ranked the conjunction above drug addiction, compared with 23% for the inclusive event. This between-group pattern supports the authors’ account of a plausible motive making a narrower allegation seem more likely.
Violence-Prone Policeman
The hypothesis that a policeman described as violence-prone was involved in the heroin trade was ranked less likely than the conjunction that he is involved in the heroin trade and recently assaulted a suspect. The assault made the combined allegation more representative of the suspect's disposition, even without a causal link.
A MOMENT TO REFLECT
Put it to work.
How could the conjunction error affect a jury's evaluation of evidence where a prosecutor presents a plausible motive?
In what real-life situations have you observed a detailed story feeling more plausible than a simpler one, despite the added details reducing probability?
Why might the representativeness heuristic lead to conjunction errors in legal contexts, and how can judges and juries be trained to avoid it?
Often confused with
References
- Rubinstein (1979) — Rubinstein (1979)
- Saks & Kidd (1981) — Saks and Kidd (1981)
CONTINUE THE CONVERSATION
Source texts
- Heuristics and Biases The Psychology of Intuitive Judgment (Thomas Gilovich, Dale Griffin, Daniel Kahneman)