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Do Brain Scans Sway Juries?

Neuroskeptic iconNeuroskeptic
By Neuroskeptic
Jan 12, 2012 7:40 AMNov 5, 2019 12:14 AM


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Does seeing a criminal's brain affect jury decisions?

Edith Greene and Brian Cahill ask this question in a new study which put volunteers in the position of jurors in a murder trial. The 'defendant' was guilty, but the question was: should they get life in prison, or death?

It turned out that seeing brain scans didn't have much of an effect - but it's not clear how far the results would generalize.

208 mock-jurors were randomly assigned to get different kinds of mitigation information about the accused. Sometimes, all they were told was that he had been diagnosed with schizophrenia, depression and a substance misuse disorder. Others were also given neuropsychological test scores showing that he did poorly on various tests of reasoning and cognition. Finally, some were shown brain scans on top of all that, scans which were described as showing left frontal lobe damage.

All these materials were based on a real 2007 court case.

What happened? When the defendent was said to have been assessed as probably "dangerous" in future, people who were only told his diagnosis of schizophrenia usually sent him to the chair. But when they were given his psychological test scores - showing that he suffered from cognitive impairments - they were far more lenient. Seeing the neuroimages had no effect on top of that.

If the guy was described as posing a low risk of future violence, the verdicts were lenient, no matter what else they were told about him. In the real case, by the way, he got life.

This suggests that brain scans don't exert a seductive allure on jury decisions, at least not over-and-above psych test scores. But I'm not sure how representative the results are. The 'jurors' were all psychology undergrads. Most were Hispanic (63%) females (67%). Are psychology students especially resistant to the allure of brain scans - and/or especially vulnerable to the allure of psychological test scores? No-one knows, but it's surely plausible.

On some level, neuroimaging evidence clearly can influence people's decisions, like any other evidence; lawyers wouldn't bother presenting it otherwise. The question is how much of an impact it has, but that is surely going to depend on the details of the case as well as the juror's background; I'm not sure how much a study like this one, focussing on one example, will be able to tell us.

Greene E, and Cahill BS (2011). Effects of Neuroimaging Evidence on Mock Juror Decision Making. Behavioral Sciences and the Law PMID: 22213023

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