We all know that neuro-imaging is not ready for courtroom persuasion yet, but did someone remember to tell the jurors? Apparently not–but maybe they already knew.

Are “pictures” of the brain so persuasive that they can sway juror decision making? Even though the credibility of the medical experts has to be factored in, many people believe that when you include brain scan photos (as opposed to bar charts or topographical maps for example) the viewer is more persuaded. Even though there isn’t a chance in the world that an average juror can make sense of the image.

So, the current researchers decided to look at the effect, if any, of neuroimages using a fact pattern of a capital murder case. They were trying to sort out whether it is useful to focus mitigation arguments on neuroimaging evidence, since it could “create the impression the defendant is ‘damaged goods’ and beyond repair.”

The researchers presented the facts of a murder along with evidence (included in nearly all capital trials) of the likelihood of future dangerousness and evidence about the psychiatric condition of the defendant. They divided participants into 3 conditions:

The first group was told the defendant was psychotic.

The second group was told the defendant was psychotic and was given the results of neuropsychological testing discussing damage to the defendant’s brain (specifically, the frontal lobe).

The third group was given the diagnostic information and the test results but also shown color photos of “structural and functional scans of the defendant’s brain” and given descriptions of likely consequences with this sort of damage to the brain.

The researchers expected that the more information participants were given, the less they would choose the death penalty IF the defendant was adjudged to be at low risk for future violence. And they were wrong.

When mock jurors were provided only a diagnosis, and were told the defendant had a high risk of future violence, they were overwhelmingly more likely to give a death sentence (65% voted for death penalty) than other mock jurors. When they were given additional information (either a psychological testing summary or the testing summary with neuroimaging evidence), their imposition of the death sentence dropped dramatically (down to 12% voting for death penalty). If jurors assessing these ‘dangerous’ defendants were given all three forms of information, the death penalty was selected only 8% of the time.

Oddly, low risk of dangerousness only affected those jurors who were only given the diagnosis (but no supportive test data or images). ‘Diagnosis only’ mock jurors assigned the death penalty about 17% of the time (down from 65%), while diagnosis plus testing mock jurors chose the death penalty about 21% of the time and mock jurors receiving all three types of evidence chose the death penalty about 14% of the time (essentially the same as the ‘high risk of violence’ condition).

The researchers concluded that with high risk for future violence defendants, both neuropsychological testing and neuroimaging evidence reduced the likelihood jurors would choose the death penalty. Rather than being ‘aggravating’, this additional evidence was truly mitigating in the case of the high-risk-to-violently-reoffend defendant.

Our view is that this is consistent with what is frequently seen in juror decision-making: Jurors are much more comfortable accepting a high-risk proposition if they are comforted that they have a valid reason for doing it.  This is most clearly seen in the high risk group, where they needed to be assured there is ‘scientific evidence’ that the defendant was impaired, not merely that the defense found an expert willing to make that claim.  It also suggests that in the population used in the study, there were about 15-20% who were in favor of the death penalty regardless of mitigating scientific  evidence.

It’s important to note that the neuropsychological testing results mitigated almost as much as the testing results plus the brain pictures. The verbal description alone (or verbal picture if you will) persuaded jurors. The researchers say it is possible that any additional information pertinent to the defendant’s condition ‘personalizes’ the defendant for jurors and thus may prove mitigating. We go with the former idea, as it seems unlikely that a psychological test profile or a description of neuroimaging studies causes anyone to become more ‘personal’. The jurors simply want reassurance that they are working off more than one person’s opinion.

For the practitioner, this says you don’t need bells and whistles [like brain scans] to successfully mitigate. But you do need solid and scientific information, well-presented using language and examples jurors can understand. That testimony can save a life.

Greene, E., & Cahill, B. (2011). Effects of Neuroimaging Evidence on Mock Juror Decision Making Behavioral Sciences & the Law DOI: 10.1002/bsl.1993

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