Neurobiological Evidence In Sentencing Decisions: A Double-Edged Sword

Brain evidence is playing an increasing role in criminal trials, and new research by Allen and colleagues (2019) suggests that such evidence may have both aggravating and mitigating effects on criminal sentencing. Indeed, legal theorists have previously portrayed physical evidence of brain dysfunction as a double-edged sword: on the one hand, capable of decreasing punitive motivations by minimizing the offender’s perceived responsibility for his transgressions, while, on the other hand, capable of increasing punitive motivations towards the offender by virtue of his ostensibly increased dangerousness. But, this effect has never been demonstrated experimentally.

Allen and colleagues (2019) sought to directly test this pattern. After presenting a large sample of 330 participants with a criminal case summary describing a sexual assault, participants were prompted to render an initial sentence recommendation. Next, participants were informed about the defendant’s mental health status using evidence of disorder described either as neurobiological or psychological, and treatable or untreatable. Participants then had an opportunity to amend their original criminal sentencing judgments and were able to allocate time between prison sentencing and involuntary hospitalization however they saw fit.

As hypothesized, neurobiological evidence elicited both shorter prison sentences and longer involuntary hospitalization terms compared to equivalent psychological evidence. These same effects were also observed when analyzing a participant’s change from baseline sentencing recommendation (see Figure 1). As hypothesized, decreased prison sentence length was accounted for by a change in deontological concerns — i.e., participants within the neurobiological condition found the defendant as less morally responsible and blameworthy for his actions, compared to those in the psychological condition.

Fig 1. Punishment change score by condition. Bars denote the percentage change in time from individual baseline punishment rating across conditions, for their revised prison recommendation (dark grey) and recommendation for involuntary hospitalization (light grey). Standard error bars shown. Image republished with permission from Corey Allen and PLOS ONE from

Overall, the experimental findings suggest that when mental health evidence is presented as having a neurobiological cause, laypeople are likely to assign more importance to it. Paradoxically, this effect seems to both favor and disfavor the defendant, depending on the punitive options available. That is, though prison sentences may be mitigated by the presentation of neurobiological evidence, the same evidence may increase the defendant’s risk of being involuntarily hospitalized.

These observed effects may have far-reaching implications for the law, which regularly confronts questions about the quality and presentation format of mental health evidence. For example, how can policymakers best manage the observed effects? Should neurobiological evidence always be accompanied by corresponding psychological/behavioral evidence or even warnings of potential biasing effects? Should jurors be made aware of treatment options or mandates in the case of excuse by mental illness of the defendant? Should judges receive a legal education on neurobiological evidence? Additional work is needed to answer these questions and explore other practical applications of this research.

These findings are described in the article entitled Reconciling the opposing effects of neurobiological evidence on criminal sentencing judgments, recently published in the journal PLOS ONE.

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