The use of neuroscience in criminal court cases has risen steeply in recent decades.
However, its actual effects on legal judgments remain unclear.
Key Facts:
- Neuroscience evidence, especially brain scans, is increasingly used in criminal trials to support mental disorder diagnoses.
- Studies find limited effects of neuroscience evidence in some cases, like reducing death penalty sentences.
- But neuroscience doesn’t consistently affect other judgments like guilty/not guilty verdicts or sentence length.
- Images of the brain itself don’t appear to hold any special persuasive power over legal decisions.
- More research is needed into the circumstances under which neuroscience does and does not affect legal judgments.
Source: Cogn Res Princ Implic.
Brain Scans as Evidence in Court
Over the last few decades, there has been an explosion in neuroscience research and technologies that enable scientists to study the living human brain.
Tools like MRI and EEG have furthered our understanding of the brain immensely.
They have also allowed experts to better characterize neurological and psychiatric disorders through biomarkers and scans showing atypical structure or function.
Naturally, these advances have made their way into courtrooms as evidence to support insanity defenses and other claims of mitigating mental conditions.
This trend is especially notable in the United States.
One analysis found a steady increase in US court cases involving neuroscientific evidence from 2005 to 2012, with over 250 judicial opinions referencing such evidence in 2012 alone.
This influx has sparked interest, and concern, over how neuroscience evidence might sway legal decisions made by judges and jurors.
Could looking at brain scans cause them to see defendants differently?
Might images of damaged brains elicit undue sympathy?
While this issue has garnered significant media attention, the research literature paints a complex picture.
Overall, it suggests limited effects of neuroscience evidence under certain circumstances.
However, many open questions remain regarding the precise conditions under which it does and does not affect legal judgments.
Early History: Neuroscience Enters the Courtroom
The use of neuroscience in courtrooms began as early as the 1940s, alongside the advent of new techniques like EEG, which records electrical activity in the brain.
By the mid-1900s, EEG had become common in criminal cases involving diagnoses like epilepsy.
One high-profile case that brought neuroscience into the legal spotlight was the trial of John Hinckley Jr. for the attempted assassination of President Reagan in 1981.
Hinckley’s defense team introduced CT scans of his brain to support their argument that he had schizophrenia and should be found not guilty by reason of insanity (NGRI).
Although controversial at the time, the judge ruled the scans admissible, and Hinckley was ultimately found NGRI.
Over the next few decades, more advanced neuroimaging tools like MRI and PET were introduced in court cases, along with extensive neuropsychological testing.
This shift accompanied the 1993 Daubert Supreme Court decision.
Daubert established a new standard governing the admissibility of scientific evidence in federal cases.
It allowed techniques not yet widely accepted to appear in courtrooms, paving the way for increased use of novel neuroscience methods.
Modern Use of Neuroimaging in Criminal Trials
So in what kinds of modern criminal cases is neuroscience evidence introduced?
A 2015 analysis reviewed 553 US criminal cases using such evidence between 1992-2012.
Two thirds began as death penalty cases, while one quarter involved a potential life sentence or decade+ in prison.
In virtually all cases, neuroscience evidence was presented by the defense as mitigating information, often supporting a disorder diagnosis.
Interestingly, almost two-thirds of the cases specifically involved neuroimaging evidence, including MRI, PET and CT scans.
This highlights the prominence of visual brain data.
Furthermore, over half the cases included arguments that the inadequate use of neuroscience evidence by defense counsel constituted ineffective assistance of counsel.
This suggests an increasing expectation that neuroscience should be introduced when relevant.
Does Neuroscience Evidence Actually Impact Legal Decisions?
This trend of rapidly increasing courtroom neuroscience sparked scientific interest in whether such evidence truly sways legal judgments.
Specifically, does it affect verdicts rendered by judges and jurors?
Overall, the research literature paints a complex picture, due to widely varying study methodologies.
However, taken together, it suggests neuroscience evidence has limited effects in some cases, but this is far from a consistent finding.
For example, a 2014 mock juror study found neuroscience testimony reduced death sentences, but only for defendants with schizophrenia – not psychopathy.
Similarly, another 2012 study reported that expert testimony describing a murderer’s brain damage reduced death sentences, but only when he was characterized as high-risk for future violence.
However, other mock juror studies have found no effects of neuroscience testimony on verdicts or sentencing.
And importantly, neuroscience did not consistently increase not guilty verdicts, although a few studies did report increases in NGRI verdicts.
Furthermore, while neuroscience may affect sentencing for the most severe crimes, it did not consistently reduce prison sentence length in these studies.
One exception found reduced sentences among judges (not jurors) presented with neuroscience testimony.
The Limited Power of Brain Images
Many studies specifically examined the persuasive power of brain images themselves.
However, overall they do not appear to hold any special influence over and above neuroscience testimony.
Multiple experiments found no differences in verdicts or sentencing when mock jurors were shown neuroscience expert testimony with or without brain images.
This contrasts with earlier studies suggesting visual brain scans uniquely swayed peoples’ judgments.
Therefore, current evidence indicates that although neuroscience evidence may sometimes elicit more lenient legal decisions, the brain images themselves do not enhance expert testimony.
Unanswered Questions and Future Directions
Clearly the influence of neuroscience on legal judgments is nuanced, with effects emerging under select conditions.
But research is still needed to clarify why effects occur in some cases but not others.
For example, no single study has examined whether a defendant’s characteristics, like race or socioeconomic status, change neuroscience’s effects.
But given wider evidence of bias in legal decisions, this seems critical to investigate.
Furthermore, we still lack data on how neuroscience evidence impacts jurors’ perceptions of defendants, including responsibility and future potential of danger.
These cognitive shifts likely explain neuroscience’s effects in some cases.
Research into moderators and psychological mechanisms would significantly advance this field.
Studying lesser crimes, beyond murder and assault, would also better match neuroscience’s real-world courtroom use.
Finally, we must be cautious not to overstate neuroscience’s legal implications.
Additional research can help educate judges and jurors on what neuroscience does and does not mean in a courtroom context.
With care, neuroscience may better inform how people think about criminal responsibility and behavior.
References
- Study: Neuroscientific evidence in the courtroom: a review
- Authors: Darby Aono et al. (2019)
