Brain scans could influence jurors more than other forms of evidence

It's surely just a matter of time until functional MRI brain scans are admitted in US and UK courts. Companies like No Lie MRI have appeared, and there have been at least two recent attempts by lawyers in the USA to submit fMRI-based brain imaging scans as trial evidence.

Functional MRI gauges fluctuating activity levels across the brain, with experts divided on the merits of using the technology as a high-tech lie detection measure (see earlier). The late David McCabe who died earlier this year, and his colleagues, have put that debate to one side. They asked: if fMRI evidence were to be allowed in courts, would it have a particularly influential effect on jurors' decisions? There's good reason to think it might. For example, a 2008 study by Deena Weisberg found that lay people and neuroscience students (but not neuroscience experts) were more satisfied by bad scientific explanations when they contained gratuitous mentions of neuroscience.

For the new study, 330 undergrads at Colorado State University read a vignette about a criminal trial in which a defendant was accused of killing his estranged wife and lover. Various points of evidence were mentioned and summaries of testimony and cross-examination were provided (the vignette amounted to two pages).

Crucially, a sub-set of the participants read a version in which fMRI evidence was cited: "... there was increased activation of frontal brain areas when Givens [the defendant] denied killing his wife and neighbour, as compared to when he truthfully answered questions." For comparison, other participants read a version that either included incriminating evidence from polygraph, from thermal imaging technology (which measures changes in facial skin temperature), or that contained no lie-detection technology.

The key finding was that participants who read the brain-imaging version were far more likely (76 per cent) to say they considered the defendant guilty, compared with participants who read the other versions (47 to 53 per cent). Moreover, the lie-detection evidence was more likely to be cited by participants in the fMRI condition as key to their decision, as compared with participants who read versions that didn't mention fMRI.

The participants were not entirely seduced by fMRI. Some of them were given a slightly different version of the fMRI vignette, in which the expert witness warned about the technology's unreliability. These participants came to a similar proportion of guilty verdicts as the participants who read the vignette versions that lacked fMRI evidence. So it seems the persuasive influence of fMRI evidence can be tempered easily enough if people are reminded of its limitations.

The researchers acknowledged the obvious weaknesses of their study: the use of students as mock jurors, the use of vignettes rather than a real trial, and so on. These caveats aside, they said their data show that fMRI evidence could be more influential than other types of evidence. "... [T]hough determining whether that indicates the evidence would lead to unfair prejudice, confusion of the issues, misleading the jury, or needless presentation of cumulative evidence is a complex issue," they said. "At the very least, it appears that juries should be informed of the limitations of fMRI evidence."
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McCabe, D., Castel, A., and Rhodes, M. (2011). The Influence of fMRI Lie Detection Evidence on Juror Decision-Making. Behavioral Sciences and the Law DOI: 10.1002/bsl.993

Further reading: The brain on the stand, by Jeffrey Rosen, New York Times magazine.

This post was written by Christian Jarrett for the BPS Research Digest.