Several years ago, Ars looked at the role of neuroscience in crime. Since then, the scientific community has continued to learn about how brain abnormalities or dysfunctions can affect reasoning and behavioral traits, and certain gene variants like monoamine oxidase have been linked to violent behavior.

But correlation isn't the same as causation, and many of those correlations fall well short of 100 percent linkage. So, while biologically reductionist arguments like "my brain made me do it" can be appealing, scientists know that in the real world things are a lot more complex.

Nevertheless, neuroscience is increasingly being used as evidence in the criminal justice system. Nita Farahany, a professor of law and philosophy at Duke University, has now conducted a systematic study of how neuroscience ends up being used in the courthouse.

Starting with over 10,000 federal and state criminal cases that produced judicial opinions published between 2005 and 2012, she found 1,585 cases where neurobiological evidence was introduced. Her findings show that there is an increasing trend by criminal defendants to use neuroscience or behavioral genetics in their trials—and that those uses are often well outside the accepted academic or scientific implications of that work.

Farahany found that about 100 judicial opinions discussed neurobiological evidence in 2005, rising to more than 250 by 2012. These cases weren't just capital murder cases (where prosecutors sought the death penalty); they included other homicides and other felonies including drug offenses, assaults, robberies, and even fraud. Contrary to expectations, that data hasn't just been used to try and make sentencing more lenient by arguing the defendant's brain left them little choice.

Show me the brain scan?

One might expect that neurobiological evidence would mean submitting brain scans to the court. However, it turns out this only happened in about 15 percent of cases. More than 40 percent "have no discussion of neurological testing the opinion, even though the defendant staked their defense in part on a claim that 'his brain made him do it.'" (Brain scans, when used, were usually MRI or CAT scans, which provide a static picture of the brain's structure, rather than EEGs or fMRI scans, which track its activity.)

The paper highlights how neurobiology has actually shown up in a number of pretrial proceedings, usually in reference to the competency—or lack thereof—of the defendant to stand trial. in all, 11.5 percent of those 1585 cases featured neuroscience being used to challenge competency. Other cases used it to challenge the defendant's competency to plead guilty, or in a few cases, the defendant's competency to confess (for example, after defendants waived their Miranda rights).

The least useful application has involved neurobiology in determining guilt (as opposed to using neuroscience during pretrial or in sentencing). There is a growing body of scientific research that challenges the underlying assumption in criminal law that all our actions are "voluntary and the product of conscious thought." And, as Farahany points out, the idea that individuals may not be able to control their actions does not "align well with our "subjective experiences of self-directed decision-making."

And, in general, it's not likely to go over well. Claiming that you got into a high-speed chase with the police because you were in a blacked-out state of shock resulting from a head injury several weeks earlier is unlikely to be accepted as evidence by a trial judge.

About 44 percent of the cases identified featured the use of neurobiology to mitigate sentencing—of these, nearly half involved defendants arguing that their lawyers did not act effectively as they failed to use this line of defense earlier in the case. This, Farahany writes, "put[s] neurobiological evidence in a rarified position of must-investigate evidence. Defense counsel are ineffective if they fail to mount a defense at all, sleep through an entire (but not just parts of) a trial, or if they fail to investigate a probable neurological abnormality in a defendant. One of these things is not like the others, and its oddity makes clear that neurobiological evidence is an embedded part of the criminal process."

Will it get me off?

How effective are these approaches? Not very—about 20 to 30 percent of defendants were successful upon appeal in part due to neurobiological evidence, meaning that in the vast majority of cases it had no effect on the outcome. That may be a good thing from the perspective of many scientists, particularly when their work is used to draw conclusions that are not scientifically warranted. After all, our understanding of neuroscience and behavioral genetics is quite good at inferring things at a population level, but not when it comes to determining why an individual did something.

Farahany calls for neuroscientists to become much more engaged with the criminal justice system and to better educate judges, lawyers, and the public who make up our juries. If science is going to be used as evidence—which this study shows is happening with increasing frequency— scientists have a responsibility to make sure that's done appropriately.

J Law Biosci 2016. DOI: 10.1093/jlb/lsv059 (About DOIs).