The role of neuroscience in criminal liability has been a topic of much debate in recent years. Some argue that neuroscience is irrelevant in criminal court, while others contend that it can help establish that many criminals lack control over their behavior. However, the truth is likely somewhere in the middle, as certain types of neuroscience evidence may prove useful and relevant in criminal trials.
There are five types of neuroscience evidence that can be applicable to criminal law:
The first type of admissible evidence includes data showing brain abnormality obtained through scans, which may diagnose the defendant with conditions such as frontal lobe disorder or traumatic brain injury (TBI). At present, this evidence is only weakly relevant to criminal liability because it fails to establish a causal link between the neurological impairment and the crime. In rare cases though, behavioral evidence may be sufficient to establish a connection between neurology and behavior, such as in cases where a brain tumor is shown to be a cause of impulsive behavior. Overall, evidence of the abnormality alone is usually insufficient to prove impairment or causation in criminal trials.
Second, research demonstrating a higher occurrence of criminal behavior among similarly impaired individuals can help to supplement evidence of a defendant's brain abnormality. One research study found that 73% of prisoners convicted of violent crimes had frontal lobe damage (FLD), while only 28% of those who committed non-violent crimes had FLD. However, this research only shows that a high proportion of violent individuals possess this brain abnormality and ultimately does not answer the central question of whether individuals with the abnormality are more likely to be violent. Courts have deemed this type of research as irrelevant and exculpatory because it does not explain how the abnormality contributed to the crime. Therefore, it tends to be no more useful than evidence of the abnormality alone.
Third, genetic and environmental factors that strongly predispose people to commit serious crimes may also be considered by criminal courts. However, even if the defense presents strong evidence that a crime is caused by neurological abnormality, evidence of planning or other factors can still undercut an insanity defense. In general, numerous difficulties in generalizing from a study to the case at hand continue to undermine the usefulness of effect-of-cause studies.
Fourth, neuropsychological testing provides insight into a particular defendant's functioning, including their capacity to self-control, ability to plan and form intent, and possess an awareness of risks. However, the testing provides limited insight into a particular defendant's functioning rather than providing the type of functioning that might accompany a particular brain abnormality. There are several obstacles to this approach such as malingering, difficulty of obtaining baseline data for relevant demographic groups, and the fact that testing results do not necessarily reflect the defendant's state of mind at the time of the crime. Moreover, science itself cannot provide a definitive answer to a normative question of how much "dysfunction" is required for legal purposes.
Lastly, individualized neuroscience findings can be used to compare the brain structure and functioning of defendants with juveniles or individuals with intellectual disabilities, which can be relevant at sentencing. The Supreme Court has exempted individuals with intellectual disability and juveniles from the death penalty, stating that they are categorically less culpable than the average criminal. Neuroscientific data can potentially be used to compare the brain structure and functioning of defendants with those of juveniles or individuals with intellectual disabilities, and this evidence can be considered relevant at sentencing. This approach is known as "scientific stare decisis." However, identifying those neurological characteristics that should be used for comparison continues to pose a scientific and legal