Culpability and accountability: The insanity defense

The insanity defense provides the possibility that a defendant who committed a crime is excused because of the presence of a mental illness at the time of the act. Typically, it is not just the presence of a mental disorder, but its specific influence that exculpates a defendant. Even though it may seem intuitive that, at least in some cases, a mental disorder excuses a person for criminal conduct, the insanity defense is one of the most debated topics in criminal law. In this chapter, we will consider the insanity defense by looking at some of the central issues under debate. These are, among others, the legal criteria for insanity, the reliability of insanity assessments, and the contribution of neuroscience to the assessments. As we will discuss, in many legal systems, the insanity criteria include both a knowledge/appreciation component and a control component. This means that the question basically is: did the defendant know that what he was doing was wrong and/or could he control his conduct? Regarding the reliability of the assessments, we address some of the specific challenges. In general, we argue, obtaining and corroborating evidence from different sources is crucial in forensic psychiatric assessments. Finally, in a limited number of cases neuroscience may be helpful to diagnose a disorder, in particular neurological disorders such as a brain tumor or dementia. If neuroimaging is being used, in principle, it is advisable to also perform a neuropsychological evaluation in order to relate imaging findings to cognitive functioning. It is good to keep in mind that a brain abnormality is not necessarily legally relevant.