31 Section 3.4: Legal Defenses
To successfully obtain a conviction, the prosecutor must show all of the elements of the crime beyond a reasonable doubt in criminal court. This is not the end of it in some cases. It must also be shown (if the issue is raised) that the actus reus and the mens rea was present, but also that the defendant committed the act without justification or excuse. Both justifications and excuses are species of legal defenses. If a legal defense is successful, it will either mitigate or eliminate guilt.
A justification consists of a permissible reason for committing an act that would otherwise be a crime. Under normal circumstances, for example, it would be a crime to shoot a man dead on the street. If, however, the man was a mugger and had the shooter at knifepoint, then the justification of self-defense could be raised. A justification means that an act would normally be wrong, but under the circumstances it was the right thing to do. An excuse is different.
The Insanity Defense
The term insanity comes from the law; psychology and medicine do not use it. The everyday use of the term can be misleading. If a person acts abnormally, they tend to be considered by many as “crazy” or “insane.” To use insanity as a legal excuse, the defendant has to show that he or she lacked the capacity to understand that the act was wrong, or the capacity to understand the nature of the act. Some jurisdictions have a not guilty by reason of insanityplea.
The logic of the insanity defense goes back to the idea of mens rea and culpability. We as a society usually only want to punish those people who knew what they were doing was wrong. Most people believe that it is morally wrong to punish someone for an unavoidable accident. Likewise, society does not punish very young children for acts that would be crimes if an adult did them. The logic is that they do not have the maturity and wisdom to foresee and understand the nature of the consequences of the act. Put in oversimplified terms, if a person is so crazy that they do not understand that what they are doing is wrong, it is morally wrong to punish them for it.
Over the years, different courts in different jurisdictions have devised different tests to determine systematically if a criminal defendant is legally insane. One of the oldest and most enduring tests is the M’Naghten rule, handed down by the English court in 1843. The basis of the M’Naghten test is the inability to distinguish right from wrong. The Alabama Supreme Court, in the case of Parsons v. State (1887), first adopted the Irresistible Impulse Test. The basic idea is that some people, under the duress of a mental illness, cannot control their actions despite understanding that the action is wrong.
Today, all of the federal courts and the majority of state courts use the substantial capacitytest developed within the Model Penal Code. According to this test, a person is not culpable for a criminal act “if at the time of the crime as