Another standard by which criminal insanity can be determined is with the substantial capacity test. In 1981 the American Law Institute created this test to provide a clear and scientific definition of mental illness and to eliminate the vague descriptions of insanity that were used in the earlier standards.

Under this test, the defendant may know that a certain action is criminally wrong, but he may not be able to completely appreciate the significance of committing such an act or he may be unable to stop himself from behaving in such a way. A person, for example, who is dominated by extreme fears and delusions may be unable to fully control his actions. Thus this test includes two basic categories of defendants who can be considered criminally insane: those with various psychological or cognitive problems, and those with serious behavior-control problems.

The substantial capacity test also clearly stated that antisocial behavior, or even extreme criminal behavior itself, cannot be used alone to prove criminal insanity. This prevented criminal psychopaths and those with antisocial personality disorders from using the insanity defense.

The most famous use of the substantial capacity test was at the trial of John Hinckley Jr., the man who shot President Ronald Reagan in 1982. The jury’s verdict was that Hinckley was not guilty by reason of insanity because the defense convinced the jury that Hinckley was not in control of his actions when he shot the president.