Hire a psychiatrist or other mental health expert. An attorney often calls upon a psychiatrist to conduct tests on an accused person to prove or disprove the defendant's mental health during the commission of a crime. Many mental health experts specialize in interviewing and evaluating defendants in court cases.
Differentiate the legal meaning of mental illness from other behavioral or psychiatric problems. The term mental illness appears in court trials along with other types of problems that influence an individual's behavior. Mental illness describes a person who doesn't understand that his actions are illegal. This description varies slightly from state to state. A mentally ill person may remain so indefinitely, whereas a legally insane person may be incapacitated at the time of the crime but recover after treatment.
Prove diminished capacity. In first degree murder cases, lawyers challenge the state's case on the basis that the defendant couldn't commit premeditated murder because she lacks the mental capacity to do so.
Apply the M'Naghten Rule. Lawyers follow this legal standard to determine if the defendant knew right from wrong when the crime was committed. It's the standard most often used to establish the insanity defense.
Employ the irresistible impulse test. This expands the criteria used in M'Naghten to include the defendant's ability to control his impulses. This gives the insanity defense more credibility.
Use the American Law Institute's (ALI) Model Penal Code Test. This test makes it much harder to ascertain mental illness or insanity and has a much broader scope that the other tests for mental competency. Using this test gives the defense team a better chance of proving mental health problems or insanity in court.
Determine if the court accepts guilty but mentally ill as a possible verdict. Certain states allow defendants to be found guilty but mentally ill, and allow them to be placed in a mental care facility in lieu of jail time. Also, they may be sentenced to prison and receive psychiatric treatment while incarcerated.