An expert witness, particularly in common law countries such as the United Kingdom, Australia, and the United States, is a person whose opinion by virtue of education, training, certification, skills or experience, is accepted by the judge as an expert. The judge may consider the witness's specialized opinion about evidence or about facts before the court within the expert's area of expertise, to be referred to as an "expert opinion". Expert witnesses may also deliver "expert evidence" within the area of their expertise. Their testimony may be rebutted by testimony from other experts or by other evidence or facts.
The insanity defense, also known as the mental disorder defense, is an affirmative defense by excuse in a criminal case, arguing that the defendant is not responsible for their actions due to a psychiatric disease at the time of the criminal act. This is contrasted with an excuse of provocation, in which the defendant is responsible, but the responsibility is lessened due to a temporary mental state. It is also contrasted with the justification of self defense or with the mitigation of imperfect self-defense. The insanity defense is also contrasted with a finding that a defendant cannot stand trial in a criminal case because a mental disease prevents them from effectively assisting counsel, from a civil finding in trusts and estates where a will is nullified because it was made when a mental disorder prevented a testator from recognizing the natural objects of their bounty, and from involuntary civil commitment to a mental institution, when anyone is found to be gravely disabled or to be a danger to themself or to others.
The M'Naghten rule(s) (pronounced, and sometimes spelled, McNaughton) is a legal test defining the defence of insanity that was formulated by the House of Lords in 1843. It is the established standard in UK criminal law. Versions have been adopted in some US states, currently or formerly, and other jurisdictions, either as case law or by statute. Its original wording is a proposed jury instruction:
that every man is to be presumed to be sane, and ... that to establish a defence on the ground of insanity, it must be clearly proved that, at the time of the committing of the act, the party accused was labouring under such a defect of reason, from disease of the mind, as not to know the nature and quality of the act he was doing; or if he did know it, that he did not know he was doing what was wrong.
The Sixth Amendment to the United States Constitution sets forth rights related to criminal prosecutions. It was ratified in 1791 as part of the United States Bill of Rights. The Supreme Court has applied all but one of this amendment's protections to the states through the Due Process Clause of the Fourteenth Amendment.
In criminal law, diminished responsibility is a potential defense by excuse by which defendants argue that although they broke the law, they should not be held fully criminally liable for doing so, as their mental functions were "diminished" or impaired.
Forensic psychology is the application of scientific knowledge and methods to assist in answering legal questions that may arise in criminal, civil, contractual, or other judicial proceedings. Forensic psychology includes research on various psychology-law topics, such as: jury selection, reducing systemic racism in criminal law, eyewitness testimony, evaluating competency to stand trial, or assessing military veterans for service-connected disability compensation. The American Psychological Association's Specialty Guidelines for Forensic Psychologists reference several psychology sub-disciplines, such as: social, clinical, experimental, counseling, and neuropsychology.
Forensic psychiatry is a subspeciality of psychiatry and is related to criminology. It encompasses the interface between law and psychiatry. According to the American Academy of Psychiatry and the Law, it is defined as "a subspecialty of psychiatry in which scientific and clinical expertise is applied in legal contexts involving civil, criminal, correctional, regulatory, or legislative matters, and in specialized clinical consultations in areas such as risk assessment or employment." A forensic psychiatrist provides services – such as determination of competency to stand trial – to a court of law to facilitate the adjudicative process and provide treatment, such as medications and psychotherapy, to criminals.
Clark v. Arizona, 548 U.S. 735 (2006), is a United States Supreme Court case in which the Court upheld the constitutionality of the insanity defense used by Arizona.
In criminal law, a mitigating factor, also known as an extenuating circumstance, is any information or evidence presented to the court regarding the defendant or the circumstances of the crime that might result in reduced charges or a lesser sentence. Unlike a legal defense, the presentation of mitigating factors will not result in the acquittal of a defendant. The opposite of a mitigating factor is an aggravating factor.
Settled insanity is defined as a permanent or "settled" condition caused by long-term substance abuse and differs from the temporary state of intoxication. In some United States jurisdictions, "settled insanity" can be used as a basis for an insanity defense, even though voluntary intoxication cannot, if the "settled insanity" negates one of the required elements of the crime such as malice aforethought. However, U.S. federal and state courts have differed in their interpretations of when the use of "settled insanity" is acceptable as an insanity defense and also over what is included in the concept of "settled insanity".
An ultimate issue in criminal law is a legal issue at stake in the prosecution of a crime for which an expert witness is providing testimony.
Insanity in English law is a defence to criminal charges based on the idea that the defendant was unable to understand what he was doing, or, that he was unable to understand that what he was doing was wrong.
United States federal laws governing offenders with mental diseases or defects provide for the evaluation and handling of defendants who are suspected of having mental diseases or defects. The laws were completely revamped by the Insanity Defense Reform Act in the wake of the John Hinckley Jr. verdict.
The Insanity Defense Reform Act of 1984 (IDRA) was signed into law by President Ronald Reagan on October 12, 1984, amending the United States federal laws governing defendants with mental diseases or defects to make it significantly more difficult to obtain a verdict of not guilty only by reason of insanity.
The criminal law of the United States is a manifold system of laws and practices that connects crimes and consequences. In comparison, civil law addresses non-criminal disputes. The system varies considerably by jurisdiction, but conforms to the US Constitution. Generally there are two systems of criminal law to which a person maybe subject; the most frequent is state criminal law, and the other is federal law.
The role of expert witnesses in English law is to give explanations of difficult or technical topics in civil and criminal trials, to assist the fact finding process. The extent to which authorities have been allowed to testify, and on what topics, has been debated, and to this end a variety of criteria have evolved throughout English case law.
United States v. Brawner, 471 F.2d 969, is decision by the United States Court of Appeals for the District of Columbia Circuit in which the Court held that a person is not responsible for criminal conduct if at the time of such conduct as a result of mental disease or defect, he lacked substantial capacity either to appreciate the criminality of his conduct or conform his conduct to the requirements of the law.
Durham v. United States, 214 F.2d 862, is a criminal case articulating what became known as the Durham rule for juries to find a defendant is not guilty by reason of insanity: "an accused is not criminally responsible if his unlawful act was the product of mental disease or mental defect."
The ALI rule, or American Law Institute Model Penal Code rule, is a recommended rule for instructing juries how to find a defendant in a criminal trial is not guilty by reason of insanity. It broadened the M'Naghten rule of whether a defendant was so mentally ill that he is unable to "know" the nature and quality of his criminal act, or know its wrongfulness, to a question of whether he had "substantial capacity to appreciate the criminality of [his] conduct". It also added a volitional component as to whether defendant was lacking in "substantial capacity to conform his conduct to the law". It arose from the case of United States v. Brawner.
Kahler v. Kansas, 589 U.S. ___ (2020), is a case of the United States Supreme Court in which the justices ruled that the Eighth and Fourteenth Amendments of the United States Constitution do not require that states adopt the insanity defense in criminal cases that are based on the defendant's ability to recognize right from wrong. It was argued on October 7, 2019 and decided on March 23, 2020.
