The M’Naghten Rule was not without criticism. Some scholars felt that the rule was too rigid and would not excuse anyone from criminal conduct except the most severely mentally ill. The law, many people felt, should be more flexible. In 1886, the Parsons v. Alabama (81 AL 577, So 854 1886 AL) decision established additional criteria for the insanity defense. The court decided that a person could utilize the insanity defense if he or she could prove that “by reason of duress of mental disease he had so far lost the power to choose between right and wrong, and to avoid doing the act in question, as that his free agency was at the time destroyed. “. It became know as the “Irresistible Impulse Test” or as an earlier court in England called it, the “policeman at the elbow test.” In other words, if the person would have committed the crime even if a policeman was standing next to him, then the act could be characterized as an irresistible impulse because no sane person would commit a criminal act in the presence of a law enforcement agent. But what exactly was an “irresistible impulse”? How is it measured and how can one tell if another could resist an impulse? Many in the legal profession felt that this rule would allow criminal defendants to escape guilty verdicts by fakery and manipulation of the courts. 

Subsequent rulings in the nation’s courts like Sinclair v. State of Mississippi (132 So. 581 1931 MS) which declared that sanity is an essential element of mens rea, Washington State v. Strasburg (110 P. 1020 1910 WA) and Leland v. Oregon (343 US 790 1952 OR) interpreted and refined the M’Naghten Rule under varying circumstances. Then in 1954, a Court of Appeals in Washington, D.C., presided over by Judge David Bazelon, heard the Durham case and transformed the insanity landscape once again.

The Durham v. United States (214 F.2d 862) decision in 1954 widened the scope of psychological assessment of defendants and opened the door for an avalanche of expert psychiatric testimony. In the Durham case, the court ruled that a person could not be held criminally responsible if his act was the “product of a mental disease or defect” (Rowe, 1984, p. 319). The courts now had new, ambiguous words to deal with: “product” and “mental defect.” Later, the phrase “diminished capacity” also began to enter into the nations’ courts. The idea that a person who commits a criminal act was acting under a lesser capacity of understanding than was acceptable took hold.

The American Law Institute (A.L.I.) established a Model Penal Code in 1970 that was  adopted by a number of states, eager to resolve the conflicts brought on by increasing frequency of the insanity defense. The A.L.I. model stated that a defendant is not responsible for a criminal offense if “at the time of such conduct as a result of a mental disease or defect, he lacks substantial capacity either to appreciate the criminality of his conduct or to conform his conduct to the requirements of the law.”  It seemed not to matter that as hard as the courts tried to define the limit and scope of a defense based on mental state of a defendant at the time of offense, there was always a problem with its application. By 1972, however, the Durham test, as an effective standard for the nation’s courtrooms, was over. It was simply too broad and gave the psychiatric profession too much power in criminal court. But no single case caused the Durham experiment to fail. Rather, it was “the accumulation of eighteen years of problems and patchwork solutions and, above all, to simple exasperation” (Maeder, 1985, p. 95). However, the newer guidelines provided the foundation for a wide array of ingenious, and some say frivolous, defense strategies. One of the most famous was the “Twinkie Defense” of the late 1970s.