The Durham product rule, which was adopted in 1954 in New Hampshire, is still used there as the official standard for determining criminal insanity. Under the Durham rule, the defendant can be acquitted of a crime if it can be proven that he was suffering from a mental illness at the time of committing the crime. This rule is sometimes known as “the products test” because the jury can decide whether the defendant’s actions were the product of his mental illness.

In contrast to the M’Naghten rule, the Durham rule is more general and can be used in a wider variety of cases because it defines insanity not just as a delusion or total loss of understanding, but as a product of many personality factors. Under the Durham rule, the defense can present the jury with different types of evidence in an attempt to prove insanity, including past history of mental illness, presence of imaginary physical pains, and even IQ score.

But the Durham rule has been strongly criticized precisely for allowing such a general definition of insanity. According to its critics, this standard is so vague and unclear that even alcoholics and compulsive gamblers could use this defense and escape punishment. Finally, the Durham rule does not provide the jury with a clear definition of insanity on which it can base its verdict, instead relying on various kinds of expert witnesses, such as psychologists and social workers, which often results in confusion for the jury.