In response to criticisms of the M'Naghten Rule, some legal commentators began to suggest expanding the definition of insanity to include more than a cognitive element. Such a test would encompass not only whether defendants know right from wrong but also whether they could control their impulses to commit wrong-doing. The Irresistible Impulse Test was first adopted by the Alabama Supreme Court in the 1887 case of Parsons v. State. The Alabama court stated that even though the defendant could tell right from wrong, he was subject to "the duress of such mental disease [that] he had ... lost the power to choose between right and wrong" and that "his free agency was at the time destroyed," and thus, "the alleged crime was so connected with such mental disease, in the relation of cause and effect, as to have been the product of it solely." In so finding, the court assigned responsibility for the crime to the mental illness despite the defendant's ability to distinguish right from wrong.

The Irresistible Impulse Test gained acceptance in various states as an appendage to the M'Naghten Rule, under which right versus wrong was still considered a vital part of any definition of insanity. In some cases, the Irresistible Impulse Test was considered to be a variation on M'Naghten; in others, it was considered to be a separate test. Though the Irresistible Impulse Test was considered to be an important corrective on M'Naghten's cognitive bias, it still came under some criticism of its own. For example, it seemed to make the definition of insanity too broad, failing to take into account the impossibility of determining which acts were uncontrollable rather than merely uncontrolled, and also making it easier to fake insanity. The test was also criticized for being too narrow; like M'Naghten, the test seemed to exclude all but those totally unable to control their actions. Nevertheless, several states currently use this test along with the M'Naghten Rule to determine insanity, and the American Law Institute in its Model Penal Code definition of insanity adopted a modified version of it.