The Definition of Insanity: Legal Terms Referenced in the Dahmer Case

Kindly contributed by retired Criminal Investigator, Criminology professor, and True Crime author Steve Giannangelo ✍️

Below are some key terms, concepts and arguments within the insanity defense, as presented by Jeffrey Dahmer and his counsel, and the prosecutors’ opposition. 

Affirmative Defense

A defense, as in most cases after the Hinckley verdict, where the defense has to prove the accused is insane rather than the prosecution proving he is not.

DSM

The Diagnostic & Statistical Manual (DSM) is the catalog of diagnostic criteria and guides for the classification and categorization of mental disorders, primarily used in the United States¹.

It’s produced by the American Psychiatric Association and is commonly referred to by clinicians in a court setting. The revised third edition (DSM-III-R), published in 1987, was used in the Dahmer trial.

Expert witnesses (such as psychiatrists Dr. Fred Berlin and Dr. Park Dietz) referred to mental conditions such as borderline personality disorder or paraphilias, terms found in the DSM. Several of the psychiatrists who testified had been part of the subcommittee responsible for defining those paraphilias.

Insanity

A legal, not medical, term.

The definition of insanity is what a specific jurisdiction (state/federal) says it is, and it is ultimately determined by jurors (or a judge in a bench trial), usually persuaded by expert witnesses on both sides of the case. 

Insanity Defense Standard

The standard for a finding of insanity in a specific jurisdiction refers to the defendant’s mental capacity and how that affects his/her ability to appreciate the criminality of their conduct and to conform to lawful conduct.

These legal standards that determine insanity in most states usually run between the strict M’Naughten Standard of just knowing right from wrong, or a more defense-friendly standard that also considers if a defendant can conform his behavior despite compulsions, delusions or other factors. This is often referred to as an ALI (American Law Institute) or MPC (Model Penal Code) standard.

U.S. federal and many state standards became much more rigid as a result of the Hinckley case in 1984, but most states have one version or the other, or a combination, to determine insanity.

Four U.S. States² have no insanity defense option at all. 

Irresistible Impulse

A concept formulated through criticism of the M’Naughten standard and borne from the belief that an offender could know the difference between right and wrong, but still be incapable of conforming his actions to the requirements of law.

This concept strikes at the heart of the Dahmer case, as his attorneys and their hired experts argued that he suffered from mental disease and compulsions to commit murders which he could not resist. 

Dahmer defense witness Dr. Carl Wahlstrom diagnosed Dahmer with “severely disabling mental disease,” (including borderline personality disorder) and referred to him experiencing delusions and psychosis, evidenced by the desire to turn a human being into a zombie. However, while delusions and psychosis often come up in insanity cases, Dahmer’s defense team focused more on the irresistible impulse of his compulsion to commit murder in order to satisfy his necrophilic fantasies.

M’Naughten Standard

In place for over 100 years in England and the U.S, this is the most common form of insanity standard and requires that the defendant simply knew the difference between right and wrong in order to have the requisite “culpable mind,” to be found legally responsible for a crime.

Wisconsin Requirements (specifically in the Dahmer case)

For a defendant to be eligible to be found Not Guilty by Reason of Insanity (NGRI), the jury, by a majority vote, must determine he:

1. Had a mental disease at the time of the offense

AND

2. Was unable to conform his actions to the requirements of the law

These were the points that defense attorney Boyle would need to prove in his affirmative defense, and that prosecutor McCann would counter.

Wisconsin also used a description of a “mental disease” which included (in Boyle’s words) a “mental condition, the results of which was an impairment of the mind, either transitory or enduring, that affected his mental or emotional processes.” 

Seasoned prosecution expert Dr. Park Dietz (who’d testified that John Hinckley was sane when he’d attempted to assassinate Ronald Reegan) deftly dodged repeated attempts by Dahmer’s Lead Attorney Gerald Boyle to get Dietz to confirm the first question: that Dahmer did, in fact, have a mental disease. Dietz instead hammered away at listing all the clear evidence of Dahmer’s awareness that he was committing crimes and that it was wrong, as well as his attempts to evade detection and arrest.

Dietz did admit that one could consider that Dahmer had a mental disease, but rebutted Boyle’s inquiry about “promoting an insanity defense for necrophilia,” with the point that “usually, no one would suggest that it’s insanity” – a point with which Boyle predictably disagreed. 

Dietz stated that at the time of the homicides:

“[The] paraphilic aspects of Mr. Dahmer do not fulfill, in my opinion, the Wisconsin criteria for a mental disease, and even if they did, that he nonetheless appreciated the wrongfulness of his actions and could conform his conduct to the requirements of the law, regardless of whether one thinks this is a mental disease.” 

Responding to a direct question on mental disease (impairment of the mind) per Wisconsin definition, Dietz directly rejected that paraphilia, personality traits or disorders “substantially affect mental or emotional processes.” Although he did allow that alcohol dependence may substantially affect emotional processes.

Dietz later admitted, “I’ve tried to avoid giving an opinion in this court about whether he suffered from a mental disease and instead address the specific language of the Wisconsin jury instructions about what it would mean for someone to have a mental disease.”

Dietz also specifically rejected that Dahmer was driven by a compulsion he could not overcome, claiming that “if he [Dahmer] had a compulsion to kill, he would not have to drink alcohol.”

He also disagreed with the idea that a person suffering with a disorder such as a paraphilia was unable to choose whether or not he would act on such desires, including murder.  

Regarding the Wisconsin requirements, in an interview, Dahmer Attorney Wendy Patrickus told me that in the beginning, “we had to get over the first hurdle (the question of whether he had a mental disease) and that we knew that this was the one that was going to be the toughest. And it certainly was.”

I found this surprising, as it seemed the question of whether Dahmer was suffering from an “impairment of the mind that affected his mental or emotional processes” was the easy part, not withstanding Dietz’ obvious motivation and gymnastics to dodge that point as someone hired for the prosecution. I assumed the tough part was combatting the point that, mental disease or not, Dahmer knew the difference between right and wrong and was not overcome by overwhelming compulsions. Ms. Patrickus felt otherwise and was proven correct. Her opinion was that they (the jury) “weren’t taking any chances…. They didn’t want to take the chance that one of their sons could have been victimized by him.” 

In a later interview conducted with defense expert witness Dr. Fred Berlin, a prevailing question I had was about the jury’s decision that Dahmer did not have a mental disease. I simply asked him, “How does that happen?” He responded that it seems to him as a form of “jury nullification” in that “in any other setting, it’s inconceivable to me that anyone wouldn’t have decided that he didn’t have some sort of mental disease or defect” and that they just “couldn’t get past the emotions at all.” 

In the end, the jury rejected Question 1 (Did Jeffrey Dahmer Have a Mental Disease?) which rendered Question 2, and all the expected arguments about Dahmer’s compulsions, moot.

These terms and comments scratch only the surface of the insanity defense topic, but this summary should enable most non-attorney readers to understand their application when used in the context of the Dahmer trial.

 – Stephen J. Giannangelo (Adjunct Professor, Psychology of the Offender & Serial Murder; Author, Irresistible Impulses: Defending the Insanity of Serial Murder (2025))

Footnotes:

1. Europe and the rest of the world primarily use the International Classification of Diseases (ICD), published by the World Health Organization (WHO) ↩︎
2. Idaho, Kansas, Montana, and Utah ↩︎