Mental illness has long occupied a difficult position within American criminal law. While criminal liability generally requires both a prohibited act (actus reus) and a culpable mental state (mens rea), the law recognizes that certain mental disorders may impair an individual's ability to appreciate the nature or wrongfulness of their conduct. The insanity defense exists as a narrow exception to criminal responsibility, but courts have historically applied it cautiously.
One of the most controversial disorders raised in criminal proceedings is Dissociative Identity Disorder (DID), formerly known as Multiple Personality Disorder (MPD). DID is characterized by the presence of two or more distinct identity states that may alternately control an individual's behavior (Bartol & Bartol, 2021). Although the disorder is recognized in the Diagnostic and Statistical Manual of Mental Disorders (DSM-5), its legal significance remains highly contested. Courts have generally required defendants asserting DID to satisfy the same demanding legal standards applicable to all insanity defenses rather than accepting the diagnosis alone as proof of legal insanity.
The prosecution of Kenneth Bianchi, famously known as the Hillside Strangler, illustrates the significant legal obstacles defendants face when attempting to use DID as the basis for an insanity defense. His case demonstrates that criminal courts focus less on the existence of a psychiatric diagnosis and more on whether the defendant can satisfy the jurisdiction's legal standard for insanity through credible expert testimony and objective evidence.
The Facts of State v. Bianchi#
Kenneth Bianchi was charged with two counts of first-degree murder after the strangulation deaths of two young women in Washington. The victims' bodies were later discovered in the Los Angeles area, contributing to the series of murders attributed to the Hillside Strangler (Bartol & Bartol, 2021).
Rather than disputing that the killings occurred, Bianchi asserted that an alternate personality named "Steve" committed the murders. According to his claim, he suffered from Multiple Personality Disorder and therefore should not be held criminally responsible for the acts committed by this alternate identity.
After the State sought the death penalty, Bianchi entered a plea of not guilty by reason of insanity. The court appointed six psychologists to evaluate both his competency to stand trial and the validity of his alleged diagnosis. Following extensive examinations, the experts unanimously concluded that Bianchi had fabricated his symptoms and was intentionally attempting to deceive the evaluators. Rather than diagnosing DID, the psychologists concluded that he exhibited psychopathic traits and was malingering in an effort to avoid criminal punishment. State v. Bianchi, No. 59268-1-II (Wash. Ct. App. Dec. 31, 2024).
Once the insanity defense collapsed, Bianchi abandoned the claim and pleaded guilty pursuant to a plea agreement, thereby avoiding the death penalty while receiving two consecutive life sentences.
DID and the Legal Standard for Insanity#
One of the most common misconceptions surrounding insanity defenses is that the presence of a mental illness automatically excuses criminal conduct. That is not the law. Courts distinguish between medical insanity and legal insanity. A psychiatric diagnosis may be relevant evidence, but it does not itself establish criminal non-responsibility. The defendant must satisfy the applicable legal test adopted by the jurisdiction, which often requires proof that, because of a severe mental disease or defect, the defendant could not appreciate the nature or wrongfulness of the conduct at the time of the offense.
Consequently, defendants asserting DID face two separate evidentiary burdens. First, they must establish that the diagnosis itself is genuine through reliable expert testimony. Second, they must demonstrate that the disorder rendered them legally insane under the governing statute or common-law standard.
The Bianchi case illustrates why this burden is exceptionally difficult to satisfy. Although Bianchi claimed that an alternate personality committed the murders, the forensic psychologists concluded that he had deliberately manufactured the symptoms. Without credible medical evidence supporting the diagnosis, the legal foundation for an insanity defense disappeared.
The Importance of Expert Testimony#
Expert testimony frequently determines whether an insanity defense succeeds or fails. Courts rely heavily on forensic psychologists and psychiatrists to distinguish legitimate mental illness from malingering, particularly when defendants present symptoms that are difficult to verify objectively.
Bianchi's evaluators found several inconsistencies that undermined his credibility. Evidence suggested that he possessed prior knowledge of psychological disorders and attempted to imitate the clinical presentation of Multiple Personality Disorder. His performance during hypnosis and interviews ultimately convinced the experts that he was feigning the disorder rather than experiencing genuine dissociative symptoms (Bartol & Bartol, 2021).
The case underscores an important legal principle: credibility matters. Courts are generally skeptical of mental health defenses that depend primarily on subjective symptoms, particularly when significant forensic evidence independently establishes guilt.
Why the Insanity Defense Failed#
Several factors contributed to the failure of Bianchi's insanity defense.
First, the defense lacked credible psychiatric support. Every court-appointed evaluator rejected the diagnosis of DID.
Second, the prosecution possessed overwhelming physical evidence linking Bianchi to the murders. Investigators recovered semen stains, pubic hair, and clothing fibers connecting him directly to one of the victims and to the crime scenes. This forensic evidence corroborated his earlier confession and significantly weakened any attempt to shift responsibility to an alleged alternate personality.
Third, Bianchi's behavior suggested calculated efforts to manipulate the legal process rather than genuine psychiatric impairment. Courts routinely distinguish between defendants who suffer from legitimate mental illness and those who attempt to exploit psychological diagnoses to avoid criminal responsibility. The unanimous conclusion that Bianchi was malingering severely damaged the credibility of his defense.
Legal Significance#
The enduring importance of State v. Bianchi is not that DID can never support an insanity defense. Rather, the case demonstrates that courts require substantial, credible evidence before permitting a psychiatric diagnosis to negate criminal responsibility.
Modern criminal law does not excuse unlawful conduct simply because a defendant has a recognized mental disorder. Instead, the legal inquiry focuses on whether the disorder deprived the defendant of the cognitive or moral capacity required under the jurisdiction's insanity standard. Even where DID is legitimately diagnosed, courts continue to scrutinize the reliability of expert testimony, the defendant's behavior before and after the offense, and the totality of the forensic evidence.
Conclusion#
State v. Bianchi remains one of the most well-known examples of a failed insanity defense based on Dissociative Identity Disorder. Rather than establishing a precedent that DID cannot support legal insanity, the case reinforces a broader principle of criminal law: psychiatric diagnoses alone do not excuse criminal conduct. The burden remains on the defendant to prove legal insanity through credible expert testimony and persuasive evidence.
When that burden cannot be met, the strategic value of an insanity defense rapidly diminishes. Faced with overwhelming forensic evidence and unanimous expert opinions rejecting his diagnosis, Bianchi ultimately chose to negotiate a plea agreement that spared him the death penalty while ensuring accountability for his crimes. The case continues to illustrate the judiciary's careful balancing of mental health considerations against the fundamental principles of criminal responsibility and public safety.
References#
Bartol, C. R., & Bartol, A. M. (2021). Criminal Behavior: A Psychological Approach (12th ed.). Pearson.
Jordan, L., & Champney, D. (1994). Multiple Personality Disorder and the Legal System.
State v. Bianchi, No. 59268-1-II (Wash. Ct. App. Dec. 31, 2024).