Settled Insanity: Conundrums of Forensic Psychology

By Dr. David Dixon, Forensic Psychologist

A common diagnostic challenge for the forensic psychologist is a crime committed by a mentally ill defendant affected by psychoactive drugs at the time of the offense. It is often very difficult to differentiate whether the psychosis diagnosed was caused by the primary mental illness or drug toxicity. Drug toxicity due to voluntary intoxication is not a mental defense in most states. Voluntary intoxication is not considered grounds for excusing the culpability of a criminal act.

 However, if the intoxication exacerbated an existing mental disorder, a mental defense may be allowed. This contribution of the substance may alter the legal outcome of the case.

Clinical definition and the psychological-legal analysis often causes confusion for a judge and jury.

 Does the ingestion of a psychoactive chemical cause intoxication and/or psychosis? The focus becomes the origin of the mental disorder at the time of the crime charged. Did the voluntary use lead to a “transient” mental state? Was this disorder caused by the substance induced intoxication, or was it a permanent and fixed mental condition?

Further salient discussion needs to explore if the condition was chronic, but not clearly observable? Was this “sub-clinical” mental disorder ever the attention of a clinical diagnosis and treatment? Was it an independent mental disease? Did the toxic drug “open the door” to the mental disorder manifesting itself and making it more observable?



These questions are all more easily answered if a psychiatric disorder has previously manifested in an individual’s history, and was not associated with use/abuse of a chemical.

Various mental states have different criteria which allow for a defense of settled insanity. According to the DSM IV, “A substance induced psychotic disorder requires a psychoactive substance to be the cause of the psychotic condition.” – and the disorder will reverse itself within a timeframe when the substance is eliminated. However, complicating this seemingly simple concept, is the fact that changes in neurotransmitters and brain activity have been shown to persist long after abstinence (defined as “chronic toxicity”) There may not be such a clear dichotomous choice in a mental state either being pre-existing and underlying, or purely induced by toxicity. 

It is important to understand the role of major psychoactive drugs, and the prevalence that these drugs cause psychotic symptoms with use/abuse.

A study by Thithalli and Bengal (2006)reports the following percentage of psychosis associated with use/abuse of psychoactive drugs:
Hallucinogens 83%, PCP 82, Cocaine 80%, Cannabis 64%, Amphetamines 56%, Opioids 54%, Alcohol 41%, and Sedatives 32%.
These percentages seem to increase with prolonged use and dependence. Effects of psychosis appeasr dose related.

 Computer tomography has examined brain changes in neurotransmitters and cerebral blood flow. Brain neuro-imaging demonstrates the disturbance of the dopaminergic system. 
Other concerns have arisen when the origins of acute psychosis, or amnestic ‘blackout” episodes, or psychotic depression are from failure to follow a treatment plan. With an abrupt discontinuation of a psychotropic treatment medication can severely exacerbated symptoms. Withdrawal may not be volitional, or even anticipated.



The bottom line, from a legal perspective, is what role did a mental condition play? Would it have “affected the defendant’s ability to understand the nature and quality of his acts.”? The assigning of causation of the aberrant and criminal behavior is crucial. Was it from a mental disorder or a mind altering drug voluntarily ingested? The answer to this question may at times boil down to the believability of the expert’s opinion than on any real science. The complexity of all the interactions are often difficult to understand.