‘Cognitive disorders’ is a broad and heterogeneous diagnostic category, which includes different disorders, each with a distinct aetiology. They affect individuals in different ways depending on the age in which they occur. The term may be applied to a child, who has experienced perinatal trauma as well as to an older person with a beginning dementia of the Alzheimer type. The scientific literature on offenders with cognitive disorders is sparse. Most authors in forensic psychiatry do not systematically differentiate between the diagnostic subcategories and tend to use broad terms, such as organic disorder, organic psychosis, organic brain syndrome, neuropsychological deficit, dementia, mental handicap, mental retardation to include a number of different disorders in their studies. The number of patients with any kind of brain disorder in forensic hospitals and institutions is comparatively small and ranges from 1 to 10 per cent of all forensic inpatients. The same numbers apply for individuals assessed for criminal responsibility or risk of reoffending.^(1,2,3) Compared to major mental disorders like schizophrenia or affective disorders or to personality disorders, patients with cognitive disorders account for only a small proportion of individuals seen by forensic psychiatrists. Subdividing this group any further would be statistically irrelevant. The way forensic psychiatry and the law deals with offenders suffering from organic brain disorders is rather derived from case reports and convention than from empirical knowledge.

DSM-IV-TR cites several disorders where aggression is either a diagnostic or associated feature and among them are four with an organic aetiology.

The psychiatric and general medical literature lists several other organic brain disorders that are either believed to be or in fact are associated with violence and offending, although their link is not as well proven.⁽⁴⁾

From the experience of the author two other disorders should be added to this list:

Systematic analyses of epidemiological data and of other research findings show that patients with clinically relevant brain damage do not commit violent crimes more often than would be expected according to their proportion in the general population.^(5,6,7) These findings do not contradict the knowledge we have about aggressive and disruptive behaviour of certain patients with brain damage. The estimates of the frequency of such behaviours range from 18 per cent in demented patients to 60 per cent in patients with frontal lobe injuries.⁽⁸⁾ Most of these patients are not seen by forensic psychiatrists, but are treated in special institutions or in outpatient settings. Apparently, the violent behaviour of patients with brain damage does not lead to interventions by the criminal justice system as often as could be expected from the above mentioned numbers. Similar findings are reported from demented patients: although not appearing in criminal court files, aggression and agitation of demented patients is a major problem in nursing homes and for caregivers of the elderly in outpatient settings. Again, exact definitions and robust data on how much violence really occurs are lacking, but estimates range from 18 to 48 per cent.⁽⁸⁾ Rabins et al.⁽⁹⁾ reported that 75 per cent of caregivers considered aggression as the most serious problem in agitated demented patients.

Offending and contact with the criminal justice system can be expected to be more frequent in patients who suffer less from cognitive impairments—which would prevent skilled or planned criminal activity—but rather from personality changes, like irritability, impulsivity, lack of concern for others, and for the consequences of one’s action which is the case in frontotemporal dementia. Offending, but rarely violent offending, occurs sometimes as a first sign of this disorder.⁽¹⁰⁾ Violent crimes are sometimes associated with cognitive disorders when delusions are among the first symptoms of a beginning dementia. Especially delusions of jealousy, envy, or revenge are prone to result in violent acts, which may leave partners or neighbours as victims. These crimes contradict the previous occupational and social life of the perpetrators and are paradigmatic examples of offending as a result of a mental disorder, leading to inculpability of the patient.

Offending can also be expected to be more frequent in patients between 18 and 35 years old and therefore in an age, where offending is statistically more frequent than in other age groups. Males of the same age group have the highest rate of traumatic brain injury. They also belong to the age group with the highest rate of criminality and especially of violent criminality. This same age and sex group also has the highest rate of substance abuse. Given the high prevalence of brain injury among young men and their propensity to use alcohol and drugs it is surprising how few are seen by forensic psychiatrists or sentenced to prison. The actual numbers of such patients found in forensic hospitals and in prisons do not reflect the high risk of violent crime by persons with brain injury. Hodgins⁽¹¹⁾ found that only 0.4 per cent of male penitentiary inmates warranted a diagnosis of organic brain syndrome (which is a much broader term than traumatic brain injury). Similarly the proportions of patients with organic brain syndrome in forensic hospitals is below 10 per cent and not greater than that in general psychiatric hospitals.^{(2, 5)}

Several studies suggest that the criminality of individuals with brain injury may, to a large extent, be attributed to premorbid personality traits, to the social disintegration which follows the injury, and hence not only to the injury itself. Kreutzer et al.⁽¹²⁾ studied a sample of 327 patients with varying severities of traumatic brain injury. Those arrested after the brain injury were more likely to have had a history of police contacts before the brain injury, than those who were not arrested.

Two disorders have to be presented in greater detail:

Epilepsy, because it was historically one of the disorders of great concern for forensic psychiatrists and served as a model of the mentally ill offender not responsible for his crimes, and ADHD, because it is one of the disorders for which a relationship to antisocial behaviour and offending is most intensively researched.