Violence in severe mental illness: is cognition missing in the associations with ethnicity,cannabis and alcohol?

The article by Dharmawardene and Menkes ¹ supports that cannabis use in an inpatient psychiatric sample is a significant predictor of lifetime violence among the severely mentally ill. Although this finding is important for clinicians as well as the researchers, an important point in the analyses of this research is missing. Did the adult psychiatric inpatients who were also cannabis users demonstrate more deficits in their neuropsychological profile than the psychiatric inpatients did who scored lower on substance use questionnaires?

Cognitive deficits have long been considered a core component of psychotic disorders. In this line, it has been supported that aggression levels differentiate primarily due to the varying levels of deficits in cognition, as researchers have found that cognitive tasks are valuable in predicting aggression and its management. ² Specifically, verbal aggression has been found to be predicted by deficits in attention in the form of increased attention towards threat and aggression, and perceptual difficulties in recognizing sad and happy faces, while physical aggression is influenced by decreased response inhibition. ² In addition to that, aggression can be predicted by well-known and widely used neuropsychological tests ³ assessing general cognitive functioning (e.g. the Wechsler Adult Intelligence Scale and the Mini Mental State Examination) and other test scores assessing more specific cognitive functions, such as attention (e.g. Trail Making Test A and Trail Making Test B), which are all found to be in significant correlation with risk factors for aggressive behavior. ⁴

Thus, the relationship of violence and substance abuse must be examined as a complex relationship, moderated by a plethora of factors in the individual and the environment. Cannabis users may be at increased risk of interpersonal violence, but it is still not clear how and to what extent the association is causal. For this reason, the assessment of the association between cannabis use and violence must be done with the use of methodological and statistical approaches that diminish the risk of confounding, following the paradigm of Norström and Rossow. ⁵

Consequently, several questions remain. Do psychiatric inpatients who are also cannabis users (or with heavy substance dependence) have a poorer cognitive performance or not when compared to psychiatric inpatients who are not substance users (or with milder substance dependence)? Does their cognitive performance or the fact that they belong to the category of users/non-users predict better violence? Answers to the above questions may clarify if affected patients should receive specific treatment for substance use co-morbidity or if cognitive rehabilitation should be a priority.