The application of non-invasive brain stimulation techniques to reduce anger and violence proneness: Results of a systematic review and meta-analysis

Since the 1990s, there has been a rise in the number of publications assessing the effects of applying non-invasive brain stimulation (NIBS) to treat patients with drug-resistant depression. This involves applying magnetic fields or electrical currents to the surface of the skull to influence the superficial neurons in the cerebral cortex. Due to the evidence regarding symptom reduction in these types of patients, such as irritability or hostility, there was a rise in the use of this technique to reduce negative mood, including anger state. This decrease in anger state could also help reduce other problems such as violence proneness. In this sense, the anger state of individuals who are prone to violence might be affected by interfering with the excitability of the prefrontal cortex (PFC), a key brain region responsible for behavioral regulation. Thus, we conducted a systematic review and meta-analysis following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) criteria. After initially identifying 2426 sources, we eventually included a total of 69 publications for the systematic review, from which 45 were employed for the meta-analysis. Only a few of them highlighted a significant contribution of using NIBS techniques on different regions of the PFC to reduce anger state or violence when compared to participants receiving sham stimulation in normative and clinical samples. Furthermore, the comparison of effect sizes between groups that received real stimulation on several regions of the PFC and those that received sham stimulation did not reveal a significant difference in reducing anger state or violence. In addition, despite most of the conclusions being consistent, considerable heterogeneity existed across studies regarding certain PFC regions, which could be explained by the type of NIBS employed. Therefore, using superficial stimulation over the PFC as a general tool for reducing violence proneness should be approached with caution, except in specific cases.