Reducing Aggression Through Non-Invasive Brain Stimulation
Reducing Aggression Through Non-Invasive Brain Stimulation
This study revealed that anodal compared with sham tDCS applied to the right dorsolateral prefrontal cortex reduced proactive aggression in men.
Exploring gender differences in our sample, we demonstrated that men reported more aggressive tendencies than women did. They also behaved more aggressively compared with women. A vast body of literature is in line with this finding. It has repeatedly been suggested that men display more physical aggression than women, who in turn tend to revert to more indirect forms of aggression (Eagly and Steffen, 1986; Lagerspetz et al., 1988; Bjorkqvist, 1994; Archer, 2004). Several biological factors such as testosterone levels contribute to this phenomenon (Book et al., 2001; Mehta and Beer, 2010). The TAP, in which the actual aggressive act is to assign to the opponent a noise feedback evoking a rather unpleasant and almost painful auditory experience, can be understood as a measure of physical aggression. It is therefore to be expected that—due to its characteristics—the TAP is well suited to generate aggression in men.
There was no relationship between total and reactive behavioral and self-reported aggression scores. It is a long and well-known problem in aggression research that behavioral measures of social constructs do not necessarily overlap with measures on a self-reported level (Scheier et al., 1978). Especially in this domain, effects of social desirability are obstacles that measurement tools have to overcome (Vigil-Colet et al., 2012). Our self-reported data were probably likewise affected as we measured exclusively university students, a sample for which it might be very difficult to admit aggressive tendencies.
We found a positive relationship between behavioral and self-reported aggression in the proactive domain for the overall sample and for men. This might hint toward the fact that conceptually, the proactive aspect of the TAP overlaps more precisely with the proactive sub-scale of the RPQ than the reactive aspect of the TAP with the reactive sub-scale of the RPQ. Biases regarding self-reported aggression might be more relevant for reactive than for proactive aggression. In our societies, it is emphasized that everyone should react rationally to provocation. Proactive aggression might be less frequent and more exceptional and, thus, less prone to social biases. The data collected in this study could give a hint in this direction. However, more empirical evidence needs to be collected in larger samples to substantiate this claim.
In line with our hypothesis, we found that the induction of right-hemispheric neural activation dominance reduced aggressive behavior compared with sham brain stimulation, although the effect was only significant in men.
Proactive aggression refers to the instrumental use of aggression to obtain a reward or a prey (Anderson and Bushman, 2002). Therefore, the motivation to approach seems central. The experimental manipulation in this study was meant to enhance activity in the right dorsolateral prefrontal cortex. This area is said to be responsible for emotional and cognitive processes generating avoidance motivation (Harmon-Jones, 2004; Carver and Harmon-Jones, 2009). The assumption that the applied brain stimulation protocol enhanced avoidance and thus lowered approach motivation fits with our finding that it reduced proactive aggression.
The current findings can also be explained in the light of social information-processing theories. It has been shown that reactive and proactive aggression revert to biases in different stages of social information processing. Thereby, reactive aggression seems to result from deviations in rather early stages, such as an increased attentional bias for angry faces or a hostile interpretation bias (Anderson and Bushman, 2002; Lobbestael et al., 2013; Brugman et al., 2014). For proactive aggression, the later stages seem more impaired and lead to a more positive evaluation of aggressive action options (Walters, 2007). A proactive attitude likely also steers coping processes, meaning that proactively aggressive individuals have the tendency to approach their goals using aggression. The evaluation of the option to act proactively aggressive is more closely related to approach motivation than to attention and interpretation biases. It seems likely that an alteration of such motivational states (on a neural level) influences proactive rather than reactive aggression.
With this study, we demonstrated that it is possible to specifically manipulate proactive aggression. Usually, this form of aggression is more difficult to deal with in clinical contexts; proactive aggression is potentially very dangerous as it is a planned behavior and not emotionally driven. It is often prevalent in patients with psychopathic traits. So far, neuroscience and especially neuroimaging research mostly neglected the differentiation between proactive and reactive aggression. In the light of the current results, it seems promising to consider the difference in further neuroscientific research on aggression. This could lead to more elaborate theories on which specific neural mechanisms underlie proactive aggression compared with reactive aggression and how these mechanisms can be manipulated in order to ultimately change behavior.
This study demonstrates that tDCS can reduce aggressive behavior. Our findings still have to be considered in light of the limitations that the current experimental setting was accompanied by. Our sample (N = 32) was restricted to university students. The field would profit from investigating larger samples and more heterogeneous populations. The lack of a stimulation effect in women might be caused by a floor effect considering that female students in our restricted sample displayed low aggression levels. Applying tDCS in the context of aggression to larger and, further, more variable female sample might lead to a clearer picture on whether aggressive behavior can or cannot be reduced in women compared with men. Furthermore, research should also zoom in on larger male samples enabling the inclusion of more control variables (such as, e.g. perception of the opponent, perception of feedback, influence of brain stimulation side effects) and different brain stimulation conditions.
To further examine the specificity of the present effects, other stimulation parameters, such as bilateral stimulation setups and frequency-dependent protocols, might be of use in further investigating the effects of cortical asymmetry on aggression. Future experiments should consider including different stimulation sides within the prefrontal cortex (based on imaging literature) in order to investigate if lateralization effects are bound to the right dorsolateral prefrontal cortex.
We demonstrated the effects of brain stimulation on aggression measured by the Taylor paradigm. The question of whether the findings are generalizable and specific to aggression remains to be answered. It is especially interesting to assess to what degree the very same mechanism plays a role in both aggressive and prosocial approach.
Discussion
This study revealed that anodal compared with sham tDCS applied to the right dorsolateral prefrontal cortex reduced proactive aggression in men.
Gender Differences
Exploring gender differences in our sample, we demonstrated that men reported more aggressive tendencies than women did. They also behaved more aggressively compared with women. A vast body of literature is in line with this finding. It has repeatedly been suggested that men display more physical aggression than women, who in turn tend to revert to more indirect forms of aggression (Eagly and Steffen, 1986; Lagerspetz et al., 1988; Bjorkqvist, 1994; Archer, 2004). Several biological factors such as testosterone levels contribute to this phenomenon (Book et al., 2001; Mehta and Beer, 2010). The TAP, in which the actual aggressive act is to assign to the opponent a noise feedback evoking a rather unpleasant and almost painful auditory experience, can be understood as a measure of physical aggression. It is therefore to be expected that—due to its characteristics—the TAP is well suited to generate aggression in men.
Relationship Between Behavioral and Self-reported Aggression
There was no relationship between total and reactive behavioral and self-reported aggression scores. It is a long and well-known problem in aggression research that behavioral measures of social constructs do not necessarily overlap with measures on a self-reported level (Scheier et al., 1978). Especially in this domain, effects of social desirability are obstacles that measurement tools have to overcome (Vigil-Colet et al., 2012). Our self-reported data were probably likewise affected as we measured exclusively university students, a sample for which it might be very difficult to admit aggressive tendencies.
We found a positive relationship between behavioral and self-reported aggression in the proactive domain for the overall sample and for men. This might hint toward the fact that conceptually, the proactive aspect of the TAP overlaps more precisely with the proactive sub-scale of the RPQ than the reactive aspect of the TAP with the reactive sub-scale of the RPQ. Biases regarding self-reported aggression might be more relevant for reactive than for proactive aggression. In our societies, it is emphasized that everyone should react rationally to provocation. Proactive aggression might be less frequent and more exceptional and, thus, less prone to social biases. The data collected in this study could give a hint in this direction. However, more empirical evidence needs to be collected in larger samples to substantiate this claim.
Effects of Brain Stimulation
In line with our hypothesis, we found that the induction of right-hemispheric neural activation dominance reduced aggressive behavior compared with sham brain stimulation, although the effect was only significant in men.
Dissociation of Proactive vs Reactive Aggression
Proactive aggression refers to the instrumental use of aggression to obtain a reward or a prey (Anderson and Bushman, 2002). Therefore, the motivation to approach seems central. The experimental manipulation in this study was meant to enhance activity in the right dorsolateral prefrontal cortex. This area is said to be responsible for emotional and cognitive processes generating avoidance motivation (Harmon-Jones, 2004; Carver and Harmon-Jones, 2009). The assumption that the applied brain stimulation protocol enhanced avoidance and thus lowered approach motivation fits with our finding that it reduced proactive aggression.
The current findings can also be explained in the light of social information-processing theories. It has been shown that reactive and proactive aggression revert to biases in different stages of social information processing. Thereby, reactive aggression seems to result from deviations in rather early stages, such as an increased attentional bias for angry faces or a hostile interpretation bias (Anderson and Bushman, 2002; Lobbestael et al., 2013; Brugman et al., 2014). For proactive aggression, the later stages seem more impaired and lead to a more positive evaluation of aggressive action options (Walters, 2007). A proactive attitude likely also steers coping processes, meaning that proactively aggressive individuals have the tendency to approach their goals using aggression. The evaluation of the option to act proactively aggressive is more closely related to approach motivation than to attention and interpretation biases. It seems likely that an alteration of such motivational states (on a neural level) influences proactive rather than reactive aggression.
With this study, we demonstrated that it is possible to specifically manipulate proactive aggression. Usually, this form of aggression is more difficult to deal with in clinical contexts; proactive aggression is potentially very dangerous as it is a planned behavior and not emotionally driven. It is often prevalent in patients with psychopathic traits. So far, neuroscience and especially neuroimaging research mostly neglected the differentiation between proactive and reactive aggression. In the light of the current results, it seems promising to consider the difference in further neuroscientific research on aggression. This could lead to more elaborate theories on which specific neural mechanisms underlie proactive aggression compared with reactive aggression and how these mechanisms can be manipulated in order to ultimately change behavior.
Limitations and Outlook
This study demonstrates that tDCS can reduce aggressive behavior. Our findings still have to be considered in light of the limitations that the current experimental setting was accompanied by. Our sample (N = 32) was restricted to university students. The field would profit from investigating larger samples and more heterogeneous populations. The lack of a stimulation effect in women might be caused by a floor effect considering that female students in our restricted sample displayed low aggression levels. Applying tDCS in the context of aggression to larger and, further, more variable female sample might lead to a clearer picture on whether aggressive behavior can or cannot be reduced in women compared with men. Furthermore, research should also zoom in on larger male samples enabling the inclusion of more control variables (such as, e.g. perception of the opponent, perception of feedback, influence of brain stimulation side effects) and different brain stimulation conditions.
To further examine the specificity of the present effects, other stimulation parameters, such as bilateral stimulation setups and frequency-dependent protocols, might be of use in further investigating the effects of cortical asymmetry on aggression. Future experiments should consider including different stimulation sides within the prefrontal cortex (based on imaging literature) in order to investigate if lateralization effects are bound to the right dorsolateral prefrontal cortex.
We demonstrated the effects of brain stimulation on aggression measured by the Taylor paradigm. The question of whether the findings are generalizable and specific to aggression remains to be answered. It is especially interesting to assess to what degree the very same mechanism plays a role in both aggressive and prosocial approach.
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