Aggression: impulsive or instrumental

We recently published a study on the association between aggression and the wiring of the brain. Aggression can be defined as behavior that intentionally causes harm to others, objects, or oneself. It comes in various forms and can be expressed physically and/or verbally. Moreover, other – more inhibited – varieties of aggression also exist, such as hostile thoughts like distrusting overly friendly people, or being convinced your friends talk about you behind your back. In humans, two subtypes of aggression are typically distinguished: 1) reactive-impulsive aggression, which occurs in response to provocation and gives rise to uncontrollable and inappropriate actions, and 2) instrumental aggression, which is a premeditated form of aggression to achieve a certain goal.

Brain connections and testosterone

Extreme forms of aggression pose a threat to both individuals and society. High levels of aggression have been linked to conduct disorder, oppositional defiant disorder and antisocial personality disorder. Although social factors play an eminent role, scientific evidence also suggests that an imbalance between brain systems may predispose people to aggressive behavior. Studies using functional MRI in adults have shown that aggression is associated with high activity in motor and subcortical brain areas dedicated to motivation, together with low regulatory capacities of the prefrontal cortex. These findings suggest that people who are unable to control aggressive urges have a lower quality of connections between subcortical and cortical brain areas.

In our study we tested this assumption by examining the association between aggressive personality, brain connections, and testosterone in a large group of participants across adolescence. We applied a relatively new brain imaging technique called Diffusion Tensor Imaging (DTI), which enabled us to reconstruct white matter connections between subcortical and cortical areas in the brain. Subsequently, we measured the ‘integrity’ along the connections. Aggressive personality was measured using the Buss-Perry Aggression Questionnaire.

‘Hypo-connected’ brain

We found that adolescents who scored high on reactive forms of aggression had relatively low ‘quality’ of white matter connections between subcortical and prefrontal brain areas. The opposite was true for adolescents who scored high on the hostility subscale: they showed a higher quality of white matter connections. In other words, we can distinguish here between people with a ‘hypo-connected’ brain (who tend to act on aggressive urges) versus people with a ‘hyper-connected’ brain (who have a better ability to inhibit aggressive urges).

Higher testosterone - lower hostility

We also investigated the role of testosterone – as this hormone has been associated with aggression. We indeed found that higher testosterone was related to higher physical aggression, and to a lower quality of white matter connections. To our surprise, however, higher testosterone was also related to lower hostility (i.e., aggressive thoughts). In seeking to explain this result, we came across studies that demonstrated a high correlation between hostility and anxiety (as hostility is characterized by a cynical attitude, feelings of resentment, and distrust of others). Interestingly, our finding of higher testosterone relating to lower hostility further supports the idea that testosterone can reduce symptoms of anxiety.

Thus, if given enough provocation, most of us probably feel like hitting another person. Our study shows that the ability to suppress these aggressive urges is related to a higher quality of brain connections and a lower level of the hormone testosterone.

Is testosterone again the usual suspect when it comes to aggression? On the contrary, we could argue that – to a certain extent – testosterone helps people to let off steam: a friendly punch on the nose could free you from feelings of resentment and jealousy.