Small amygdala - high aggression? The role of the amygdala in modulating aggression in healthy subjects

Objective. Several lines of evidence suggest an association between the amygdala and the modulation of aggressive behaviour. Previous morphometric brain imaging studies have focused on the role of the amygdala in the context of pathologic neuropsychiatric conditions like depression, personality disorders, and dysphoric and aggressive behaviour in epilepsy. In order to better understand the physiological role of the amygdala in modulating aggressive behaviour we investigated the relationship between amygdala volumes and lifetime aggression in healthy subjects. Methods. Morphometric brain scans were obtained in 20 healthy volunteers. Amygdala volumes were measured by manually outlining the boundaries of the structure following a well established and validated protocol. Careful psychiatric and psychometric assessment was done to exclude any psychiatric disorder and to assess lifetime aggressiveness with an established and validated psychometric instrument (i.e. Life History of Aggression Assessment (LHA)). Results. All volunteers scored in the normal range of lifetime aggression. Volunteers with higher aggression scores displayed a 16-18% reduction of amygdala volumes. There was a highly significant negative correlation between amygdala volumes and trait aggression. Conclusion. The extent of volumetric differences in this study is remarkable and suggests that amygdala volumes might be a surrogate marker for the personality property of aggressiveness in healthy human beings.