Structural covariance of amygdala subregions is associated with trait aggression and endogenous testosterone in healthy individuals

Many studies point toward volume reductions in the amygdala as a potential neurostructural marker for trait aggression. However, most of these findings stem from clinical samples, rendering unclear whether the findings generalize to non-clinical populations. Furthermore, the notion of neural networks suggests that interregional correlations in grey matter volume (i.e., structural covariance) can explain individual differences in aggressive behavior beyond local univariate associations. Here, we tested whether structural covariance between amygdala subregions and the rest of the brain is associated with self-reported aggression in a large sample of healthy young students (n=263; 51% women). Salivary testosterone concentrations were measured for a subset of n=76 participants (45% women), allowing us to investigate the influence of endogenous testosterone on structural covariance. Aggressive individuals showed enhanced covariance between superficial amygdala (SFA) and dorsal anterior insula (dAI), but lower covariance between laterobasal amygdala (LBA) and dorsolateral prefrontal cortex (dlPFC). These structural patterns overlap with functional networks involved in the genesis and regulation of aggressive behavior, respectively. With increasing endogenous testosterone, we observed stronger structural covariance between centromedial amygdala (CMA) and medial prefrontal cortex in men and between CMA and orbitofrontal cortex in women. These results speak for structural covariance of amygdala subregions as a robust correlate of trait aggression in healthy individuals. Moreover, regions that showed structural covariance with the amygdala modulated by either testosterone or aggression did not overlap, speaking for a more complex role of testosterone in human social behavior rather than the simple assumption that testosterone only increases aggressiveness.