Aggression

Amygdala

The threat detector. Dense in androgen receptors, and the structure most implicated in the anger, vigilance and fear side of what these compounds do. Splitting it out of the limbic system matters because threat processing and reward processing sit in the same blob otherwise, and compounds pull them in opposite directions.

Amygdala Volume

The size of the threat detector on MRI. Long-term users had a larger right amygdala than non-using weightlifters, alongside weaker functional coupling between it and the regions that are supposed to regulate it.

Impulsivity

Acting before the cost has been weighed. Distinct from aggression: aggression is a response to a trigger, impulsivity is the absence of the pause that would have stopped it. The imaging literature points at a weaker brake.

Lateral Anterior Hypothalamus

The convergence point of the aggression circuit and the best-characterised site of a steroid effect on behaviour anywhere in the literature. Adolescent exposure changes vasopressin, dopamine, serotonin and GABA signalling here together, which is why the behaviour change is not undone by acting on any one of them.

Periaqueductal Grey

Where a threat becomes an action: freeze, flee or attack. It receives the amygdala's alarm and executes it. Relevant here because it explains why the reported anger is sudden and physical.

Executive Function

Planning, switching between tasks, and holding back a response you are about to make. The last of those is the one that matters here: reported aggression on these compounds is often better described as a failure of the brake than as an increase in anger.

Raphe Nuclei

The brain's serotonin source. It sets patience, satiety with a situation and the threshold at which irritation becomes an outburst. Adolescent steroid exposure changes serotonin signalling in animals, and the change reverses direction on withdrawal.

Serotonin Signalling

How strongly serotonin is signalling. The system most often named when 19-nor androgens are linked to impulse control, irritability and sleep. Almost all of the evidence is rodent work; nothing has been measured at these receptors in a human on these compounds.