Contrary to popular belief, testosterone's effects on aggression in male mice are often mediated by its conversion to estrogen via the enzyme aromatase. Researchers found that estrogen implants alone can restore aggression in castrated mice, completely bypassing the direct need for testosterone.

The impact of estrogen on aggression is context-dependent. During long days with ample sunlight, estrogen does not evoke aggression. However, during short days, which are associated with higher stress hormones and lower dopamine, increased estrogen levels heighten the predisposition for aggressive behavior.

The consistent pattern of men committing mass violence is rooted in biological evolution. Men are wired for aggression and physical confrontation, a trait historically selected for by women seeking protectors. This is a biological reality, not a surprising social anomaly.

Cannabinoids (THC, CBD) don't directly reduce testosterone. Instead, the act of smoking marijuana increases the enzyme aromatase, which converts testosterone into estrogen. Higher estrogen then signals the pituitary to reduce testosterone production, creating an indirect negative feedback loop.

Research shows that the same genetic predispositions for physical aggression (e.g., fighting) in boys can manifest as relational aggression (e.g., social exclusion, reputation damage) in girls. This highlights a common biological root for sex-differentiated expressions of aggression, which can be equally damaging.

Proactive aggression can stem from a neurological difference where the brain doesn't learn from mistakes through fear. The negative consequences that deter most people don't register. Instead, the harmful behavior might produce a reward signal, motivating the individual to continue rather than stop.

Stimulating specific aggression neurons in the ventromedial hypothalamus (VMH) of male mice elicits offensive aggression that they find rewarding. Mice will learn to perform tasks, like pressing a lever, for the opportunity to attack a subordinate male, indicating the behavior has a positive valence.

The actions of armed individuals at protests are not just about aggression but are a form of testosterone-fueled status-seeking. In a subculture where pushing back against authorities grants clout, testosterone compels that specific behavior to climb the social hierarchy.

Conrad Lorenz's model of aggression as a "hydraulic pressure" accurately reflects its biological underpinnings. Hormones, neurotransmitters, stress levels, and external stimuli all converge to increase or decrease this internal pressure, biasing an individual toward or away from an aggressive outburst. This pressure build-up is often observable.