Evolution designed an economical system where a single, subconscious "kinship estimate" for each person dictates both altruism towards them and sexual aversion. It's one calculation for two different social behaviors, determining how close your heart should be and how far your genitals should be.

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 feelings of love and attachment arise from three collaborating neural circuits: the autonomic nervous system (our physiological state), empathy circuits (our ability to match another's state), and, surprisingly, circuits associated with positive delusions—the belief that our partner is uniquely special and irreplaceable.

The neural systems evolved for physical survival—managing pain, fear, and strategic threats—are the same ones activated during modern stressors like workplace arguments or relationship conflicts. The challenges have changed from starvation to spreadsheets, but the underlying brain hardware hasn't.

Neurons for fear and offensive aggression are located closely together in the hypothalamus. Activating these fear neurons can immediately stop a fight, causing the animals to freeze. This reveals a functional hierarchy where the fear state is dominant and can override aggressive impulses.

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.

In mice, prolonged social isolation causes a dramatic increase in the neuropeptide tachykinin. This neurochemical surge is directly responsible for increased aggression, fear, and anxiety. A drug that blocks the tachykinin receptor can completely reverse these isolation-induced effects.

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.