Chapter 4: Hours to Days Before

Sapolsky examines how hormones influence behavior in the hours to days leading up to an action. The chapter begins with testosterone, often assumed to be the root of aggression. Sapolsky shows that while testosterone does facilitate aggressive responses, its effects are far more nuanced and context dependent than popular belief suggests. Castration studies confirm testosterone’s role in aggression, but social learning and preexisting behavioral tendencies often matter more. Normal fluctuations in testosterone rarely predict aggression; instead, the hormone amplifies existing patterns. Sapolsky introduces the “challenge hypothesis,” which states that testosterone rises during status challenges, pushing individuals toward behaviors—sometimes aggressive, sometimes prosocial—that maintain dominance. Thus, testosterone does not create aggression but magnifies context-specific responses, including generosity or cooperation when status depends on fairness. The chapter then shifts to oxytocin and vasopressin, neuropeptides popularly dubbed the “love hormones.” These chemicals foster mother-infant bonding, pair-bonding in species like prairie voles, paternal care, and trust, but their effects are not universally prosocial. Oxytocin promotes in-group bonding while simultaneously amplifying ethnocentrism, envy, and hostility toward out-groups. Research demonstrates that oxytocin increases generosity within teams but can enhance preemptive aggression and bias against outsiders, showing its dual role in both empathy and exclusion. Sapolsky also explores the endocrinology of female aggression, debunking myths that females are inherently passive. Hormones such as estrogen, progesterone, and oxytocin can promote maternal aggression, while androgens play a role in competitive aggression across species. In some cases, female mammals—including bonobos, lemurs, and hyenas—exhibit dominance and heightened aggression, shaped by complex hormonal interactions. Perimenstrual shifts in mood and irritability are biologically real, though often pathologized as PMS; however, evidence linking them directly to aggression remains limited. The chapter further analyzes stress hormones, highlighting the contrast between adaptive acute stress and harmful chronic stress. Acute stress mobilizes energy and sharpens cognition, but sustained stress over days impairs the frontal cortex, strengthens the amygdala, and biases behavior toward fear, impulsivity, displacement aggression, and diminished empathy. Stress can make people more self-centered in moral decisions and less charitable, with glucocorticoids narrowing the scope of who is considered an “Us.” Nonetheless, moderate stress in benevolent contexts can be pleasurable and motivating, as in play or challenge. Finally, Sapolsky addresses alcohol’s effects, challenging the assumption that drinking universally causes aggression. Alcohol only heightens aggression in predisposed individuals or those who expect it to do so, underscoring the role of belief and social learning in shaping biology. Overall, this chapter demonstrates that hormones like testosterone, oxytocin, estrogen, and glucocorticoids rarely dictate behavior outright. Instead, they act as amplifiers, heightening sensitivity to social triggers and preexisting tendencies, revealing the profound interplay between biology, context, and culture in shaping behavior.