7 ideas
Every behavior traces back to biology and prior environment that the agent did not choose, leaving no causal gap where an uncaused 'free' decision could insert itself. What people call free will is simply the portion of behavior whose biological causes we have not yet identified, which shrinks as science advances.
The prefrontal cortex's core job is to make you do the harder thing when it is the right thing: overriding impulses, regulating emotion, and weighing long-term consequences. It is the last brain region to fully mature, not until the mid-twenties, which helps explain adolescent risk-taking and impulsivity.
To explain a behavior, ask what happened in the brain one second before, what sensory cues triggered it seconds to minutes before, what hormones primed it hours to days before, and keep going back through adolescence, childhood, the fetal environment, genes, culture and evolution. Each level is shaped by the ones before it, so no single discipline or cause is enough to explain what someone did.
Testosterone does not create aggression from nothing. It lowers anxiety and raises confidence, and it magnifies whatever behavior already wins status in a given setting. In experiments where generosity earns status, raised testosterone makes people more generous, so the hormone's effect depends on context.
Humans divide others into Us and Them within milliseconds, and the amygdala responds to other-race faces before conscious thought. Which categories count as Us and Them is highly flexible, though. Emphasizing a shared identity, such as the same team, can quickly override racial categorization, so the target of tribalism can be changed even if the tendency cannot be removed.
Every behavior results from prior biological and environmental causes that the person did not choose, so the idea of a freely willed choice that deserves punishment falls apart. Sapolsky argues we should treat harmful behavior as we now treat epilepsy, once blamed on demonic possession: protect society from the danger without moralized retribution.
Genes code for proteins whose effects depend on the environment, so a gene's influence is conditional: 'if in environment X, then tendency Y.' Gene-environment interactions such as MAO-A variants raising antisocial risk only when paired with childhood abuse show that asking whether a trait is genetic or environmental is usually the wrong question.