Neuroscience Psychology Big Ideas

Behave: The Biology of Humans at Our Best and Worst (Summary)

by Robert M. Sapolsky

You pull a trigger. In the seconds before, what did the amygdala, the brain's fear and aggression center, do? An hour before, were you stressed and hungry? Years before, what did your adolescence do to shape the frontal cortex that's supposed to control the amygdala? And what did your ancestors' culture, thousands of years ago, teach you about who is 'us' versus 'them'? The simple act of pulling a trigger is the end result of a biological and cultural cascade that began centuries before you were born.

Testosterone Doesn't Create Aggression, It Magnifies It

Hormones like testosterone don't magically create feelings or actions from scratch. Instead, they act as 'volume knobs,' amplifying pre-existing social tendencies and learned behaviors. They make us more of what we already are.

Giving a surge of testosterone to a monastery of pacifist monks won't start a brawl. It might, however, make them compete more fiercely to see who can perform the most acts of charity. The hormone amplifies the pre-existing social goalā€”in this case, generosity, not aggression.

Your Brain is a Battlefield

Much of human behavior can be understood as a constant struggle between the impulsive, ancient amygdala (fear, aggression) and the rational, modern prefrontal cortex (PFC), which acts as a brake on our worst instincts.

In the famous case of Phineas Gage, a railroad worker whose PFC was destroyed by an iron rod, his personality changed overnight from responsible and well-mannered to impulsive and profane. His amygdala was still intact, but the part of his brain responsible for controlling it was gone.

The 'Us vs. Them' Switch is Biological and Instantaneous

Our brains are hardwired to rapidly categorize people as 'Us' or 'Them,' a process that happens automatically and unconsciously, deeply influencing our capacity for both empathy and cruelty.

Brain scans show that when we see the face of someone from a different race (a classic 'Them' trigger), the amygdala activates in less than a tenth of a secondā€”before we are even consciously aware of the face. This isn't destiny, but it is a powerful default setting we have to consciously overcome.

Your Decisions Aren't Really Yours

From the smell in the air to your blood sugar levels to the culture your ancestors built, countless biological and environmental factors influence your decisions in ways you are completely unaware of, seriously challenging the concept of free will.

A famous study showed that judges are statistically far more likely to grant parole to prisoners right after their lunch break than right before it. A decision that dramatically alters a person's life is measurably influenced by something as simple as the judge's blood glucose level, not just the legal facts of the case.

Go deeper into these insights in the full book:
Buy on Amazon
Listen to the full audio book with an Audible Free Trial.
As an Amazon Associate, qualifying purchases help support this site.