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July 31, 2025
quote of Freud’s emphasized by Elie Wiesel, the Nobel Peace Prize winner and concentration camp survivor: “The opposite of love is not hate; its opposite is indifference.”
Limbic function is now recognized as central to the emotions that fuel our best and worst behaviors, and extensive research has uncovered the functions of its structures (e.g., the amygdala, hippocampus, septum, habenula, and mammillary bodies).
The autonomic nervous system has two parts—the sympathetic and parasympathetic nervous systems, with fairly opposite functions.
The sympathetic nervous system (SNS) mediates the body’s response to arousing circumstances, for example, producing the famed “fight or flight” stress response. To use the feeble joke told to first-year medical students, the SNS mediates the “four Fs—fear, fight, flight, and sex.”
In contrast to the SNS and the four Fs, the PNS is about calm, vegetative states. The SNS speeds up the heart; the PNS slows it down.
the brain region most involved in feeling afraid and anxious is most involved in generating aggression.
In PTSD sufferers the amygdala is overreactive to mildly fearful stimuli and is slow in calming down after being activated.
The amygdala is linked to social uncertainty in other ways. In one neuroimaging study, a subject would participate in a competitive game against a group of other players; outcomes were rigged so that the subject would wind up in the middle of the rankings.15 Experimenters then manipulated game outcomes so that subjects’ rankings either remained stable or fluctuated wildly. Stable rankings activated parts of the frontal cortex that we’ll soon consider. Instability activated the frontal cortex plus the amygdala. Being unsure of your place is unsettling.
We’ve now got learning to be afraid under our belts.*21 Now conditions change—the tone still occurs now and then, but no more shock. Gradually the conditioned fear response abates. How does “fear extinction” occur? How do we learn that this person wasn’t so scary after all, that different doesn’t necessarily equal frightening? Recall how a subset of BLA neurons respond to the tone only once conditioning has occurred. Another population does the opposite, responding to the tone once it’s no longer signaling shock (logically, the two populations of neurons inhibit each other). Where do these
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the default state is to trust, and what the amygdala does is learn vigilance and distrust.
The dissociation between fear and aggression is evident in violent psychopaths, who are the antithesis of fearful—both physiologically and subjectively they are less reactive to pain; their amygdalae are relatively unresponsive to typical fear-evoking stimuli and are smaller than normal.
In a world in which no amygdaloid neuron need be afraid and instead can sit under its vine and fig tree, the world is very likely to be a more peaceful place.*
called von Economo neurons (aka spindle neurons). At first they seemed to be unique to humans, but we’ve now found them in other primates, whales, dolphins, and elephants.* That’s an all-star team of socially complex species.
The second is an equally interesting area called the anterior cingulate. To give a hint (with more to come), it’s central to empathy.
Among humans, the larger someone’s social network (measured by number of different people texted), the larger a particular PFC subregion (stay tuned).
And there’s another nonpathological circumstance where the PFC silences, producing emotional tsunamis: during orgasm.
criminal psychopaths have decreased activity in the frontal cortex and less coupling of the PFC to other brain regions
(c) chronic stress or pain depletes dopamine and decreases the sensitivity of dopamine neurons to stimulation, producing the defining symptom of depression—“anhedonia,” the inability to feel pleasure.
In one study a subject would play an economic game with someone, where a player is rewarded under two circumstances: (a) if both players cooperate, each receives a moderate reward, and (b) stabbing the other person in the back gets the subject a big reward, while the other person gets nothing. While both outcomes increased dopaminergic activity, the bigger increase occurred after cooperation.*
Other research examined the economic behavior of punishing jerks.82 In one study subjects played a game where player B could screw over player A for a profit. Depending on the round, player A could either (a) do nothing, (b) punish player B by having some of player B’s money taken (at no cost to player A), or (c) pay one unit of money to have two units taken from player B. Punishment activated the dopamine system, especially when subjects had to pay to punish; the greater the dopamine increase during no-cost punishment, the more willing someone was to pay to punish. Punishing norm violations
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Dopaminergic responses to reward, rather than being absolute, are relative to the reward value of alternative outcomes. In order to accommodate the pleasures of both mathematics and orgasms, the system must constantly rescale to accommodate the range of intensity offered by particular stimuli. The response to any reward must habituate with repetition, so that the system can respond over its full range to the next new thing.
Schultz demonstrated that following a reward, the dopamine system codes for discrepancy from expectation—get what you expected, and there’s a steady-state dribble of dopamine.
If we were designed by engineers, as we consumed more, we’d desire less. But our frequent human tragedy is that the more we consume, the hungrier we get.
work by my Stanford colleague Brian Knutson has shown dopamine pathway activation in people in anticipation of a monetary reward.91 Dopamine is about mastery and expectation and confidence. It’s “I know how things work; this is going to be great.” In other words, the pleasure is in the anticipation of reward, and the reward itself is nearly an afterthought (unless, of course, the reward fails to arrive, in which case it’s the most important thing in the world). If you know your appetite will be sated, pleasure is more about the appetite than about the sating.* This is hugely important.
This explains context-dependent craving in addiction.93 Suppose an alcoholic has been clean and sober for years. Return him to where the alcohol consumption used to occur (e.g., that rundown street corner, that fancy men’s club), and those potentiated synapses, those cues that were learned to be associated with alcohol, come roaring back into action, dopamine surges with anticipation, and the craving inundates.
In other words, dopamine is not about the happiness of reward. It’s about the happiness of pursuit of reward that has a decent chance of occurring.*
Low serotonin didn’t predict premeditated, instrumental violence. It predicted impulsive aggression, as well as cognitive impulsivity
mainly because I think that a term like “forgiveness,” and others related to criminal justice (e.g., “evil,” “soul,” “volition,” and “blame”), are incompatible with science and should be discarded.
Animals vocalize to intimidate, proclaim, and seduce. Birds sing, stags roar, howler monkeys howl, orangutans give territorial calls audible for miles. As a subtle example of information being communicated, when female pandas ovulate, their vocalizations get higher, something preferred by males. Remarkably, the same shift and preference happens in humans.
The more expensive a supposed (placebo) painkiller, the more effective people report the placebo to be.
So if whites see a black face shown at a subliminal speed, the amygdala activates.10 But if the face is shown long enough for conscious processing, the anterior cingulate and the “cognitive” dlPFC then activate and inhibit the amygdala. It’s the frontal cortex exerting executive control over the deeper, darker amygdaloid response.
As we saw, there’s that shortcut from the thalamus directly to the amygdala, such that while the first few layers of, say, the visual cortex are futzing around with unpacking a complex image, the amygdala is already thinking, “That’s a gun!” and reacting. And as we saw, there’s the trade-off: information reaches the amygdala fast but is often inaccurate.16 The amygdala thinks it knows what it’s seeing before the frontal cortex slams on the brakes; an innocent man reaches for his wallet and dies.
pain does not cause aggression; it amplifies preexisting tendencies toward aggression.
Various studies, predominantly by Roy Baumeister of Florida State University, show that when the frontal cortex labors hard on some cognitive task, immediately afterward individuals are more aggressive and less empathic, charitable, and honest. Metaphorically, the frontal cortex says, “Screw it. I’m tired and don’t feel like thinking about my fellow human.”
Subjects decide whether to administer a hypothetical drug. If they’re told, “The drug has a 95 percent survival rate,” people, including doctors, are more likely to approve it than when told, “The drug has a 5 percent death rate.”*
being aggressive stimulates testosterone secretion;
testosterone makes people less adept at identifying emotions by looking at people’s eyes, and faces of strangers activate the amygdala
Testosterone also increases confidence and optimism, while decreasing fear and anxiety.
Success in everything from athletics to chess to the stock market boosts testosterone levels.
Testosterone makes people cocky, egocentric, and narcissistic.
Testosterone boosts impulsivity and risk taking, making people do the easier thing when it’s the dumb-ass thing to do.7 Testosterone does this by decreasing activity in the prefrontal cortex and its functional coupling to the amygdala and increasing amygdaloid coupling with the thalamus—the source of that shortcut path of sensory information into the amygdala.
testosterone’s actions are contingent and amplifying, exacerbating preexisting tendencies toward aggression rather than creating aggression out of thin air.
Then there was a study that is embarrassingly similar to stereotypical human couples. Among pair-bonding tamarin monkeys, lots of grooming and physical contact predicted high oxytocin levels in female members of a pair. What predicted high levels of oxytocin in males? Lots of sex.
these neuropeptides appear to play a role in human pair-bonding.28 For starters, circulating oxytocin levels are elevated in couples when they’ve first hooked up. Furthermore, the higher the levels, the more physical affection, the more behaviors are synchronized, the more long-lasting the relationship, and the happier interviewers rate couples to be.
oxytocin caused males in relationships to spend less time looking at pictures of attractive women. Importantly, oxytocin didn’t make men rate these women as less attractive; they were simply less interested.
oxytocin and vasopressin facilitate bonding between parent and child and between couples.*
the brains of humans and domesticated wolves evolved a new response to oxytocin: when a dog and its owner (but not a stranger) interact, they secrete oxytocin.
oxytocin elicits prosocial behavior, and oxytocin is released when we experience prosocial behavior (being trusted in a game, receiving a warm touch, and so on). In other words, a warm and fuzzy positive feedback loop.
oxytocin enhances charitability—but only in people who are already so. This mirrors testosterone’s only raising aggression in aggression-prone people.
When playing against strangers, oxytocin decreases cooperation, enhances envy when luck is bad, and enhances gloating when it’s good.
