Science of Stress, Testosterone & Free Will | Dr. Robert Sapolsky

KEYPOINTS

• 01

  Short-term stress helps, chronic stress harms, and the right amount is 'stimulation'

  Sapolsky describes two curves. Over time, acute stress brings benefits (escaping the predator, sharper focus for a presentation) and chronicity is 'downhill from there'; most modern stressors, like 20 years of traffic or an abusive boss, are clearly chronic. The other curve is an inverted U: too little challenge is boredom, too much is stress, and the optimum, which we pay for with scary movies and roller coasters, is stimulation.

• 02

  The amygdala decides whether arousal feels like excitement or terror

  A racing heart and fast breathing produce roughly the same brain activation whether the event is wonderful or awful. The one difference is whether the amygdala is recruited; when it is, the experience is counted as adverse. Sapolsky calls it the checkpoint for valence.

• 03

  Testosterone doesn't cause aggression; it turns up the volume on existing tendencies

  Give the number-three monkey in a five-monkey hierarchy lots of testosterone and he fights more, but only with numbers four and five; he keeps deferring to one and two. In the amygdala, testosterone raises firing rates only in neurons that are already active and shortens their after-hyperpolarizations. It lowers the threshold for provocation rather than creating aggression.

• 04

  Behavior drives testosterone more than testosterone drives behavior

  Sexual activity and aggression raise testosterone, while levels beforehand barely predict behavior. Raising an adult's testosterone by 30% has essentially no effect on sexual behavior. After castration, sex and aggression drop but not to zero, and how much remains depends on prior experience, so social learning carries much of the behavior.

• 05

  The challenge hypothesis: testosterone rises when status is threatened

  John Wingfield proposed that testosterone promotes whatever behaviors maintain status. In baboons that means aggression. In an economic game where status comes from trustworthiness, testosterone made people more generous. Sapolsky's conclusion is that the problem in a violent society is how much status it hands out for aggression, not testosterone.

• 06

  Testosterone breeds confidence, and sometimes overconfidence

  In economic games it makes people more confident and therefore less cooperative, cocky and impulsive. Sapolsky speculates that this kind of altered risk assessment fed miscalculations like those that started World War I. Testosterone and dopamine are intertwined: testosterone boosts energy, alertness and motivation within minutes (glucose uptake into muscle rises), which is why replacement can help aging men. Rats will lever-press to infuse testosterone into the range that optimizes dopamine release.

• 07

  Prenatal hormones organize the brain; adult levels mostly act as an on-off signal

  The ratio of the second to fourth finger reflects fetal androgen exposure. It is visible on fetal sonograms and in other primates, but samples of around 1,000 people are needed to detect effects. Huberman was an author, as a Berkeley master's student, on Marc Breedlove's paper on it. Hormone levels during development make a big difference; adult fluctuations matter little.

• 08

  Estrogen protects the brain and blood vessels, but timing is everything

  Estrogen boosts hippocampal neurogenesis and glucose and oxygen delivery, protects against dementia and reduces oxidative damage to vessels, whereas testosterone worsens these. A large NIH trial of postmenopausal estrogen was stopped after it showed more cardiovascular disease, stroke and dementia. Sapolsky's explanation is that women started estrogen only after menopause was complete, by which time receptor patterns had shifted. Monkey studies that simply maintained ovulatory levels showed protection.

• 09

  Control and predictability buffer stress, but only within narrow limits

  A rat that thinks its lever reduces shocks gets fewer ulcers, even if the lever is disconnected. A warning light 10 seconds before a shock helps; one second before does nothing, and two minutes before makes things worse. A sense of control helps with mild to moderate stressors but adds guilt after catastrophes, so telling a refugee or someone with terminal cancer to 'take control' is, in his words, privileged heartlessness.