Robert Sapolsky
Stanford neuroscientist; argues in 'Determined' (2023) that free will does not exist
13 videos analysed · 72 quotes checked word for word · updated 2026-10-22
Key facts
- Born in Brooklyn in 1957; Harvard degree, PhD from Rockefeller University; professor at Stanford (biology, neurology, neurosurgery). Wikipédia · Robert Sapolsky (Wikipedia)
- Books include Why Zebras Don't Get Ulcers (1994), Behave (2017) and Determined (2023). Wikipédia · Robert Sapolsky (Wikipedia)
- He admits that living without belief in free will is very hard: he says he manages it 'once a month or so for about three minutes'. Article · How to live in a world without…
Positions
click a row to see the evidenceHe says plainly: "I'm an atheist and I also don't believe there's free will." He dates his turning point to a night at age 14. Wikipedia adds that he was devoutly religious as a child.
- He reached this view young: at 14 he concluded that there is no free will, no God and no purpose in a big, indifferent universe, and neuroscience only added facts. Video 2024 · Do We Have Free Will? with…
- He explains religion through biology: many rituals, he thinks, came from people struggling with obsessive-compulsive tendencies, since the lists of religious and OCD rituals match. This does not make believers 'crazy', he adds. Video 2026 · 31. Religious Belief by Robert…2
- For him, believing in God and believing in free will are two separate axes, and he rejects both. Video 2025 · Tourette's, Free Will vs God,…
Explaining religion through biology treats believers as sick or crazy.
He stresses that a biological explanation of religiosity does not mean believers are crazy, and that the biology of losing faith is as interesting as that of keeping it.
Whatever begins to exist has a cause, so the universe has a divine cause.
He says we are biological machines, 'materialist machines', and that animals and humans alike are machines, only humans are more complicated. Any free will that is 'in your brain, but not of your brain' has no place in it, and he calls an extra step beyond physical causes 'magic'.
- Humans, like animals, are biological machines, only more complicated. He says Descartes was wrong to set humans apart. Video 2023 · The Great Simplification #88…2
- Failures of self-control under stress show that we are biological machines and that consciousness is only a small part of what drives us. Video 2023 · Robert Sapolsky on Free Will and…
- No defender of free will offers a mechanism that does not rely on a magical step: nothing in the brain could be 'in the brain but not of it'. Video 2023 · Free Will, Morality, Self…23
The mind is a thinking substance distinct from the body, and animals are mere machines while humans are not.
He says Descartes was wrong: animals and humans alike are biological machines, only humans are more complicated.
Consciousness is more than a machine process: we can consciously decide and control ourselves.
He sees self-control failures under stress as evidence that we are biological machines, with consciousness only a small part of the picture, and says no defender of free will offers a mechanism that avoids a magical step.
Defends : Man is a machine that winds its own springsCriticizes : Mind distinct from body
He rejects free will outright. We can act on our choices, he says, but we cannot choose what we choose, because everything that shaped us was outside our control. He is also critical of compatibilist philosophers. In his words: "We don't change, we don't choose to change. But we sure are changed." He admits that 95 to 99 percent of the time he is a "flaming hypocrite" who cannot function as if there were no free will.
- Whatever you do comes from your brain at that moment, and the brain was shaped by earlier causes: the senses a second ago, hormones, childhood, genes, evolution. He challenges anyone to find a neuron that caused a behavior independently of all that. Video 2021 · Science of Stress, Testosterone…23
- Even a conscious intent does not save free will. For him the real question is how you became the person who has that intent, and the answer leads back to a biology and an environment you did not choose. Video 2024 · You have no free will at all |…23
- Quantum randomness does not help: it would give chance, not freedom. And a 'higher-level' self that reaches down to change how its parts work is, in his words, invoking magic. Video 2024 · Free Will FIGHT!! Who Has the…23
- Against compatibilists, he says they draw an artificial line between biology and a separate self, and mostly want to keep free will because a world without it feels scary. The causal biology applies to everyone, all the time. Video 2023 · Is There Free Will? The…2
- The practical conclusion: without free will, blame, hatred and deserved punishment make no sense, and people can still change, because we are changed by what happens to us and can use our knowledge of how that happens. Video 2023 · Is There Free Will? The…234
Free will is compatible with a world of causes: what matters is that we act from our own reasons.
He says this rests on the intuition that choices feel free and on fear of a world without free will, and finds Dennett's idea of generating options at random and choosing the rational one unconvincing.
Free will can emerge at the level of the whole brain, even if the neurons themselves follow physics.
He says no brain mechanism can show free will in the ordinary sense, at a reductive or an emergent level: a macro property cannot reach down and change how its parts work.
See 1 more objections
Nobody can really live as if there were no free will, so the view cannot be taken seriously.
He admits he almost never lives that way, but says that stepping away from the idea of agency is hard and worth it because it makes a more humane world.
Defends : To be responsible, one would have to be the cause of oneselfMixed : The brain acts before we decideCriticizes : The only freedom worth wanting
At 14 he concluded there is no God and no purpose, 'just a big empty and indifferent universe'.
- From his early conclusion, the universe has no given purpose. He admits this is hard to live with, yet he does not treat it as a reason for despair. Video 2023 · Free Will, Morality, Self…2
- What remains is to be kind: we are odd machines, yet kindness still feels good to us, and self-esteem only makes sense when it makes us kinder and more caring. Video 2023 · Is There Free Will? The…2
A universe with no God, no purpose and no free will is a bleak view that leaves nothing to live for.
He agrees it is demoralizing and says he could not get through the last 200 pages of his book at first, but says that we are weird machines for whom good things still feel good, and that is what remains.
No sourced position yet on: Source of knowledge, Certainty, Status of morality, Faith and reason, Human nature.
On the big questions
Does God exist?Atheism
He describes himself as an atheist, and as a 'strident' one alongside Dennett.
- He reached this view young: at 14 he concluded that there is no free will, no God and no purpose in a big, indifferent universe, and neuroscience only added facts. Video 2024 · Do We Have Free Will? with…
- He explains religion through biology: many rituals, he thinks, came from people struggling with obsessive-compulsive tendencies, since the lists of religious and OCD rituals match. This does not make believers 'crazy', he adds. Video 2026 · 31. Religious Belief by Robert…2
- For him, believing in God and believing in free will are two separate axes, and he rejects both. Video 2025 · Tourette's, Free Will vs God,…
Explaining religion through biology treats believers as sick or crazy.
He stresses that a biological explanation of religiosity does not mean believers are crazy, and that the biology of losing faith is as interesting as that of keeping it.
Whatever begins to exist has a cause, so the universe has a divine cause.
Does life have a meaning?A meaning to create
He says the view of no God, no free will and an empty, indifferent universe without purpose is demoralizing: he spent years teetering on the edge of an existential void. It is still his own view, and he says that if it bums you out, you are one of the lucky ones.
- From his early conclusion, the universe has no given purpose. He admits this is hard to live with, yet he does not treat it as a reason for despair. Video 2023 · Free Will, Morality, Self…2
- What remains is to be kind: we are odd machines, yet kindness still feels good to us, and self-esteem only makes sense when it makes us kinder and more caring. Video 2023 · Is There Free Will? The…2
A universe with no God, no purpose and no free will is a bleak view that leaves nothing to live for.
He agrees it is demoralizing and says he could not get through the last 200 pages of his book at first, but says that we are weird machines for whom good things still feel good, and that is what remains.
Are we free?No free will
His whole book defends this camp: biology and circumstances determine human actions.
- Whatever you do comes from your brain at that moment, and the brain was shaped by earlier causes: the senses a second ago, hormones, childhood, genes, evolution. He challenges anyone to find a neuron that caused a behavior independently of all that. Video 2021 · Science of Stress, Testosterone…23
- Even a conscious intent does not save free will. For him the real question is how you became the person who has that intent, and the answer leads back to a biology and an environment you did not choose. Video 2024 · You have no free will at all |…23
- Quantum randomness does not help: it would give chance, not freedom. And a 'higher-level' self that reaches down to change how its parts work is, in his words, invoking magic. Video 2024 · Free Will FIGHT!! Who Has the…23
- Against compatibilists, he says they draw an artificial line between biology and a separate self, and mostly want to keep free will because a world without it feels scary. The causal biology applies to everyone, all the time. Video 2023 · Is There Free Will? The…2
- The practical conclusion: without free will, blame, hatred and deserved punishment make no sense, and people can still change, because we are changed by what happens to us and can use our knowledge of how that happens. Video 2023 · Is There Free Will? The…234
Free will is compatible with a world of causes: what matters is that we act from our own reasons.
He says this rests on the intuition that choices feel free and on fear of a world without free will, and finds Dennett's idea of generating options at random and choosing the rational one unconvincing.
Free will can emerge at the level of the whole brain, even if the neurons themselves follow physics.
He says no brain mechanism can show free will in the ordinary sense, at a reductive or an emergent level: a macro property cannot reach down and change how its parts work.
See 1 more objections
Nobody can really live as if there were no free will, so the view cannot be taken seriously.
He admits he almost never lives that way, but says that stepping away from the idea of agency is hard and worth it because it makes a more humane world.
Defends : To be responsible, one would have to be the cause of oneselfMixed : The brain acts before we decideCriticizes : The only freedom worth wanting
The question on the site: all the argumentsWhat makes something right?Morality questioned
Asked whether his view is amoral, he agrees in a sense: you cannot do good or bad things to a machine. Yet he points to a centuries-long widening of the moral circle.
- Without free will, nobody deserves blame or praise in the moral sense: each person is the outcome of biology and luck they did not choose. Video 2023 · The Great Simplification #88…23
- History shows that moving a behavior from 'moral failing' to 'biology' (witchcraft, dyslexia) makes the world more humane, so he thinks we must keep revising our moral intuitions, including about responsibility. Video 2023 · Free Will, Morality, Self…23
- When we judge others, we should remember the unearned luck behind our own success. Video 2024 · You have no free will at all |…
Without free will nobody can be blamed or praised, so there is no way to hold anyone accountable.
He answers that change is still possible and worth striving for, because we are changed by what happens to us and can use what we know about how that change comes about, and that blame makes as little sense as hating an earthquake.
Is the mind the brain?The mind comes from matter
Our beliefs, values and behaviour arise from genes, brain chemistry and environment, with no separate self in charge.
- Humans, like animals, are biological machines, only more complicated. He says Descartes was wrong to set humans apart. Video 2023 · The Great Simplification #88…2
- Failures of self-control under stress show that we are biological machines and that consciousness is only a small part of what drives us. Video 2023 · Robert Sapolsky on Free Will and…
- No defender of free will offers a mechanism that does not rely on a magical step: nothing in the brain could be 'in the brain but not of it'. Video 2023 · Free Will, Morality, Self…23
The mind is a thinking substance distinct from the body, and animals are mere machines while humans are not.
He says Descartes was wrong: animals and humans alike are biological machines, only humans are more complicated.
Consciousness is more than a machine process: we can consciously decide and control ourselves.
He sees self-control failures under stress as evidence that we are biological machines, with consciousness only a small part of the picture, and says no defender of free will offers a mechanism that avoids a magical step.
Defends : Man is a machine that winds its own springsCriticizes : Mind distinct from body
The question on the site: all the argumentsAre humans good by nature?Both at once
He sees the same basic wiring behind our cruelty and our kindness, shared with other social animals; evolution does not only reward aggression, and organized war is probably recent.
- We have the same basic blueprint for being awful or wonderful to each other as other social species: chimps even carry out organized killing of neighbours, yet we and rats share the neurobiology of empathy. What is unique is how we use it. Video 2020 · #384 Robert Sapolsky: Behavior,…
- Evolution is not only 'nature red in tooth and claw': among baboons, the male who walks away from stupid provocations leaves more copies of his genes over a lifetime than the one who fights his way to alpha. Video 2024 · No Freewill, No Purpose, No God?…
- He is convinced organized violence is a recent invention (since farming, about 12,000 years ago), yet individual killing over mates, rejection or honor seems as old as our species: and that side of us is very long-lasting and hard to change. Video 2024 · No Freewill, No Purpose, No God?…
War is part of human nature: the archaeological record shows organized violence throughout our evolutionary history.
He calls this hugely controversial among anthropologists and says he is convinced by the evidence that organized war only appeared in the last 10,000 to 12,000 years, with farming and surplus.
Does history have a direction?Progress
He sees a long-term trend in which the circle of people whose needs count keeps widening, but he thinks it is smaller than Pinker says and far from finished.
- The group we count as 'us' has been expanding for centuries: the Geneva Convention and rules of war, Doctors Without Borders or animal protection leagues could not have existed in the 18th century. Video 2023 · Free Will, Morality, Self…
- He agrees with Pinker that the world has become more humane over the last four centuries, but thinks this has happened nowhere near as much as Pinker says, and that Pinker mostly looks at the West. Video 2020 · #384 Robert Sapolsky: Behavior,…
- A consistent trend does not prove it is the right one, and he says we still have a hell of a way to go: we will have to keep revising what seems morally obvious, such as who is responsible for what. Video 2023 · Free Will, Morality, Self…
A steady trend over centuries does not show it is correct: history could swing back and forth between societies that accept slavery or atrocity and ones that despise them.
He agrees a trend does not imply its correctness, and says only that we might be on the uphill swing and have a long way to go.
In their words
We have no ground to praise or blame anyone in a moral sense.
All we are is whatever brought us to that moment and made us form that intent.
Praise, blame, punishment, reward make no sense at all if there's no free will and there isn't.
Hating somebody for anything makes as little sense as hating an earthquake.
1 more quotes
How did you become the sort of person who would have that intent at that point?
How the ideas evolved
Click a step for details1971A night at 14: no free will, no God, no purpose
He says that at 14 he had a night when he suddenly decided there is no free will. As long as he was at it, he decided there is no God and no purpose either. Neuroscience, he says, only added facts to this view.
In their words« I was 14 when I had this incredibly epipal night where I suddenly decided there's no free will »He decided at age 14 that there is no free will, and on the same night no God and no purpose. watch
« I also as long as I was at it that night decided there's no God and there's no purpose and it's just a big empty and different Universe »He also concluded that night that there is no God and no purpose in a big, indifferent universe. watch
« we're just machines we're biological machines »Neuroscience only added facts to his view that we are biological machines. watch
2017Behave ends on 'no free will'
His book Behave explains behaviour through many layers, from brain chemistry and genes to culture. According to a secondhand summary, it concludes that humans lack free will.
2021'Not a shred of free will'
He says he does not think we have a shred of free will, against most philosophers. He challenges anyone to show a neuron that caused a behavior independently of everything before it.
In their words« I don't think we have a shred of freewill despite 95% of philosophers »He says he does not think we have any free will at all, against most philosophers. watch
« show me a neuron that just caused that behavior »He challenges anyone to find a neuron or network that caused a behavior independently of all earlier influences, from the senses a second ago to evolution. watch
« there's no space in there to fit in a freewill concept that winds up being in your brain, but not of your brain »No room in the brain for a free will that would be in the brain but not of it. watch
2023Determined: a book entirely on free will
He devotes a whole book to the question: Determined: A Science of Life Without Free Will. He argues that free will is absent, or far more limited than people assume, and that this should change how we think about crime and punishment.
2023Debate with Kevin Mitchell
Facing the neuroscientist Kevin Mitchell, he repeats that no brain mechanism, reductive or emergent, can show free will in the ordinary sense. He asks how anyone became a person who values self-reflection.
In their words« show me a brain that has just produced one of these behaviors that we are impressed with and show me how it did it independent of everything that came before »His reply to Mitchell: show a brain producing a behavior independent of everything that came before. watch
« it is simply not possible to show something that would pass for free will in the sense that we understand it »No brain mechanism can show free will in the ordinary sense, whether on a reductive or emergent level. watch
« how did you become the sort of person who values self-reflection »He answers Mitchell: even the capacity for self-reflection is something one did not choose. watch
2024Why he wrote Determined, and the rejection of quantum randomness
He says audiences missed the point of Behave, so he wrote a book 'hitting people over the head' with the message that there is no free will. He also says randomness is not what you build a moral philosophy from, and calls Dennett's compatibilist account an 'unconvincing circus trick'.
In their words« the issue of consciousness is not terribly relevant to assessing free will »He says consciousness is not relevant to judging free will; the real question is where the intent came from. watch
« where did that intent come from? How did you turn out to be the sort of person who would have that intent at that moment? »Even a conscious intent leaves open how you became the person who has it; there is no space for free will there. watch
« randomness is not the stuff you're going to build your your moral philosophy out of »He rejects quantum indeterminacy as a basis for free will: it gives randomness, not freedom. watch
2026A biology of religious belief
In a lecture on religious belief, he argues that many religious rituals came from people with obsessive-compulsive tendencies, not from committees. He adds that explaining religiosity biologically does not mean believers are crazy.
In their words« they didn't come from committees. They were coming from people, I believe, struggling with OCD. »He argues that many religious rituals were invented by people with obsessive-compulsive tendencies, since the lists of religious and OCD rituals match. watch
« nowhere in there am I saying anything nutty like »He stresses that explaining religiosity biologically does not mean believers are crazy; the biology of losing faith is as interesting as that of keeping it. watch
The arguments 5
The site's arguments they defend or criticize, and why. Click a title to see the full argument.
He says people are still arguing about Libet 40 years later, but the timing of conscious intent is irrelevant. The real question is how you became the person who has that intent.
His core argument: "How did you become the sort of person who would have that intent at that point?" Even a conscious intent traces back to biology and environment he did not choose.
He says humans and animals alike are biological machines, only more complicated, which is close to La Mettrie's idea.
He rejects compatibilism. He argues that who we are depends on genes and environment, so merit is largely luck. A reviewer notes that he answers Dennett's idea of 'grit' by saying grit is shaped by the same factors. In his words, Dennett "puts way too much faith in intuition" and offers an "unconvincing circus trick".
He says Descartes was wrong: animals and humans alike are biological machines, only humans are more complicated.
Allies and opponents
Sam HarrisClose
He calls Harris a 'kindred soul' in how extreme their no-free-will position is, and credits him with attacking the idea that quantum randomness could ground free will.
Steven PinkerSparring partner
He credits Pinker with documenting a more humane world, but thinks Pinker exaggerates how much, and disagrees on how old organized war is.
Daniel DennettDisagrees
Dennett defended compatibilism; Sapolsky rejects it. A reviewer notes he gives no sustained argument against compatibilist accounts. They are both 'strident atheists'.
KKevin MitchellDisagrees
The neuroscientist Kevin Mitchell argues for a biological basis of free will; they debated it in 2023.
Debates and big conversations
Signature ideas
Quarantine instead of punishment
He proposes treating dangerous offenders like a carrier of a disease: keep them away from others without harming them. He criticises the 'excessive, visceral satisfaction' people take in punishing.
Nobody earned it
No one deserves better or worse treatment than anyone else, because nobody earned how they turned out. Punishment and reward make sense only as practical tools for changing behaviour.
Closest and furthest
how it is computedAmong the philosophers
Among today's voices
Take the Compass: 5 minutes to find out which philosophers you are closest to.
All sources (59)
- Robert Sapolsky (Wikipedia) Wikipedia
- How to live in a world without free will (Stanford Neurosciences) Stanford Neurosciences Institute
- Determined by Robert Sapolsky (review by Philip Badger) Philosophy Now
- An attack on free will (review of Determined) Psychology Today · 2023
- A Summary of 'Determined' by Robert Sapolsky - Does Free Will Exist? Psychiatry & Psychotherapy Podcast *
- Review of Determined: A Science of Life Without Free Will (Notre Dame Philosophical Reviews) Notre Dame Philosophical Reviews · 2024
- Determined: A Science of Life without Free Will - key figures SuperSummary *
- The Great Simplification #88 (animals and humans as machines) YouTube (The Great Simplification) · 2023
- Robert Sapolsky vs Kevin Mitchell: The Biology of Free Will (Philosophical Trials #15) YouTube (Philosophical Trials) · 2023
- Free Will, Morality, Self Awareness | Robert Sapolsky YouTube · 2023
- No Freewill, No Purpose, No God? - How Society Makes Us Feel Lost In Life YouTube · 2024
- Free Will FIGHT!! Who Has the Stronger Argument, Sapolsky or Me? (395) YouTube · 2024
- You have no free will at all | Stanford professor Robert Sapolsky YouTube · 2024
- Robert Sapolsky: "The Brain, Determinism, and Cultural Implications" (The Great Simplification #88) YouTube (The Great Simplification) · 2023
- Is There Free Will? The Unsettling Science Behind Our Everyday Decisions YouTube · 2023
- Free Will, Morality, Self Awareness | Robert Sapolsky YouTube · 2023
- Free Will, Morality, Self Awareness | Robert Sapolsky YouTube · 2023
- You have no free will at all | Stanford professor Robert Sapolsky YouTube · 2024
- You have no free will at all | Stanford professor Robert Sapolsky YouTube · 2024
- The Great Simplification #88 (animals and humans as machines) YouTube (The Great Simplification) · 2023
- The Great Simplification #88 (animals and humans as machines) YouTube (The Great Simplification) · 2023
- The Great Simplification #88 (animals and humans as machines) YouTube (The Great Simplification) · 2023
- The Great Simplification #88 (animals and humans as machines) YouTube (The Great Simplification) · 2023
- Robert Sapolsky vs Kevin Mitchell: The Biology of Free Will (Philosophical Trials #15) YouTube (Philosophical Trials) · 2023
- Robert Sapolsky vs Kevin Mitchell: The Biology of Free Will (Philosophical Trials #15) YouTube (Philosophical Trials) · 2023
- Robert Sapolsky vs Kevin Mitchell: The Biology of Free Will (Philosophical Trials #15) YouTube (Philosophical Trials) · 2023
- Robert Sapolsky vs Kevin Mitchell: The Biology of Free Will (Philosophical Trials #15) YouTube (Philosophical Trials) · 2023
- Is There Free Will? The Unsettling Science Behind Our Everyday Decisions YouTube · 2023
- Is There Free Will? The Unsettling Science Behind Our Everyday Decisions YouTube · 2023
- Is There Free Will? The Unsettling Science Behind Our Everyday Decisions YouTube · 2023
- Free Will, Morality, Self Awareness | Robert Sapolsky YouTube · 2023
- Free Will, Morality, Self Awareness | Robert Sapolsky YouTube · 2023
- Free Will, Morality, Self Awareness | Robert Sapolsky YouTube · 2023
- Free Will, Morality, Self Awareness | Robert Sapolsky YouTube · 2023
- Free Will, Morality, Self Awareness | Robert Sapolsky YouTube · 2023
- Free Will, Morality, Self Awareness | Robert Sapolsky YouTube · 2023
- No Freewill, No Purpose, No God? - How Society Makes Us Feel Lost In Life YouTube · 2024
- Free Will FIGHT!! Who Has the Stronger Argument, Sapolsky or Me? (395) YouTube · 2024
- Free Will FIGHT!! Who Has the Stronger Argument, Sapolsky or Me? (395) YouTube · 2024
- Do We Have Free Will? with Robert Sapolsky & Neil deGrasse Tyson YouTube · 2024
- Do We Have Free Will? with Robert Sapolsky & Neil deGrasse Tyson YouTube · 2024
- Tourette's, Free Will vs God, Sexual Maturation | Father-Offspring Interviews #60 YouTube · 2025
- 31. Religious Belief by Robert Sapolsky YouTube · 2026
- 31. Religious Belief by Robert Sapolsky YouTube · 2026
- Science of Stress, Testosterone & Free Will | Dr. Robert Sapolsky YouTube · 2021
- Robert Sapolsky on Free Will and Determinism YouTube · 2023
- Robert Sapolsky on Free Will and Determinism YouTube · 2023
- Robert Sapolsky on Free Will and Determinism YouTube · 2023
- #384 Robert Sapolsky: Behavior, Self-control, Morality, Primates, Humans, and Religion YouTube · 2020
- No Freewill, No Purpose, No God? - How Society Makes Us Feel Lost In Life | Robert Sapolsky YouTube · 2024
- No Freewill, No Purpose, No God? - How Society Makes Us Feel Lost In Life | Robert Sapolsky YouTube · 2024
- Free Will, Morality, Self Awareness | Robert Sapolsky YouTube · 2023
- #384 Robert Sapolsky: Behavior, Self-control, Morality, Primates, Humans, and Religion YouTube · 2020
- Free Will, Morality, Self Awareness | Robert Sapolsky YouTube · 2023
- #384 Robert Sapolsky: Behavior, Self-control, Morality, Primates, Humans, and Religion YouTube · 2020
- #384 Robert Sapolsky: Behavior, Self-control, Morality, Primates, Humans, and Religion YouTube · 2020
- #384 Robert Sapolsky: Behavior, Self-control, Morality, Primates, Humans, and Religion YouTube · 2020
- No Freewill, No Purpose, No God? - How Society Makes Us Feel Lost In Life | Robert Sapolsky YouTube · 2024
- Free Will, Morality, Self Awareness | Robert Sapolsky YouTube · 2023