Does your brain decide before you do?
In 1983 a neuroscientist found brain activity that began before people felt they had decided to move. What it shows about free will is still fought over.
- Reading time
- 4 min
- Difficulty
- Intermediate
- Branch
- Free will
- First posed
- Benjamin Libet and colleagues, 1983, building on Kornhuber and Deecke, 1965
The setup
Imagine you are sitting in a laboratory with electrodes on your scalp. In front of you is a clock with a dot circling the face quickly, once every 2.56 seconds. Your task is simple: whenever you feel like it, flex your wrist. Afterward, report where the dot was at the moment you first felt the urge or decided to move.
This was the experiment run by Benjamin Libet and colleagues at the University of California, San Francisco, published in 1983. Electrodes measured a slow build-up of electrical activity known as the readiness potential, first described by Hans Kornhuber and Lüder Deecke in 1965. Muscle sensors timed the movement itself.
The result: the readiness potential began, on average, roughly half a second before the movement. People's reported moment of conscious decision came only about 200 milliseconds before the movement. In other words, the brain appeared to start preparing the action about 350 milliseconds before the person was aware of deciding to act.
Later studies seemed to push the gap further. In 2008, Chun Siong Soon, John-Dylan Haynes and colleagues used brain scanning to predict which of two buttons people would press, with about 60 percent accuracy where chance was 50, up to ten seconds before they reported deciding.
Why it matters
These experiments are the most famous scientific challenge to free will. They are often summarised in headlines as 'your brain decides before you do', implying that conscious will is a bystander, informed after the fact of a decision already made.
If that interpretation were right, it would undermine the intuitive picture in which conscious deliberation causes our actions. Whether it is right turns on details of method and interpretation that matter enormously.
Key terms
- Readiness potentialA slow build-up of electrical activity in the brain that begins before a voluntary movement, first recorded in 1965. Libet's experiments timed it against the reported decision to move.
- Free willThe capacity to choose between options in a way that makes us the real authors of our actions and responsible for them. What exactly that requires is the central dispute.
- Free won'tLibet's suggestion that even if the brain starts preparing a movement before we decide, we can still consciously veto it at the last moment.
- EpiphenomenalismThe view that mental states are caused by physical states but have no effects of their own on the physical world, like the whistle of a steam engine.
The positions
Conscious will is an illusion
The brain initiates actions unconsciously. The feeling of deciding comes afterward and does not cause the action.
Libet's own view: free won't
The brain may initiate actions unconsciously, but consciousness can still veto them in the final fraction of a second.
The readiness potential is not a decision
The readiness potential reflects random background fluctuations in brain activity, not an unconscious decision. The experiments do not show what they seem to.
Arguments and objections
Conscious will is an illusion
- Best argument
- The timing data appear to show that the causal process leading to action is well under way before awareness of a decision. Daniel Wegner argued in The Illusion of Conscious Will (2002) that the experience of willing is the mind's interpretation of its own behaviour, not its cause. This fits other findings in which people confidently claim authorship of actions they did not cause, or deny authorship of ones they did.
- Strongest objection
- The interpretation rests on contested assumptions: that the readiness potential reflects a decision, that people can accurately time their awareness against a moving clock, and that flicking a wrist at a random moment tells us anything about real choices.
Libet's own view: free won't
- Best argument
- Libet himself did not conclude that free will was an illusion. He noted that there was a window of about 150 milliseconds between conscious awareness and movement, during which a person could still cancel the action. Free will might operate as a free won't: we may not choose our impulses, but we choose which to act on.
- Strongest objection
- If the initiation of actions is unconscious, why would the veto be any different? A decision to veto might itself be preceded by unconscious brain activity.
- The reply
- Later experiments by Matthias Schultze-Kraft and colleagues (2016) found that people could cancel a movement after the readiness potential had begun, up to a 'point of no return' about 200 milliseconds before movement, which gives some support to the idea of a late veto.
The readiness potential is not a decision
- Best argument
- In 2012 Aaron Schurger and colleagues showed that the readiness potential can be modelled as neural noise drifting toward a threshold. When it happens to cross the threshold, the movement is triggered. Averaging many trials backward from the movement creates the appearance of a build-up even though no decision was made early. In arbitrary choices with no reason to prefer one moment over another, it is unsurprising that noise tips the balance.
- Strongest objection
- This explains the readiness potential away for arbitrary movements, but leaves open what happens in meaningful decisions, which might involve similar unconscious processes.
- The reply
- Uri Maoz and colleagues (2019) found that the readiness potential was present for arbitrary choices but absent or much weaker for deliberate choices that mattered to the participant, suggesting the original experiments studied the wrong kind of decision.
Who argued what
- Conscious will is an illusion Daniel Wegner, Sam Harris
- Libet's own view: free won't Benjamin Libet
- The readiness potential is not a decision Aaron Schurger, Uri Maoz, Alfred Mele
Variations
A meaningful choice
Imagine the experiment repeated, but instead of flicking your wrist at a random moment, you choose which of two charities receives a real thousand-dollar donation. Do you expect the same unconscious build-up? Maoz's results suggest not. If arbitrary and deliberate choices work differently in the brain, the original experiments may tell us little about the choices we care about.
The timing problem
How accurately can you report when you became aware of an urge? Studies show that such reports are easily shifted, for example by playing a tone after the movement. If people's reports of 'the moment of decision' are partly reconstructed, the measured gap may be an artefact of how we introspect, not of when we decide.
Go deeper
What would refute free will?
Philosopher Alfred Mele argues that no current experiment has the right shape to refute free will. To show that conscious intentions play no role, we would need to show that actions would occur just the same without them, across meaningful choices. So far, the experiments show that unconscious processes contribute to decisions, which almost every theory of free will already accepts.
Related ideas
Further reading and sources
- EncyclopediaStanford Encyclopedia of Philosophy: search “Libet free will neuroscience”
- WikipediaBenjamin Libet
- WikipediaNeuroscience of free will
- BookFree: Why Science Hasn't Disproved Free Will, Alfred Mele
- BookThe Illusion of Conscious Will, Daniel Wegner
- BookMind Time, Benjamin Libet
Open questions
- If an unconscious process starts every action, is the decision any less yours?
- Can science ever settle whether conscious will causes behaviour?
- Is the power to say no a kind of freedom, or just another output of the brain?