Free Will and the Brain

Libet's famous experiment, the readiness potential, brain scans that predict choices seconds early, newer reinterpretations — and what neuroscience can and can't say about free will.

The readiness potential

In 1965 the German neuroscientists Hans Helmut Kornhuber and Lüder Deecke discovered that a slow negative voltage builds up over the motor areas of the scalp around a second or more before a voluntary movement. They called it the Bereitschaftspotential — the readiness potential.

Libet's experiment

In 1983 Benjamin Libet and colleagues asked people to flex their wrist whenever they felt the urge, while watching a fast-moving clock dot and noting where it was when they first felt the intention. The readiness potential began roughly half a second before the movement — but people reported the conscious urge only about 200 milliseconds before moving. The brain activity seemed to start several hundred milliseconds before conscious intention. Libet himself suggested conscious will might still "veto" an action already set in motion.

Predicting choices

In 2008 Chun Siong Soon, John-Dylan Haynes and colleagues used fMRI and pattern analysis to predict which of two buttons people would press up to several seconds before they reported deciding — though the prediction accuracy was only modestly above chance (around 60%).

Reinterpretations

The headline "your brain decides before you do" has been challenged. Critics question the reliability of people's timing reports. In 2012 Aaron Schurger and colleagues proposed that the readiness potential largely reflects the averaging of random, slowly fluctuating background activity: when no external cue forces the timing, a movement occurs when noise happens to cross a threshold. On this view, the early buildup isn't a hidden decision but the noise that tips a spontaneous movement. Studies comparing arbitrary choices with meaningful, deliberate ones found the readiness potential is much weaker or absent for deliberate decisions.

What it means

Many philosophers and neuroscientists argue that these experiments test a narrow kind of free will — arbitrary, spontaneous flicks of the wrist. Compatibilists hold that free will means acting on your own reasons and values without coercion, which is compatible with decisions being produced by brain processes. Others argue that because choices are caused by brain states we didn't choose, no deep free will exists. Neuroscience informs the debate but hasn't settled it.