IN THE late 1990s a previously blameless American began collecting child pornography and propositioning children. On the day before he was due to be sentenced to prison for his crimes, he had his brain scanned. He had a tumour. When it had been removed, his paedophilic tendencies went away. When it started growing back, they returned. When the regrowth was removed, they vanished again. Who then was the child abuser?

His case dramatically illustrates the challenge that modern neuroscience is beginning to pose to the idea of free will. The instinct of the reasonable observer is that organic changes of this sort somehow absolve the sufferer of the responsibility that would accrue to a child abuser whose paedophilia was congenital. But why? The chances are that the latter tendency is just as traceable to brain mechanics as the former; it is merely that no one has yet looked. Scientists have looked at anger and violence, though, and discovered genetic variations, expressed as concentrations of a particular messenger molecule in the brain, that are both congenital and predisposing to a violent temper. Where is free will in this case?

Free will is one of the trickiest concepts in philosophy, but also one of the most important. Without it, the idea of responsibility for one's actions flies out of the window, along with much of the glue that holds a free society (and even an unfree one) together. If businessmen were no longer responsible for their contracts, criminals no longer responsible for their crimes and parents no longer responsible for their children, even though contract, crime and conception were “freely” entered into, then social relations would be very different.

We, the willing

For millennia the question of free will was the province of philosophers and theologians, but it actually turns on how the brain works. Only in the past decade and a half, however, has it been possible to watch the living human brain in action in a way that begins to show in detail what happens while it is happening (see survey). This ability is doing more than merely adding to science's knowledge of the brain's mechanism. It is also emphasising to a wider public that the brain really is just a mechanism, rather than a magician's box that is somehow outside the normal laws of cause and effect.

Science is not yet threatening free will's existence: for the moment there seems little prospect of anybody being able to answer definitively the question of whether it really exists or not. But science will shrink the space in which free will can operate by slowly exposing the mechanism of decision making.

At that point, the old French proverb “to understand all is to forgive all” will start to have a new resonance, though forgiveness may not always be the consequence. Indeed, that may already be happening. At the moment, the criminal law—in the West, at least—is based on the idea that the criminal exercised a choice: no choice, no criminal. The British government, though, is seeking to change the law in order to lock up people with personality disorders that are thought to make them likely to commit crimes, before any crime is committed.

The coming battle

Such disorders are serious pathologies. But the National DNA Database being built up by the British government (which includes material from many innocent people), would already allow the identification of those with milder predispositions to anger and violence. How soon before those people are subject to special surveillance? And if the state chose to carry out such surveillance, recognising that the people in question may pose particular risks merely because of their biology, it could hardly then argue that they were wholly responsible for any crime that they did go on to commit.

Nor is it only the criminal law where free will matters. Markets also depend on the idea that personal choice is free choice. Mostly, that is not a problem. Even if choice is guided by unconscious instinct, that instinct will usually have been honed by natural selection to do the right thing. But not always. Fatty, sugary foods subvert evolved instincts, as do addictive drugs such as nicotine, alcohol and cocaine. Pornography does as well. Liberals say that individuals should be free to consume these, or not. Erode free will, and you erode that argument.

In fact, you begin to erode all freedom. Without a belief in free will, an ideology of freedom is bizarre. Though it will not happen quickly, shrinking the space in which free will can operate could have some uncomfortable repercussions.