What can a brain scan tell us about free will?

We all think we have control of our actions but if a brain tumour or injury can completely change our personality, what does that tell us about free will, asks David Edmonds. Go to the gym or sit in front of the telly with a family-sized packet of crisps? Hmm. Gym or crisps? Gym or crisps? Gym or crisps?
Well, we’ve all been there. We may decide that what we really want to do is go to the gym – yet find ourselves reaching for that tasty salt-and-vinegar snack, followed by the inevitable feelings of self-loathing.
Neuroscientists and psychologists are making tremendous strides in understanding our drives and motivations. Weakness of will – eating those crisps when we don’t really want to – is one intriguing phenomenon. Another is addiction, whether it be to gambling, sex, booze or cigarettes. Much is being learnt about the physiological mechanisms that underlie our compulsive appetites.
There’s a growing recognition of the importance of the subconscious in our decision-making. We may not even be aware of the influence that a surrounding smell or noise is having on our choices. And some neuroscientists have even claimed that by examining patterns in the brain, they can predict decisions that we will take six or seven seconds before we ourselves consciously choose to take them. All this raises a question for the philosopher – what are the implications of advances in knowledge about human decision-making for our conception of free will? Will scientific progress undermine our sense that we have free will? Will it eventually lead us to conclude that free will is an illusion?
Take the following notorious real-life case from a decade ago. A once happily married middle-aged man begins to develop a fascination for child pornography and prostitution. Until this moment he has exhibited no unusual sexual appetites. Matters deteriorate, his wife becomes alarmed, and when he makes an advance on his step-daughter, his wife informs the police. Her husband is compelled to go and have therapy – but that doesn’t deter him. Indeed, he harasses women at the centre at which he is being treated.
A prison term seems inevitable. But just before he is due back in front of the courts, he begins to complain of headaches. He is rushed to hospital, where a scan reveals a massive tumour. Once this tumour is removed, his behaviour returns to normal. The story has a further twist. After several months, his aberrant behaviour returns. An investigation reveals that the tumour has not been entirely eradicated. A further operation sees the man back to his old self once again. In this case most people will probably feel that the man with the deviant sexual behaviour wasn’t really free. It was, as it were, the tumour that was responsible.
But we are all physical beings in a (largely) deterministic universe. Why is one physical cause – a tumour – different from any other? Might, in future, neuroscientists be brought into court to explain away all manner of transgressions, for example: “This man can’t be held responsible for his shoplifting – it was due to his unusually high levels of dopamine.” There is evidence that some people treated with dopamine for Parkinson’s have problems with impulse control, sex and gambling, for example.