It is often supposed that all shreds of human agency succumb in the face of advances in the understanding of evolutionary process, genetics and brain function. Conventional wisdom collapses and all responsibility for the consequences of our actions is diminished to the point at which, it is claimed, no blame can be attached to anything we do.

Or so the argument goes, but is it really the case that science has had such serious implications for the way we should think about our own capacity for choice? The importance of the emotions in controlling human behaviour certainly suggests to some that all of us are in the grip of our instincts and our genes. We seem to be surrounded by examples of irrational behaviour, such as when people are in love, in lynching mode or maddened with war fever. The brain (and the genes that contribute to its construction) are such that, when people make conscious choices, they don't really know what they are doing and if so the presumptions of law, morality and common sense must be wrong.

In 1979 the Mayor of San Francisco and one of his officials were gunned down by one Dan White. At his trial White was convicted of manslaughter instead of the first-degree murder of which he was accused. His lawyers produced an original argument which came to be known as the ‘Twinkie defence’.