The idea that parents shape their children's personalities is so ingrained, and still supplies so many psychoanalysts with their livelihoods, that any challenge to it is bound to meet a lot of resistance. Yet the evidence has been getting more and more clear: variations in personality are determined by a combination of genes and random influences, but not by parents. The central premise of Freudian analysis - that childhood events cause adult psychological problems - has been shown to stand on no good evidence whatsoever. Says Harris: 'The evidence does not support the view that talking about childhood experiences has therapeutic value.' Remember, in the early twentieth century all the advice to parents stressed discipline; in the later part of the century, all the advice stressed indulgence. Yet there is absolutely no evidence that this caused a shift in human personality in the Western world. Because people wanted there to be something they could do about our actions and tendencies, they argued that there must be an agent to blame. The nurture assumption was fuelled by many factors - worries about a return to Nazi eugenics, Rousseau-esque idealism, the doctrines of Marx, Freud and Durkheim - but the root of its appeal lay in the need to think of somebody being in charge. Instead, the truth is that personality unfolds from within, responding to the environment - so in a very literal sense of the word, it evolves.