The blank slate is not an uplifting image. Nobody admits to believing in it.

And yet it is the vessel in which a secular society has placed much of its hopes for freedom, equality and progress. How much can a person achieve, given the right opportunities? How much freedom of choice is theirs to exercise? We are not sure, and the blank slate contains the benefit of the doubt.

These questions are not nearly so open as they used to be, though. Psychologists of various stripes, drawing upon genetics and a little neuroscience, have accumulated a daunting body of evidence in support of the case that our slates are run up for us before we ever see daylight. Personalities, they say, can be divided up into five or six dimensions, which are largely inherited and resist modification thereafter. Intelligence, likewise, is variable and largely inherited. Human nature is universal. Male and female psychology differs by evolutionary design. And much more along these lines, they promise us, will be revealed in the next few years, thanks to the Human Genome Project and the other ships in the genetical fleet.

It's all rather a lot to swallow, as is Steven Pinker's insistent compendium. Of course, the evolutionary psychologist is riding a wave. As his colleagues said to him when he outlined his project, nobody really believes that children come into the world with minds uncoloured and unmarked. It seems reasonable to suppose a consensus among Pinker's potential readers, that children seem to be born with personalities, and that boys are born different from girls. Beyond a few agreed generalisations, though, all is enveloped in an angsty fog. Did you think Jade [a contestant in the TV show Big Brother] was like that because she had not enjoyed a rich educational environment, or because she was born too close to the lower end of the intelligence scale? If the former, would you have been so ready to give her the benefit of the doubt had she been a Sloane? 'She comes from a deprived background; he is an upper-class twit' may have a superficially attractive moral veneer. But there is no virtue in wishful thinking or woolly contradiction.

Wishful thinking and woolly contradictions appear to be human nature, though; and therein lies the value of The Blank Slate. Pinker poses a bracing challenge to most readers' ideas about what people are and what they could be. If you do not agree with his propositions, his arguments require that you work out what you do believe - or admit the charge of denial.

There will be those who find Pinker himself guilty of this universal offence. One reason to be suspicious of Pinker's vision of human nature is its uncanny correspondence to the world as mainstream opinion currently takes it to be. Pinker's reading of the data insists that individuals are innately unequal, but dismisses differences in intelligence between men and women as trivial. He skips over the fundamental question in the IQ debate, the gap in scores between black and white Americans, in a paragraph: his nerves show in the brackets he puts around it, as if trying to fence it off from the rest of the text. Others interpret the same data very differently, interpreting differences in test scores as indications that there are significant differences in abilities between the sexes, and between races. The first might just be acceptable to the audience Pinker is trying to reach; the second absolutely would not.

Pinker also leaves himself open to charges of a more transparent form of denial, in his reluctance to admit that his 'new sciences' of human nature have anything to learn from social sciences. He also has a tendency not to give the opposition - for that is how he seems to see it - its due. When discussing rape, for example, he dismisses outright the notion that it is about power rather than sex. Rapists, he says, "tend to be losers and nobodies": he does not acknowledge the evidence that they tend to be the victims' partners, for that would be acknowledging a situation in which it is easy to see that rape could be used as a means to intimidate and control. With Pinker it has to be one or the other: in rejecting the "rape-is-not-about-sex doctrine", he rejects the possibility that it might be about both sex and power.

Then there is the question of sex and power in the workplace. What if we allow that women are naturally less inclined than men to compete for high status, and more concerned about spending time with their families? As both his ideology and his psychological science are based on individuals, Pinker is inclined to be content with this state of affairs. But it is certainly not as far as evolutionary psychology can go, or as far as a decent liberal should. If you accept that men and women have different psychologies, evolved to serve different interests (as even Stephen Jay Gould did) you should not be comfortable with a situation in which institutions are controlled by members of one sex, since they are likely to favour their own sex's interests. And from what we understand of ourselves as primates, the males of our species are apt to form coalitions that exclude females, which might also have something to do with the lack of women in the boardroom.

So it is not simply a question of naturalists against nurturists, but also of how one interprets the lessons of nature. Pinker divides ideologies into the Tragic Vision, which broadly corresponds to the Right, and the Utopian Vision, which is broadly of the Left. Tragedians think that there is little point in trying to do things for others, as their fortunes depend overwhelmingly upon their own efforts and abilities. Utopians think that human nature can be changed. The tragedy of their error is all too clear now that communism is history; and the decline of non-utopian leftism makes it easy to blind readers with totalitarian atrocity. But it is still possible to want to make things better, and to use the 'new sciences' as a guide to new possibilities.

These new sciences lack resolution, though. Even if Pinker is right about the innateness of personality, and parents' lack of influence over their children - a point which readers will be likely to reject from experience, rather than through denial - these are only the outlines of a person. Instead of a blank slate, we might think of the inherited self as an unfurnished house: what we put in it does not change the structure, but it makes all the difference.