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One thing that modern Darwinism can pride itself on is the quality of its popular writing. Charles Darwin set the precedent for this when Alfred Wallace forced him to abandon his great work on natural selection and quickly compose a summary, The Origin of Species, that instantly became a best-seller. Since then, a surprisingly large number of writers have followed with books that bear comparison with the best science writing in any field and whose standard of literacy, clarity and sheer readability put much non-scientific writing, popular or otherwise, to shame. Today excellent books on evolution come out so often that even the most avid reader finds difficulty keeping up. A survey of the shelves of any bookshop stocking popular science shows a large proportion of titles directly or indirectly to do with Darwinism.

Robert Wright's new book not only continues the tradition of outstanding popular science writing about Darwinism, but carries it to a new high. It is by far the best and most complete single account of evolutionary psychology presently available. He uses Darwin's own life and writings as examples to illustrate and explain some often complex and counter-intuitive ideas. The result is that we get not only a comprehensive summary of the present state of knowledge on almost all aspects of the subject but something of a new biography of Darwin too. For example, Wright illustrates current evolutionary thinking on human co-operation and self-deception by contrasting the accounts that Huxley and Hooker sent to Darwin about their famous confrontation with Bishop Wilberforce over Darwin's theory. "Both Huxley and Hooker were telling stories that would do two things: raise their stature in Darwin's eyes, and leave him indebted to them," observes Wright.

This works well most of the time and Wright portrays Darwin as very much a modern Darwinist, at least in spirit, if not always in word. But making Darwin an evolutionary psychologist is more difficult. "Natural selection," claims Wright, "never promised us a rose garden. It doesn't 'want' us to be happy. It 'wants' us to be genetically prolific." Darwin, however, wrote in his autobiography that "according to my judgement, happiness decidedly prevails". He added that "some other considerations, moreover, lead to the belief that all sentient beings have been formed so as to enjoy, as a general rule, happiness". This is what Freud was to call the pleasure-unpleasure principle: "An animal," said Darwin, "may be led to pursue that course of action which is most beneficial . . . by suffering . . . or by pleasure."

As Wright remarks, "evolutionary analysis often lets us figure out the role of genes without worrying about the nuts and bolts of their influence". The result is a kind of genetic behaviourism, with genes replacing conditioning and adaptation playing the role of reinforcement. But the problem is that genes cannot exercise moment to moment control of an organism's behaviour. There is no instruction written in the human genome that says: "Go forth and multiply." Genes are coded instructions for constructing and maintaining the organism, not control programmes like those that run robots. In a world without contraceptive technology, where natural selection sets the parameters for human behaviour, the intensity of pleasurable sensations associated with sex would usually have had the effect of making us want to be genetically prolific, even if such a thought never consciously occurred to us. You would think that Freud's discovery of the importance of sex in human motivation would find as ready an endorsement from evolutionary psychology as Darwin's anticipation of the pleasure principle would find a central role in explaining how genes influence behaviour. But, as Wright's book shows, you find neither.

The reason it is difficult to make Darwin into an evolutionary psychologist is that Darwin's own approach was much more psychological and more concerned with the personality traits and emotions that classical behaviourism anathematized and that genetic behaviourism largely ignores. Knowing nothing of genes, Darwin surprisingly believed in the inheritance of acquired characteristics and passed both this and his concern with emotions on to his principal 20th-century psychological heir, Freud. Here, as elsewhere in the book, Wright, "who is a senior editor of The New Republic, shows himself to have a better grasp of the issues than some of the authorities he has consulted. He frequently sees the importance of Freud and wants to bring him in: "Like the new Darwinism, Freudian thought finds sly unconscious aims in our most innocent acts. And like the new Darwinism, it sees an animal essence at the core of the unconscious." But Wright has Hookers and Huxleys of his own with stories about Freud, so Darwin's greatest psychological successor remains peripheral and Wright ambivalent. Nevertheless, he generously concedes that "for all the criticism it has drawn in recent decades, Freudianism remains the most influential behavioural paradigm - academically, morally, spiritually - of our time. And to this position the new Darwinian paradigm aspires."

Wright criticises Freud's model of the mind as "insufficiently labyrinthine" and claims "a cynicism deeper than Freudian cynicism" for the new evolutionary psychology. When faced with many questions about particular psychological adaptations such as remembering and forgetting, evolutionary psychologists "can relax and come up with different explanations for each one". Essentially, this is what classical behaviourism did. Behaviourists such as Ivan Pavlov claimed that any cue could be used as a conditioning stimulus for any response. But experience has shown that a conditioned reflex that approached too close to an innate behavioural trait would soon coalesce with it. Pigeons, for example, could not be conditioned to stop pecking at keys, even when the pecking resulted in the pigeon starving to death. Pigeon behaviour, it seems, is not quite as labyrinthine as behaviourists believed because pecking means finding food for a pigeon, however it has been conditioned. What explains this is, of course, evolution. It would be ironic indeed if it turns out that present-day evolutionary psychologists have made a similar mistake where human behaviour is concerned.

Christopher Badcock is reader in sociology, University of London.