Evolution may be driven as much by attraction as by fitness, Ayala Ochert reports.
Skim the personal ads of any newspaper and you will find people looking for partners with a "good sense of humour", an "interesting personality" or "intelligence".
For Geoffrey Miller, 30, one of the new breed of evolutionary psychologists, this counts as evidence of the important criteria in human mate choice.
Just as peahens find a long, brightly coloured tail irresistible in a peacock, men and women demand "creative intelligence" in their choice of partner, says Dr Miller, who completed his PhD three years ago and is a lecturer at Nottingham University.
Dr Miller regards language, art, music, humour, even ideology as cultural courtship displays, and therefore science can explain how they arose by using evolutionary theory in the same way they use it to explain the existence of peacock tails.
Peahens had a preference for big tails and peacocks had genes that produced big tails: thus, the peahens' preference whirled out of control as the peacocks' tails grew bigger and bigger. This positive feedback process is well known to biologists as "runaway sexual selection". Sexual selection is a kind of offshoot of the theory of natural selection: males and females choose each other as mates because of particular characteristics which thus get passed on to their offspring. Individuals whose characteristics are not so desirable to the opposite sex do not get to mate so often so their traits get passed on to fewer members of the next generation.
Though evolutionary psychologists now study male and female mate choice (Darwin considered female choice), the theory was scorned by Victorian male scientists.
Dr Miller says this sexism marginalised the concept of sexual selection and natural selection was the only model used.
"About five years ago I thought that runaway sexual selection must have something to do with the evolution of the human brain, because, aside from academics, what a lot of people use their intelligence, their wit, their creativity for is courtship when they are young adults," he says.
The size of the human brain is too great, and language and creativity too sophisticated, to be explained by natural selection alone. Brain size has tripled over the past two million years for no apparent reason. Miller looks to love of novelty, or "neophilia" for an explanation. Darwin observed that many animals, particularly humans, have this neophilia. "A preference for novelty in our hominid ancestors just happened to be strong enough to produce runaway sexual selection for the production of novelty in behaviour, or what we call creativity," says Dr Miller.
He is critical of an intellectual climate that has suppressed what he takes to be rather an obvious suggestion: "Academics have a completely skewed, biased view of what intelligence is," he says.
"The long dichotomy between sex and reason, between passion and intelligence, between human powers of abstraction and our animal powers of lust has forced people to think that there could be no possible connection between sexual selection and human intelligence. Sex is just not considered an appropriate, respectable subject in psychology, even today, because it's too passionate, it's too emotional.
"But, if you were to step back, you would say that precisely those things that people get most passionate and emotional about should have been the most important selection pressures. Precisely because people talk about sex and romance all the time in all media, in all cultures, one might think that it's an important part of human life and human evolution."
Miller stresses that his own theory does not imply big differences between the sexes: "Even if it was only females that were choosing mates for their creative intelligence, the daughters as well as the sons of highly intelligent men would tend to inherit intelligence." But, though he rejects the idea that there is any difference in mental capacities between the sexes, he does suggest that men might have a greater motivation for producing "public cultural displays" such as being politically active, public speaking or writing books. One reason is that our male ancestors had the advantage of time that our female ancestors were forced to spend either pregnant or breast-feeding.
Dr Miller is concerned about how his research might be interpreted by those with less liberal views. "In modern society there are lots of sexist, institutional biases that keep women from producing culture to the extent that they would like. So, in no sense should a sexual selection theory of culture be taken to imply that women can't or shouldn't or won't produce culture."
Like many evolutionary psychologists, Dr Miller is on guard against any connection between his work and past political uses, such as the eugenics movement (which advocated enforced sterilisation), which have given the Darwinian approach such a bad name. "There are lots of groups that will pick up on snippets of scientific hypotheses and use them tactically as part of their ideological warfare. Our defence against this is not to take the ivory-tower approach and say that science is value-free. The best we can do is to develop the widest possible range of interpretations of our findings."
He says that a crucial difference between real evolutionary psychology and the critics' version is that evolutionary psychology is not interested in whether variation between individuals can be accounted for by genetic differences: he is only interested in those human qualities that distinguish us from other animals. Evolutionary psychologists think that all modern humans are practically identical in genetic terms.
When Dr Miller met some of the founders of evolutionary psychology, Leda Cosmides, John Tooby and David Buss five years ago, he was still at graduate school in Stanford, California. He calls it the turning point in his intellectual life. But it is Darwin whom he truly admires for providing "an understanding of universal human nature".
Dr Miller had become increasingly dissatisfied with the inability of conventional psychology to come up with such an explanation: "For 100 years psychologists have agreed that psychology is not unified, but I thought, 'there is no reason why it should not be. We are all the same species, we all have similar interests, aptitudes and concerns. There is a human nature to be found, and psychologists are not finding it.'" Human nature "seemed to be falling through the cracks between the arts and the human sciences". Miller hopes that evolutionary psychology can now provide the cement. Its universal theory of human nature could, Miller believes, usefully underpin political and economic theories.
Dr Miller is ambivalent about using insights from evolutionary psychology in his everyday life. "We do not need to keep a tight rein on our mental adaptations. If we fall in love, then we fall in love just like anyone else, and our little love module works and it produces courtship behaviours and commitment." But, equally, "it can be tremendously useful to have some psychological insight when you are in a relationship, because at least you can override some of your knee-jerk reactions."
Born in Ohio in the US, he is soon off to be a research fellow at the Max Planck Institute for Psychological Research in Munich in the new Centre for Modelling Adaptive Cognition. There he hopes to study "human happiness and how it is actually achieved".
He advises young researchers to do as he did and "find a problem that has an obvious solution that nobody has dared to talk about before because of social and cultural biases".