Sara Abdulla explores the burgeoning subject of evolutionary psychology. In human behaviour heredity furnishes the warp and cultural habits the woof; the warp remains everywhere much the same", said George Peter Murdoch in The Science of Culture. Philosophers have struggled with the "warp" or "human universals" for two millennia; nearly 30 years ago they were joined by academics from a range of disciplines exploring a new approach to understanding human behaviour. The result: evolutionary psychology.

Behaviourists, who held sway for nearly 50 years, thought that learning was accomplished by a system of punishment and reward operating on a few general purpose mental circuits (eg love, fear, rage). Evolutionary psychologists, by contrast, seek out the highly specialised mental subsystems that they believe arose to solve particular problems humans faced in adapting to their changing environment over millions of years.

Few would dispute that molars and incisors are adapted to specific tasks of grinding and cutting. But to identify specialised cognitive mechanisms for remorse, honour or gratitude is much more difficult - some would say impossible. Roger Shepard, the cognitive psychologist at Stanford University who discovered marked differences in male/female mental rotation ability draws comfort from the fact that "great strides have been made in carving peripheral mental processes, such as vision and more recently, language into constituent subprocesses".

One of the key misconceptions of evolutionary psychology arises from our view of history. History, as most of us conceive it, stretches not much further than the ancient Chinese, Egyptian, Indian and Sumerian civilisations. But before that our ancestors spent two million years as Pleistocene hunter-gatherers and before that several hundred million years as foragers. Evolutionary psychology is predicated upon a belief that the human mind is adapted to cope with life as a Pleistocene man, and may therefore be ill-equipped to deal with many aspects of modern life.

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The other commonly misinterpreted axiom of evolutionary psychology is Herbert Spencer's now infamous tenet, "the survival of the fittest". To understand evolutionary theory we must, explains Martin Daly in his book Homicide, acknowledge that "personal survival is not the bottom line on the natural selection ledger. Over generations it is successful traits that survive, not individuals. This sort of survival depends not only upon the longevity of those carrying the traits but also on the abundance of their progeny". Which explains why parents (throughout the animal kingdom) sacrifice their own health or safety to increase that of their offspring.

There are theorists across the spectrum from biology to the social sciences who dismiss much of evolutionary psychology as "common sense". They see many of what evolutionary psychologists define as "human universals" as nothing but learned cultural artefacts. Daly is incensed by this kind of attitude: "There is no more mischievous dichotomy than 'social' versus 'biological' - sociality has no meaning outside of the biological world". Leda Cosmides, one of the founders of evolutionary psychology, based at the University of California, Santa Barbara, agrees: "Learning cannot take place without some kind of innate hardware".

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But the critics will not be silenced without some hard proof. As Canadian psychologist David Sherry puts it: "To establish a real credibility evolutionary psychology must answer questions that could not be answered any other way". He, however, is cautious, saying firmly that "it is scientifically incorrect to infer human adaptations from those of non-human species". Which makes it difficult to use Sherry's studies into increased hippocampal (the area of the brain responsible for special ability) size in certain types of bird to conclude that a clutch of similar adaptive motors may have shaped the human mind.

Marc Hauster, of Harvard University's neuroscience programme, uses a mixture of philosophy and experimental evidence to highlight the similarities and differences between humans and the rest of the animal kingdom. He summarises the contemporary position thus: "Because of language, humans are expected to have concepts that non-human animals lack. And those concepts that they do share with non-human animals should be theoretically more sophisticated in their power to account for both real and imagined events."

He explores this by focusing on one conceptual domain - number - that is often assumed to require language. Many non-human animals have some concept of number, which they use for assessing group sizes and food quantities. But a series of studies indicate that prelinguistic infants and chimpanzees seem to have primitive computational awareness, at least up to the number four. Even more fascinating is the discovery that adult rhesus monkeys have the ability to differentiate between types of objects, a skill which infants gain only when they have mastered object-naming at around one year old. This suggests that language is not actually necessary for property/kind discrimination as was previously assumed.

Is evolutionary psychology not a rather deterministic, pessimistic and even possibly eugenic discipline? Leda Cosmides again: "Is a myopic doomed to live in a blurry world? No - thanks to the study of the eye. Was TB cured by denouncing it? No - a cure was found by knowing about how it worked. In the same way by understanding what kind of mental programs we have you might be able to figure out what method of teaching maths will be the most effective. Or you might figure out how to change the way people think about gender. But if you don't know the structure of our cognitive programs and you try to change the world, you are like a surgeon operating with a blindfold - there is likely to be more blood than healing."

Sara Abdulla is science writer in residence at the Ciba Foundation, which this week organised a symposium on "Characterising Human Psychological Adaptations."

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On the trail of selfish genes.

Martin Daly and his wife, Margo Wilson, have been seeking "ecologically valid windows on real world behaviour" since the late 1970s, primarily through close analysis of contemporary and historical murder statistics. Wilson originally "had the brainwave of using homicide data as a window on the sources of marital conflict". But both of them were soon hooked on the idea that "police records and data archives provide an assay of interpersonal conflict in general".

The couple met in 1977 while doing their postdoctoral studies in psychology at McMaster University, Ontario (where they still work). They did not become especially interested in human evolutionary psychology until a student in a sociobiology seminar questioned the cross-cultural ubiquity of the "Cinderella" syndrome: are stepchildren really mistreated or is it a myth? Astonished to find that the question had not been addressed in the child abuse literature, Daly and Wilson set about answering it themselves.

Using the national homicide archive and the Chicago police department's homicide reports they made some disturbing discoveries. It emerged that, "young children incurred about seven times higher rates of physical abuse in step-plus-genetic-parent homes than in two genetic parent homes and the difference in fatal abuse was 100-fold". Does this indicate that violent personalities often re-marry or that step-relationships predominate in high risk-groups such as the urban poor? Why then do abusive step-parents typically spare their own children? Why were 82 per cent of children slain by step-fathers beaten to death when the majority of children slain by genetic fathers were killed by less gruesome means?

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"Current theory implies that natural selection shapes social motives and behaviour on behalf of blood kin. Parental care is costly and animals have evolved a variety of psychological mechanisms to protect parents against parasitism by unrelated young," the pair conclude. In humans Daly and Wilson interpret step-parental investment as "mating effort" (part of the cost of courting the prospective mate) but even so "step-parents do not typically experience the same child-specific love or commitment nor reap the same emotional rewards from unreciprocated parental investment as do genetic parents". Many societies have found cultural solutions to this problem, for example the practice of a widow marrying her dead husband's brother ensures that her new husband has some "genetic interest" in her children.

In their book, Homicide Daly and Wilson widened the psychology of the homicide-data-searchlight to illuminate many other areas of social interaction including courtship, marriage, kinship, and warfare, leading them to posit that "several distinct types of interpersonal relationships are sufficiently ancient and important to have affected the functional organisation of our evolved social psyche". "Although there is intriguing cross-cultural diversity in human kinship systems, there are also some telling universals", they say.

Their assessments of fatherhood and male/female relations (men commit the vast majority of all homicides) is the most compelling. Examined in conjunction with an abundance of animal data Wilson and Daly's homicide analyses go a long way to explain why patriarchy is globally the social norm. If we agree that homo sapiens's number one motivation is to continue his genetic line then this throws up two fundamental problems for men. First, a man, unlike a woman may never be 100 per cent certain that a child is his. Second, if some men have multiple partners, many men will have none and most women will have at least one. Daly and Wilson's analyses provide some of the most convincing evidence that this fundamental schism in male/female reproductive confidence has shaped the human mind the world over.

Virginity and chastity in pre-menopausal women is fiercely guarded and socially hallowed the world over. Why? To minimise wasted paternal investment. Likewise society condemns female adultery and is relatively indifferent to male infidelity. Why? Again because it compromises a man's paternal certainty. And young adult males are the most violently competitive section of society. Why? Because at this age they must establish their social position in order to strengthen their chances in the competition for limited resources - ie women!

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