Science offers a framework for understanding human behaviour and a basis for moral choice. This, Paul Davies says, could point the way to a new ethics

In a few days, General Augusto Pinochet will leave these shores for his native Chile amid a welter of recriminations. He stands accused of the kidnap, torture and murder of thousands of his fellow citizens. On the grisly scale of genocidal horror, the world has seen far worse than Pinochet - Hitler, Stalin and Pol Pot killed millions. The recent slaughter in Rwanda and ethnic cleansing in the Balkans are timely reminders that deliberate, systematic human savagery is never far beneath the surface veneer of civilisation.

The conduct of the men involved in terror and extermination campaigns is so extreme it can only be described as evil, which raises the question of how these predilections became part of human nature. Why do people have a capacity to inflict such misery on others?

The problem of evil has baffled theologians and philosophers for centuries. If there is an omnipotent God who is supremely good, why does he not intervene to prevent gross wrongdoing? One traditional answer was to portray the universe as a battleground between opposing forces of good and evil, with humans caught in the crossfire. Another was to argue that evil is the price paid for human free will, which is on balance a greater good. Today, however, more rational explanations are called for.

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From the scientific viewpoint, human wrongdoing is not hard to understand. We are the products of Darwinian evolution, with its central mechanism of natural selection. Oxford zoologist Richard Dawkins coined the term "the selfish gene" to make the point that we dance to the tune of the genes that are good survivors, even if that means we may sometimes act ruthlessly to ensure they reach the next generation.

If rape, murder and theft prove good reproductive strategies (they often do), it is no surprise to a Darwinist that people resort to these acts given an opportunity.

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But doing bad is only half of the story. What about good? For every Hitler, there is a Mother Teresa, for every Stalin an Albert Schweitzer. How do we explain the many individual acts of selfless generosity, the sacrifices people make for others, the pervasive influence of brotherly love in shaping our social conventions and institutions? How do decency, democracy and the rule of law triumph in the face of innate selfishness?

The easiest examples of altruism to explain are those involving kinship. In crude terms, it makes sense for a mother to sacrifice herself for her children because each child carries half her genes. As siblings share genes, it pays for them to "look after their own". And kinship altruism is not confined to humans. Many birds and mammals endure sacrifices to protect close relatives, while some insects are far more altruistic than we are.

More puzzling are cases of altruism directed at unrelated individuals and even total strangers. Some people are prepared to engage in acts of humanitarian charity, even to rivals and enemies. Why? In the 1970s, the biologist John Maynard Smith applied game theory to animal behaviour.He proved that cooperation between competing individuals can sometimes be mutually advantageous.

One easy-to-understand example involves reciprocal altruism. I do you a favour today, you do me a favour tomorrow. A great deal of social order, such as government welfare, falls into this category. Since cooperative strategies can work well and aid survival, we might expect the genes that prompt us to adopt altruistic behaviour to be selected in the great Darwinian lottery.

Although genetics and game theory can explain the broad features of good and bad in animal species, some scientists are still baffled by the enormous propensity for evil that human beings possess. Chimpanzees are observed to murder and rape when the opportunity arises, male lions regularly kill the young of new female recruits into their harems, and many birds are highly accomplished at sexual cheating. But only humans deliberately torture or systematically slaughter helpless individuals.

Significantly, Homo sapiens belongs to the only family of mammals with just one representative. All other species of Homo, such as Neanderthal man, were annihilated by our ancestors. Did something go wrong along the evolutionary way, leaving Homo sapiens uniquely prone to extreme transgression?

Whatever the explanation for the dark side of human nature, not many people use the terms good and evil these days. The concepts seem somewhat anachronistic when one is appealing to psychology, neurology and genetics to explain human behaviour.

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This abandonment of old categories of judgement has been accompanied by a reappraisal of traditional moral values. Right and wrong have been replaced by socio-political categories such as rights, responsibilities, entitlements, freedoms and equity.

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Morality is being sought not in ancient scriptures, but within us, in our genetic and psychological make-up. Moral absolutism has been replaced by moral relativism - a world in which alternative value systems are deemed worthy of equal consideration.

Even in my lifetime, I have witnessed moral reversals. Homosexuality was once regarded by many as wicked. Today we view attempts to suppress sexual freedom as morally offensive. Not only are advances in science and technology reshaping our moral universe, they are presenting us with new ethical choices. Is it right or wrong to clone a human being? Should genes for some diseases or psychological defects be removed from the human genome? Divine guidance is of little help: the answers will not be found in the scriptures. Since nobody will tell us for sure what is right or wrong, we have to work it out for ourselves.

To appreciate the magnitude of the task, consider the issue of genetic engineering applied to humans. At the moment, tinkering with the human genome is widely regarded as dangerous and repugnant, and it is unlawful in many countries. Mostly the objections are a reaction to scare stories about designer babies, or creating races of super-athletes, mad scientific geniuses or Hitler clones. But what if gene manipulation could turn humans into a species of Good Samaritans? Wouldn't the world be a better place?

The project to map the human genome is forging ahead, and soon a complete recipe for a human being, written in genetic code, should be available. Just suppose it were possible to identify a set of "evil genes". This raises a profound ethical conundrum. Would it be right to eliminate these genes permanently from the human genome? To solve it, we must confront a stark question: is our grotesque capacity for evil part of what makes us human and therefore to be regarded as sacrosanct and preserved?

A more immediate dilemma concerns individual choice in determining the make-up of future generations. Most people would sanction an abortion on the grounds that the future adult was at high risk from disease. But what if the parents wanted an abortion because genetic screening showed the foetus failed to measure up to their required standards of sporting or musical potential?

Parental selection on grounds of sex has long been practised in societies in which male children are prized. This takes place either by aborting female foetuses or by infanticide. In China, men outnumber women by millions. The systematic elimination of females provides a salutary lesson that if fashion and prejudice are allowed to dictate the genetic mix, gross distortions of society's make-up may result.

There is an urgent need to reappraise our concepts of right and wrong and to develop an ethical framework appropriate to the scientific opportunities and challenges of the future. Science may never replace the secure ethical certainties of traditional religion, but it does offer a rational basis for moral choice and a framework for understanding human selfishness. The father of sociobiology, Edward O. Wilson, has a vision in which ethics is brought within the scope of systematic scientific inquiry, forming a grand synthesis of science and the humanities for which he has appropriated the term "consilience". He leads a new breed of social commentators who argue that the inner core of human goodness should not be ignored, but nurtured by informed contemporary analysis. To meet the awesome ethical challenges of the future, a form of moral progress is needed that embraces the findings of science, rather than shies away from them.

Paul Davies is visiting professor of physics at Imperial College, London, and honorary professor of physics at the University of Queensland.

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