Natural selection has imprinted the capacity to commit murder on the human psyche according to a controversial new theory, Stephen Phillips writes

They appeared to be the model couple, yet by the end of the evening, David Buss found himself trying to talk the husband - his friend - out of doing anything rash to his wife. He had become incensed after she had publicly belittled him at a party and flew into such a "homicidal rage" that Buss started to fear for his own life.

Then his friend seemed to cool down. But later that night, the couple got into a blazing row that ended with him threatening to kill her and shattering a mirror with his fist. He later told Buss that if he hadn't got out of there right then, there was no telling what he might have done.

His wife fled and the couple haven't spoken since; their divorce was negotiated through lawyers.

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The incident left Buss chastened - but it has since come to make rather horrifying sense to him. The new theory of murder that the professor of psychology at the University of Texas at Austin is about to publish suggests that only his friend's prudence in removing himself from the situation that night - and his wife's instinct for self-preservation - averted tragedy. Buss argues that not only his friend but everyone is a potential killer. We have, he says, been hardwired with the capacity to kill.

Buss, tall and slim, in sports jacket and slacks, resembles an off-duty business executive as he dissects the "design features" of homicides and "decision hierarchies" in the minds of prospective murderers. Then the computer slide show that he uses to explain his unsettling theory flashes up images of bone caches and a sabre-wielding warrior brandishing the severed head of a child, cutting through the corporate-style presentation with a graphic reminder of the subject.

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Historically, Buss says, killing has conferred such powerful advantages in terms of the overall fitness of our species that natural selection has ingrained the impulse in the human psyche. Slaying a rival gave an individual better access to mates or heightened their status, with a corresponding boost to their ability to attract mates. In the lawless Pleistocene epoch, when modern human minds were forged, a capacity to kill represented a useful behavioural gambit. Hence murderers' genes reproductively prevailed over those of their victims and their traits were transmitted to future generations.

The advent of law and order has criminalised killing in most societies, invoking stiff sanctions and public opprobrium to reduce the pay-off. But the instinct to kill lives on today, a mental circuit that lies largely dormant but may be tripped by certain events, Buss says. "I believe we all have the capability to commit murder."

It is a disturbing (some might say appalling) theory, dispensing with the received wisdom that murder is an aberration, born of pathological disturbance or environmental brutalisation. The theory, expounded in full in Buss's forthcoming book, The Murderer Next Door: Why the Mind Is Designed to Kill , is billed as "the most penetrating, most comprehensive and most scientifically sound theory of murder ever proposed" - a sort of grand, unified thesis of the ultimate crime.

Buss is no crank. Texas is the latest stop for the 52-year-old in an academic career spanning Harvard and Michigan universities and the University of California, Berkeley. He is now a leading figure in the burgeoning but controversial field of evolutionary psychology, but Buss stumbled into the area. One of his undergraduate essays postulated that men were driven to better their status to gain access to women, with the express intention of scandalising his professor. "I was just being controversial," recounts Buss. "But he asked me to present it to the class."

It wasn't until he was a young assistant professor at Harvard, having established himself with contributions to personality psychology, that he turned to neo-Darwinism in earnest as a way to understand human motivations. "Results kept coming back confirming hypotheses, so I thought, I'm on to something," he recalls. Still, it was a risky move. At the time, he was without tenure and it entailed mastering a whole new body of knowledge.

His work was largely on mating. But driven by a hunch that mating was perhaps heavily involved in murder, he pondered a separate study. This hunch was given further momentum by the results of a questionnaire that he innocently tendered to his undergraduates: "Had they ever considered killing someone and, if so, what brought it on and what thoughts had they entertained?"

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It was just a "thought experiment" to stir their interest in his class on murder, he says. But leafing through page after page of lurid homicidal fantasy, uncannily corresponding to the distribution of situations in which actual murders occurred, Buss was bowled over. "I had to follow this up," he says. He was being pulled "more and more into the dark side of human nature".

Dissatisfied with existing explanations, he cast a dragnet over the annals of murder, combing case files, poring over perpetrators' testimonies, crunching data from international studies and weighing evidence from archaeological finds and the animal kingdom. He considered records of "chimpicide", lions usurping rival males, "vicious" hyenas and "50 per cent of wolves dying at the hands of other wolves". He mulled over human skulls exhumed from ancient burial sites that have fractured left temples, indicating right-handed attackers. And in his office, he hefts a Maori war club called a "mere" (pronounced "merry") and a Hawaiian headsplitter as examples of the universality of killing across cultures throughout history.

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From his studies, it emerges that most killings are committed across a narrow range of archetypal situations where mating and reproductive interests are at stake - for instance, a killer taking out a romantic rival, a lover who jilted him (so no one else can have her) or avenging a public slight that derogated his status. The masculine gender is not incidental. Murder is an overwhelmingly male phenomenon, Buss found.

The vast majority of killings in the 37 countries he looked at were male on male.

Moreover, court records, personal testimonies and interviews suggest that, far from being deranged psychopaths, most killers are sane, both legally and by wider societal standards. The Murderer Next Door marshals an impressive weight of data to support its disquieting conclusions.

At one point, Buss almost abandoned the project. Called to testify as an expert witness in the case of a Michigan man charged with stalking and killing his ex-lover, he sat through six hours of taped phone conversations between the accused and the victim. "It was heart-rending to listen to the agony in her voice, then realise she was dead," he recalls. The experience prompted him to consider quitting his research. But he persevered. Far from inuring him to human suffering, Buss says his work has given him greater empathy for victims.

He expects his theory to get a rough reception from some. Evolutionary psychology remains deeply contentious for mainstream psychologists and social scientists, who prefer to see the mind as a blank slate shaped by social and cultural conditioning rather than as the seat of innate drives.

Buss is unperturbed. Discussing the current sense of excitement and discovery among evolutionary psychologists, he sheds his slight air of academic abstraction. "I feel like an explorer finding a new continent and being the first to discover a new mountain range."

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Anticipating criticism that he's absolving murderers of personal responsibility - what he calls the "my client couldn't help it, the homicidal circuits made him do it" defence - Buss denies setting up a deterministic world where people are enslaved by compulsive murderous tendencies. Killing is just one of myriad finely calibrated, deeply evolved human behavioural strategies that significantly also include co-operation, altruism and self-sacrifice, he counters. "We're in the grip of these deep-seated psychological circuits, but the saving grace is that there are many of them. Some are gratifying, some horrific. Some give us homicide, others love."