Doctors theorised that the tumour had restricted blood supply to the area of the brain associated with impulse control. Last year the tumour came back, and so did the paedophiliac urges. He has had a second operation and, for the time being, appears to be his old decent self.

Most cultures in the world believe that virtue and vice involve an individual’s ability to distinguish right from wrong, and to freely choose one or the other – unless they are insane or acting under unbearable duress or intoxication. The laws of most societies assume moral decisions are made in the conscious, rational mind. But an influential number of brain researchers disagree. They point to cases like that of Mr C to argue that it is not people, or minds, that commit bad acts, but their brains.

As Roy Fuller, one of the men who invented Prozac, notoriously declared, “Behind every crooked thought there lies a crooked molecule.” We would say, “The man raped a child,” but the brain scientist would say something like: “A decrease in the subject’s serotonergic neurotransmission, due to a decrease in his level of serotonin, led to behaviour disinhibition.”

Believing that brains cause responsible acts, good and bad, focuses sharp attention on a rapidly expanding discipline called neuro-ethics: the brain science of morality. Are the scientists about to offer explanations – solutions, even – for crime, brutality and violence? Or are they talking dangerous nonsense?

“What the late 20th century was for molecular genetics,” says Professor Martha Farah, a leading researcher in neuro-ethics at the University of Pennsylvania, “the early 21st century is proving to be for neuroscience.” Rapid advances in non-invasive brain-imaging are enabling scientists to study moral and emotional processes, including paedophiliac behaviour, in individual brains.

We already have the drugs to enhance mood (lithium and Prozac), concentration (Ritalin) and memory (Aricept). Will we one day have a drug to regulate moral behaviour? “It brings closer the untoward potential consequences of biologically engineered morality,” says Dr Laurence Tancredi, a psychiatrist who practises law in New York.

He predicts a “new society” in which “moral” aberrations will be predicted, and corrected by drugs. “Neuro-ethics,” says Professor Nikolas Rose of the London School of Economics (LSE), “is raising important questions about how we configure the boundaries of the normal and the pathological, the treatable and the acceptable… the kind of humans we want to be.”

Crude connections between the brain and our behaviour have long been familiar. Take the railroad worker Phineas P Gage, a decent chap who in 1848 suffered a prefrontal-lobe injury when an iron rod shot through his skull while he was dynamiting a tunnel in Vermont. He survived but became a foul-mouthed lout. Then there was the 1979 “Twinkie defence” trial, in which Dan White, who had shot dead the mayor and the city supervisor of San Francisco, George Moscone and Harvey Milk, was found guilty of manslaughter rather than murder because, as the public saw it, the jury accepted that White had simply eaten too many cupcakes that day, the sugar in his brain turning him into a killer.

In 1992, also in America, a Mr Weinstein was charged with strangling his wife to death during an argument and throwing her body out of the 12th-floor window of their apartment to make it look like suicide. A positron emission tomography (Pet) scan, revealing “sliced” images of Weinstein’s brain, showed up an arachnoid cyst. His psychiatrist claimed that the tumour had caused metabolic imbalances leading to poor impulse control; allowing the scan led to the lesser plea charge of manslaughter. Three years later, a charity manager, William Aramony, was charged with embezzling hundreds of thousands of dollars to fund a lavish lifestyle. But his defence lawyers argued that it was his brain that stole the money and not him. He could not form the requisite criminal intent for embezzlement, argued the defence, because his brain had been shrinking during most of the late 1980s and early 1990s, a fact that could be substantiated by a magnetic resonance imaging (MRI) scan. In the event, the prosecution agreed to a plea bargain favourable to the defence.

It is universally accepted that alcohol and drugs can be detrimental to moral behaviour. Evidence that our diet can also alter our actions has been demonstrated in a research trial at a maximum-security British prison when a zinc supplement was added to prisoners’ food, culminating in a significant reduction, it was claimed, of the rate of reoffending. At a recent symposium it was seriously proposed that zinc be introduced into drinking water, like fluoride, to combat criminality.

Quite how cupcakes, or zinc, make good people bad and bad people good is by no means clear. But with every passing week, scientists learn more about the complex action of brain chemicals known as neurotransmitters on our emotions and our behaviour, including learning, memory, decision-making and anger. At the same time, brain scans taken while people perform a variety of cognitive and emotional processes, including lying and fantasising about sex, have located specific areas in the brain crucial to decision-making. What neuroscience is telling us, moreover, is that while brains are broadly similar, they are also highly individual. There are crucial processes in early foetal development that owe less to genes than to “epigenetic” (beyond genetic) competition between migrating cells, thus guaranteeing the uniqueness of each member of the human species – even in the case of monozygotic twins.

Arguably, liars, thieves, muggers and paedophiles, like saints, philanthropists and Good Samaritans, are not bad or good – just “different”, their brains having disposed them to behaviour outside the moral norm. The eminent American neuroscientist Professor Terry Sejnowski once told me his work had made him less prone to judge others: “Neuroscience teaches us that all our drives and compulsions are unequal.” The religious doctrines of original sin – meaning we are prone to prefer wrong by nature – conscience and free will have been eroded by rationalism, science and secularism over the past two centuries, yet a powerful belief in responsibility for our actions remains – in family life, friendships, soap operas, newspapers and the criminal-justice system. The tendency to find excuses for bad behaviour was inherent in Freud. But it was neuroscience, which took off in the mid-1980s, that accelerated the process.