Truman Capote's description of the 1959 murder of four members of the Clutter family in their Kansas home is often described as the first ‘true crime novel’. The two killers had met in prison where one of them, Richard Hickock, had been told of a wealthy farming family with a cash-laden safe ripe for ransacking. But when Hickock and his partner Perry Smith arrived, armed with a shotgun, a knife and ropes, there was no safe in the house. They had decided in advance to leave no witnesses. The four members of the family were tied up. Herbert Clutter's throat was cut before he, his wife and two teenage children were each shot in the head. Based on extensive interviews carried out by Capote and his assistant Harper Lee, the book contrasts an idyllic mid-Western community with the horror of the killings, describes the aftermath and culminates with the hanging of the killers.

While Hickock was the organiser, Smith did most, perhaps all, of the killing, but he also prevented Hickock from raping the 16-year-old daughter. Hickock is described as a philandering, manipulative smooth talker, who made grand plans, liked to run down dogs in his car, and was so callous that he seemed his usual self to his family immediately after the murders. His upbringing was settled, without trauma or abuse; his psychopathy arising de novo, without any familial or environmental contribution.

Smith, on the other hand, was brought up in an orphanage after his alcoholic mother choked on her own vomit. He was physically abused by the nuns for persistent bedwetting. His brother and sister died by suicide. As an adult he had intense attachments to fleeting figures in his life. His cognitive style was characterised by superstition, magical thinking, and a dispositional sensitivity that led to problems with authority and violent loss of temper. While Hickock was certain that he was normal, denying both his callous nature and his tendency to paedophilia, Smith was concerned that he had had no compunction about killing. His own formulation – that the Clutters represented all those who had mistreated him over the years – is implicitly endorsed by Capote. That his psychopathy can be understood as the consequence of his experiences seems to make him less culpable than the intrinsically evil Hickock.

The medical evidence came from two sources: court-appointed generalists from the local community and a keen, young, forensic psychiatrist from out of town. All came to the same conclusion on the single dichotomous question asked of them, rooted in the rigidly cognitive M'Naghton rules: the killers did indeed know right from wrong. But Capote is clear that the forensic psychiatrist wanted to say more, to give an explanation in mitigation for Smith, who had had such a traumatic life. The court allowed him no opportunity to do so.

In UK Crown Courts, the psychiatrist who has been called by the defence often wants to give a narrative; it is the explanation, rather than a categorical diagnosis, that mitigates responsibility. The defence psychiatrist is the Trojan Horse that carries psychology into the courtroom. The prosecution (psychiatrist) tends to take a more reductionist approach, dismissing psychological nuance with a demand for medical certainty. In Kansas, the local doctors for the prosecution may have been biased by their familiarity with the case and the community; the forensic psychiatrist for the defence may have been biased by his academic enthusiasm. Perhaps the court was right to take such a restrictive approach to the admission of psychiatric evidence.

Underlying these courtroom politics are more fundamental questions about the nature of criminal responsibility and mitigation. For Capote, Smith was less culpable because he was understandable. Hickock was born bad, presumably through no fault of his own, yet is regarded as more deserving of punishment. Why should mitigation depend on our ability to understand? And when psychiatry has explained the truth about the criminal, rather than the crime, what then for justice?