A defendant stands in the dock. An expert describes them as a “psychopath”. In an instant, one word threatens to eclipse their history, circumstances and the crime itself.
In Ireland, England and Wales, judges are not supposed to add years to a sentence because someone has been described as a psychopath. But the label can still enter criminal justice through expert reports, risk assessments, parole, mental disorder cases and preventive detention.
In the US, the Psychopathy Checklist-Revised (PCL-R) – an assessment tool used for diagnosing psychopathic traits – has been used in death penalty cases, parole hearings, preventive detention and pre-sentence evaluation. These cases usually concern future risk, with evidence most often introduced by the prosecution or state. Research from the US has shown a marked increase in the use of PCL-R over time.
The “psychopath” label has been applied in a number of cases involving adolescent offenders, and occasionally raised by the defence, alongside brain-imaging evidence, to argue for reduced culpability or impaired control.
In the 1980s case of American serial killer Brian Dugan, psychopathy and brain-imaging evidence were used to argue that he could not control his “killer impulses”. He was ultimately sentenced to death in 2009, but the testimony transformed it from a “slam dunk for the prosecution into a much tougher case”.
In Scotland, where an order for lifelong restriction (an indeterminate, lifelong sentence for serious violent or sexual offenders assessed as posing a continuing risk) is considered, the High Court can commission a risk assessment report. Psychopathy may be assessed in the report, but should not determine the sentence by itself. In one recent sentencing statement, the court noted that the offender scored just above the UK cutoff for psychopathy, but imposed the order only after considering the full risk assessment.
Proper use of the PCL-R to assess defendants for psychopathy requires specialist training, interviews and file review. Even then, scoring and interpretation are not mechanical: they depend on professional judgment, typically forensic psychologists or psychiatrists.
In my recent research, I argue that psychopathy can be legally relevant, but not as confidently as it is sometimes presented. If judges and policymakers treat psychopathy as a hard scientific fact rather than an evolving construct, the result can be unfair labelling, distorted sentencing and overconfident predictions.
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Psychopathy: some experts now say it doesn’t exist – here’s why we may be looking at it all wrong
What is psychopathy?
Psychopathy is a cluster of traits, not a standalone clinical diagnosis. Unlike antisocial personality disorder, it doesn’t appear in either the American Psychiatric Association’s diagnostic manual (the DSM) or the World Health Organization’s equivalent (the ICD), although the concepts overlap considerably. Traits include callousness, lack of empathy, shallow emotions, manipulativeness, deceitfulness, impulsivity and antisocial behaviour.
Psychopathy tends to matter most in legal settings focused on a defendant’s future behaviour: sentencing, mental disorder cases, parole and preventive detention. But it is not a legal category. It is a clinical and research construct entering law through expert evidence and assessment tools like the PCL-R. That matters because law asks different questions from science. Courts ask about culpability, fairness and risk, while researchers debate what psychopathy is, how to measure it and how stable the concept is.
An assessment of psychopathy can seem more precise than it is. One reason is neuroscience. For years, brain imaging studies appeared to show that people with psychopathic traits had abnormalities in areas linked to emotional processing and impulse control. That fed the idea of a distinctive “psychopathic brain”.
But newer and larger studies have made that story harder to sustain. Findings are often inconsistent, weak or inconclusive. Apparent brain differences are not unique to psychopathy. Brain scans do not tell simple stories about cause and effect. A brain difference may shape behaviour – but behaviour, life experience and environment also shape the brain.
There is no agreed neural fingerprint that courts can safely treat as objective proof of psychopathy. And yet, psychopathy evidence is used to answer legal questions about culpability, future risk, treatability and detention. Psychopathy research is useful, but courts should be wary when experts present isolated findings as consensus.
Assessing psychopathy
Then there are the assessment tools. The best known is the PCL-R, developed by forensic psychologist Robert Hare and first published in revised form in 1991. It scores people across 20 items covering traits and behaviour, using interviews and file reviews. It is widely used as a gold-standard instrument, but easier to overstate than many realise.
The cutoff points used to classify someone as a psychopath are practical thresholds that vary across jurisdictions. The tool also gives significant weight to past criminal behaviour, so two people with similar traits can receive different scores depending on their history. In adversarial settings like a courtroom, experts instructed by different sides do not always agree. A psychopathy score is therefore not a self-explaining fact.
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There is a deeper problem. The best current evidence suggests psychopathy is not all-or-nothing. People vary by degree across traits such as callousness, poor impulse control and manipulativeness. There is no clear line separating “the psychopath” from everyone else.
Law, however, prefers binaries: disordered or not, high-risk or not. When courts force a dimensional condition into a yes-or-no category, they risk creating false certainty.
A better approach is to stop asking: “Is this person a psychopath?” and ask more practical questions instead: “Which traits are elevated, how extreme are they, and how do they matter here?”
The legal system need not ignore psychopathy. It must, however, resist being overawed by it. Courts should not treat psychopathy as something that can be read off a brain scan or reduced to a single numeric score. They should demand clarity about the limits of the evidence and favour careful, trait-based evidence over sweeping labels that carry enormous stigma.
Used carefully, psychopathy research can help the law make better decisions. Used carelessly, it can turn a contested scientific construct into a shortcut for fear, blame and exclusion. In a courtroom, that is too much power to hand to one word.
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