Psychophysiological Investigation of Psychopathy: From Robert Hare to Contemporary Research

Hervey Cleckley described in a subgroup of his inpatients (Cleckley, 1941), which he termed psychopathic, a diminishment of reactivity in situations that normally evoke anxiety or fear, and difficulty learning from experiences involving punishment. These descriptions initiated research on reinforcement learning mechanisms, including punishment based learning (Hare 1965b, 1972), in order to understand the underlying mechanisms. Why was it that psychopathic individuals exhibited deficient emotional reactivity? Would their ostensible cold-blooded mask of sanity be reflected in physiological correlates?

Intrigued by these questions, Robert D. Hare conducted pioneering studies on the psychophysiological underpinnings of fear-based learning using classical conditioning, and due to this work undertook to develop an innovative criterion based assessment tool for psychopathy, the Psychopathy Checklist (PCL; Hare, 1980). Characteristics of low anxiety or fear and failure to learn from punishment were reflected in the PCL’s callous/lack of empathy and shallow affect items, and also in its need for stimulation item (Hare, 1980, 2003). Hare’s seminal early work on classical conditioning and physiological reactivity to threat cues contributed to the development of fear-based theories of psychopathy including Fowles’ (1980) impaired behavioral inhibition system (BIS) conceptualization and Blair’s (2008) amygdala deficit model. Since, the nature and bases of affective response deficits in psychopathy remains a topic of intense interest among researchers, with such deficits being suggested as a driving factor in aggressive and antisocial behavior (Blair, 2007, 2022; Lykken, 1993).

As a tribute to the seminal work of Robert Hare, this article reviews the literature that speaks to some key psychophysiological mechanisms underlying psychopathic behavior, in particular as assessed by the PCL family of assessments (Forth et al., 2003; Hare et al., 2019). We pay particular attention to the ways in which the seminal research contributions from Dr. Hare’s laboratory have flourished and significantly shaped contemporary contributions to the etiology of psychopathy. In doing so we highlight a variety of different psychophysiological perspectives that reflect the diverse and distinctive contributions that Dr. Hare has made to the field, and which promise to have traction in the future. We begin by considering psychopathy as a neurodevelopmental disorder and somatic aphasia as potential core underlying mechanisms and follow this with a discussion of the role of low fear, reduced error-processing, and stress-resistance as psychophysiological characteristics of psychopathy. After a section discussing successful psychopathy as an aspect that was highlighted by Robert Hare, we finish with an outlook on unresolved questions that remain regarding the underlying mechanisms of psychopathy as reflected in its psychophysiological correlates.

The PCL, first published in 1980 (Hare, 1980), was not the only critical contribution that Robert Hare made. Ten years earlier in 1970 Hare published “Psychopathy: Theory and Research” (Hare, 1970). As the first empirically-oriented book on psychopathy, this slim volume quickly became the go-to publication for psychopathy researchers in the 1970’s. The beauty of the book was that in just 118 pages of text, it succinctly summarized everything that was known at the time on not just psychophysiological correlates and the assessment of psychopathy, but also its social processes and treatment. While it went on to influence a great deal of psychophysiological research in the ensuing decades, it contained a rough diamond that was overlooked in the gold-rush pursuit for the etiology of psychopathy: the idea that psychopathy may have a neurodevelopmental basis.

Robert Hare was a pioneer in the psychophysiological study of emotional processing in psychopathy. His earliest studies on this topic were inspired by two precedent-setting works. One was the portrayal of psychopathy in Cleckley’s (1941, 1976) book, “The Mask of Sanity”, which emphasized affective deficits including lack of remorse and inability to learn from punishment. The other was David Lykken’s (1957) experimental study of anxiousness and fear reactivity in youthful offenders exhibiting psychopathy as described by Cleckley. Drawing on Lykken’s findings and concepts from learning theory (e.g., Miller (1978); Mowrer (1960); Hare (1965a) postulated that psychopathy involves an unusually steep gradient of fear arousal, such that threat-predictive cues fail to evoke the normal anticipatory fear that inhibits actions resulting in punishment. He tested this hypothesis in a series of classic laboratory studies with incarcerated offenders (e.g., Hare 1965b, c) in which he operationalized fear in terms of increases in electrodermal activity (“galvanic skin response”). One of these studies (Hare 1965c) introduced the “countdown” paradigm, entailing measurement of physiological reactivity during anticipation of a signaled noxious event such as shock, which became a cornerstone of theorizing in the psychopathy area (Fowles, 1980; Hare et al., 1978; Lykken, 1995; Patrick, 1994).

Research over the years since Hare’s seminal studies demonstrating reduced anticipatory fear response in psychopathic offenders has examined reactivity to external stressors of other types such as threat cues, physical pain, and social pressure. In addition, another way in which affective reactivity in psychopathy has been studied is in the context of making mistakes, where the recognition of errors can lead to a stress response. Of note, reduced learning from mistakes, along with lack of stress reactivity, were notable features described by Cleckley in his clinical account of psychopathy. We review research along each of these lines in the subsections that follow.

Reduced physiological responsiveness to stressors/threat seen in those with high psychopathic traits may be associated with their low sensitivity to internal physiological changes and thus lack of fear experiences. In another seminal work of Hare, he pointed to the potential relevance of resting state neural activity (1970). He suggested that resting state neural activity might be related to a classical clinical description of psychopathic traits, which involves the disconnect between physiological and self-report indicators of emotional reactivity. According to Cleckley’s (1976) original conceptualization of psychopathy, one definitive diagnostic marker for this condition is a disconnect between linguistic and experiential components of emotion, a condition he called “semantic aphasia”. Cleckley’s idea was that psychopaths have difficulty understanding the emotional content of words, an impairment that could underly many of their symptoms, in particular their inability to understand the significance of their antisocial actions.

Taking this idea further, interoception, which includes awareness (one’s conscious feeling of physiological processes in the body) and accuracy (one’s ability to accurately perceive the physiological process), has been argued to be crucial to the subjective experience of emotion (e.g., Damasio, 1994). The inability to recognize and understand one’s own bodily signals may contribute to emotion processing deficits and lack of empathy (e.g. Ainley et al., 2016), important features of psychopathy (Northam & Dadds, 2020; Waller et al., 2020). Neuroimaging evidence has also implicated a primary interoception network, including the insular-frontotemporal region, in emotion processing and regulation (Adolfi et al., 2017), and its disruption has been found in individuals with psychopathic traits (Yang & Raine, 2009). However, the notion that psychopaths have fundamental issues with recognizing and understanding their internal bodily signals and emotional experiences has not been examined widely until relatively recently.

Interoceptive awareness and accuracy can be assessed through self-report or objective measures such as skin conductance levels and heart rate (Murphy et al., 2019). In one of the earliest studies to directly examine interoception in psychopathy, 138 adult men recruited from local temporary employment agencies completed a stressor in which they were asked to give a 2-min speech about their worst faults while their skin conductance responses and heart rate were recorded (Gao et al., 2012). Psychopathic traits were assessed using the PCL-R. A disconnect which the authors termed “somatic aphasia” was documented between perceived bodily reactions and objective measures of physiological reactivity in those high in psychopathic traits. In particular, this deficit was found to be associated with the affective-interpersonal factor of psychopathy. In a later study attempting to replicate these findings, male and female participants from college and community settings (n = 114) completed a social stressor task in which they were socially excluded by peers while their skin conductance and heart rate responses were recorded (Gao et al., 2026). They were then asked to rate their bodily sensations during the task, and self-reported psychopathic traits on the Psychopathic Personality Inventory – Revised (PPI-R; Lilienfeld & Widows, 2005). It was found that lower interoceptive accuracy, reflected by a disconnect between psychophysiological reactivity and self-reports of bodily reaction, was associated with the self-centered impulsivity dimension of psychopathy.

A few studies have also found the interoception deficits to be associated with the impulsivity-antisocial factor or secondary psychopathy. Nentjes et al. (2013) found that reduced heartbeat detection accuracy was associated with the PCL-R Facet 4 (antisocial) but no other facet scores in 75 male personality disordered offenders. In a study using the Triarchic Psychopathy Measure (TriPM; Patrick, 2010) to assess psychopathic traits, Ellis, Schroeder, Patrick, and Moser (2017) found that individuals who scored high on boldness demonstrated reduced emotional reactivity, as indexed by smaller late positive potential response to negative versus neutral picture stimuli, although their self-reported emotional reactivity was similar to those with low boldness. Instead, those high on boldness tend to over-report experiencing greater emotional reactivity when asked to increase or decrease their emotions. This over-reporting may reflect their efforts to conceal blunted emotional responses, or their enhanced attention to the emotional content of the pictures when cued to (Ellis et al., 2017).

In one recent study of 64 male and female inmates (Lamoureux & Glenn, 2021), psychopathic traits assessed by the PCL-Screening Version (Hart et al., 1995), and interoceptive ability was assessed using a heartbeat tracking task and a self-report measure. No significant relationships were found between levels of psychopathic traits and interoceptive awareness or accuracy.

Taken together, the empirical evidence to date has generally supported the notion of Hare that psychopaths show a disconnect between physiological activity and experiences of emotion, in terms of interoception deficits. Individuals high on psychopathic traits seem to have difficulty accurately perceiving their own bodily signals, although findings are not entirely consistent. Using self-report combined with objective psychophysiological measurements, studies have generally found that reduced interoception is associated with the Factor 2 of PCL-R or secondary psychopathy. Future studies with larger samples and multi-modality methods (i.e., self-report, behavioral performance, and physiological recording) are needed to further determine the interoception deficits in psychopathy. Pressing questions include what brain mechanisms may underlie this condition, and whether longitudinal research can establish whether interoception deficits are part of the characteristics of a neurodevelopmental disorder of psychopathy.

One line of Hare’s work has focused on a specific psychopathic population that is successful beyond not being in prison or a forensic institution, such as in the workplace or in specific social contexts (Babiak & Hare, 2007; Hare, 1994; Mullins-Sweatt et al., 2010). Hare (2002) stated that “Not all psychopaths are in prison. Some are in the board room”, and in fact, studies have demonstrated that the base rate of psychopathy in the upper ranks of corporations may be as high as 3% compared with 1% in the general population (Babiak et al., 2010). Further, his work has suggested that the latent dimensions and general nature of psychopathy are same across a wide diversity of samples, including offenders, psychiatric, community, and corporate samples (Babiak et al., 2010; Babiak & Hare, 2006; Hare & Neumann, 2008). Inspired by this work, increasing number of studies have been conducted to explain the differences and similarities between successful and unsuccessful psychopathy.

One of the major challenges faced in this research area of successful psychopaths concerns the inconsistency or ambiguity in the literature on conceptualizing, operationalizing, and measuring “successful” psychopathy (Glenn & Raine, 2014; Steinert et al., 2017; Wallace et al., 2022). In one of the earlier definitions, successful psychopaths were defined as individuals scoring high in psychopathy who have never been convicted of a crime. Using this definition, Ishikawa, Raine, Lencz, Bihrle, and LaCasse (2001) recruited successful (higher PCL-R score and no prior convictions) and unsuccessful psychopaths (higher PCL-R score with at least one criminal conviction) together with non-psychopathic controls from local employment agencies in the Los Angeles area. It was found that although unsuccessful psychopaths showed reduced heart rate stress reactivity and impaired executive functioning (Wisconsin Card Sorting task) as typically seen in incarcerated psychopaths, successful psychopaths from the community showed heightened stress reactivity and significantly outperformed non-psychopathic controls on the executive functioning task. In a different sample using the same recruitment technique and operationalization of “successful psychopathy”, Gao et al. (2011) found that unsuccessful psychopaths reported experiencing more childhood physical abuse, and more importantly, they showed reduced P3 amplitudes to target stimuli, demonstrating their difficulties in allocating attention to task-relevant stimuli. In contrast, successful psychopaths had larger P3 amplitudes and shorter P3 latency to irrelevant non-target stimuli, suggesting enhanced cognitive abilities in this subgroup. It is therefore hypothesized that enhanced autonomic and CNS functioning, along with better executive functioning, may protect a subgroup of psychopaths from being detected and arrested, while allowing them to still perpetrate significant harm to others in the community (Gao et al., 2020; Gao & Raine, 2010).

In this regard, Patrick et al. (2009)’s triarchic model of psychopathy emphasizes three core phenotypic constructs (Disinhibition, Boldness, Meanness) and provides a framework that is particularly relevant to the workplace. According to the moderated-expression model, successful psychopathy is explained by protective factors that play a buffering role against psychopathic manifestations of maladjustment and deviance (Hall & Benning, 2006; Lilienfeld et al., 2015). Specifically, it proposes that external factors or certain psychopathic traits such as boldness or fearless dominance, moderate the effect of core psychopathic traits such as affective deficits on severe antisocial behaviors. Therefore, enhanced autonomic responding and executive functioning (Ishikawa et al., 2001), superior brain functioning (Gao et al., 2011), and positive childhood experiences (Waller et al., 2013), characteristics likely associated with the boldness and fearless dominance features of psychopathy, may help these individuals adopt more socially adaptive strategies and protect them from manifesting serious antisocial behavior. This is turn decreases the likelihood of being incarcerated or premature death (Gao & Raine, 2010). High intelligence has also been theorized to play such a protective role, although empirical studies are needed to test if higher levels of intelligence may reduce the risk of deviant behaviors exhibited by those with psychopathic traits (Persson & Lilienfeld, 2019; Wall et al., 2013).Alternatively, some have proposed that successful psychopaths show a pattern that is different from the pattern of other psychopaths. According to this differential-configuration model, some adaptive features, such as fearless dominance or boldness, may be the key to distinguish those who are successful from others (Lilienfeld et al., 2015). Finally, a few studies have failed to support the notion that some psychopathic traits contribute to a successful life and questioned the existence of successful psychopaths (Kiehl & Lushing, 2014). It is important to note that psychophysiological evidence for the latter two theories is lacking.

In conclusion, despite the burgeoning interest in successful psychopathy, there is not a unified approach to define and conceptualize this construct, likely due to lack of clear conceptualization of “success” for psychopaths (Wallace et al., 2022). Overall, some adaptative features, such as boldness and other features from Factor 1 of the PCL-R, may contribute to their success in workplace and other social contexts. Nonetheless, more search is needed to allow a better understanding of the theoretical mechanisms together with a working definition of successful psychopathy. Most of the research to date has relied on a diverse mixture of self-report measures and rating inventories. Given that psychopathic individuals may be manipulative and dishonest, future work should integrate self-report and other-observant methods to provide more reliable and valid measures of psychopathic traits. Furthermore, significant amount of the research to date has utilized undergraduate students who by definition have achieved significant success; as such establishing the correlates of further success may be susceptible to ceiling effects. Finally, almost all research is cross-sectional, precluding inferences regarding temporal precedence. Prospective longitudinal research is needed to understand the directionality of the relationships.

Though some questions that Robert Hare initiated with his aim to understand reinforcement learning in psychopathy have been answered, differences in learning as reflected in psychophysiological measures are still under investigation. In fact, throughout this paper, we have argued that there are a lot of open questions that need further investigation to clarify the developmental pathways of psychopathy reflected in psychophysiological correlates and to derive indications for treatment (Kyranides et al., 2017).

There are wider implications from the perspective of psychopathy as a neurodevelopmental disorder. First, if the field was to reconceptualize psychopathy as a neurodevelopmental disorder, that classification could potentially be used as a mitigating factor in criminal cases. In much the same way that an autism-spectrum diagnosis may influence jurors’ perceptions (Berryessa et al., 2015), viewing psychopathy as a condition that emanates from very early factors beyond the individual’s control could influence decisions in death penalty cases. Whether this would be construed as a welcome or unwelcome development is a matter of some debate.

Second, there could be implications for risk assessment. On the one hand, if psychopathy is viewed as neurodevelopmental in nature, sentencing could be longer for psychopaths if it were felt that their neurodevelopmental nature meant that the individual could not be treated. Similarly, parole prospects may be diminished if it were believed that the neurodevelopmental underpinnings of the condition precluded future change in behavior. On the other hand, treatment opportunities need to be further investigated as the current knowledge about successful treatment in adults (as opposed to young people) is very limited.

Third, considering adult psychopathy as neurodevelopmental in origin could result in most children with significant callous-unemotional traits also being considered to suffer from a neurodevelopmental condition. This in turn may attract greater clinical attention to this condition and result in a redoubling of efforts to develop interventions for this childhood condition based on empirical findings for effective treatments (Fleming et al., 2023; Perlstein et al., 2023), and potentially prevent its development into adult psychopathy. Indeed, if psychopathy is a neurodevelopmental disorder, then interventional efforts would likely be most effective when they start in early childhood or infancy, or indeed as some have called for, as early as the newborn period (Brady et al., 2023). Whether it would be ethically advisable to screen and intervene at an early age is again a matter of debate.

If we were to derive a list of priorities for future research based on the open questions, these should include the following considerations.

There are several conclusions that can be drawn from the psychophysiological research on psychopathy to date. First, there is strong evidence for psychopathy demonstrating characteristics of a neurodevelopmental disorder, which needs to be considered in future investigations into psychophysiological correlates of PCL-assessed psychopathy. Second, low resting heart rate seems to be an important indicator for psychopathy and as a precursor of callous-unemotional traits. Third, the evidence for a reduced interoceptive awareness in psychopathy might relate to low resting heart rate and the related low physiological reactivity to fear-eliciting cues. Fourth, subjective and physiological responding to emotional stimuli cannot be equated, but this dissociation between responses within the same person to a single cue is actually providing us not only with insights into the nature of psychopathy but also into the nature of human emotional reactivity more broadly (Marsh, 2013). Last but not least, increasing our understanding of the psychophysiological correlates in successful (compared to non-successful) psychopathy needs to advance to be able to differentiate psychopathy from its related but yet different construct, antisocial personality disorder (Leistico et al., 2008). As Robert Hare has described the move towards integration of models, theories using neuro- and psychophysiological measures: “A notable trend is the interest shown by neuroscientists in using psychopathy as a vehicle for evaluating their own models of behavior, personality, and brain function. The result may ultimately be an integration of psychopathy theory and research with more general psychobiological, behavioral genetic, developmental, and personality models.” (Hare & Neumann, 2008 p. 240).