Call Me Irresponsible Is Psychopaths’ Responsibility a Matter of (Data) Preference?

The philosophical debate over psychopaths’ moral and criminal responsibility is increasingly evidence based. However, as we noted, such arguments are misleading if philosophers only consider evidence that supports their own positions. In his response, Glannon counters our argument by introducing new evidence—neuroimaging data—and so demonstrates the exact problem we outlined; Strijbos, in contrast, offers a workable solution.

Glannon’s response is a succinct summation of the strengths and weaknesses that philosophers bring to the debate. Although Glannon accurately portrays the potential role of neuroimaging data in determining responsibility as merely supplementary to behavior, he simultaneously mistakes the strength of the actual data. Glannon (2017, p. 13) cites Gregory et al. (2012) as evidence that psychopaths have reduced gray matter volume in “brain regions associated with empathic processing, prosocial emotions and moral reasoning.” In fact, the only reduction unique to psychopaths in that study was in temporal pole regions, whose relevance to empathy and moral reasoning is unclear.

Of the 26 structural magnetic resonance imaging studies on psychopathy (assessed via total score on the Psychopathy Checklist or the Psychopathy Checklist: Screening Version) published to date, only two (Ermer, Cope, Nyalakanti, Calhoun, & Kiehl, 2012; Yang, Raine, Colletti, Toga, & Narr, 2010) found psychopaths to have reduced volumes in both the amygdala and ventromedial prefrontal cortex (vmPFC), regions usually implicated in empathy, emotion processing, and moral reasoning. However, even these findings have caveats; in Ermer et al. (2012), the effects disappeared in peak height whole brain analysis, and in Yang et al. (2010) the results applied only to unsuccessful psychopaths. Although other studies of psychopaths reported abnormalities in amygdalae or vmPFC (but not both), the findings were not consistent: One study described reduced amygdala volume in psychopaths (Yang, Raine, Narr, Colletti, & Toga, 2009), two found increased volume (Boccardi et al., 2011; Schiffer et al., 2011), and four found no differences at all (Cope et al., 2012; de Oliveira-Souza et al., 2008; Gregory et al., 2012; Tiihonen et al., 2008). After correcting for multiple comparisons and controlling for substance abuse, three additional studies reported volumetric reduction in psychopaths’ vmPFC (Boccardi et al., 2011; Tiihonen et al., 2008; Yang, Raine, Colletti, Toga, & Narr, 2011), but two found no differences (Cope et al., 2012; Gregory et al., 2012). A 2009 meta-analysis showed that Psychopathy Checklist-Revised scores do not moderate prefrontal cortex volume or function in antisocial individuals (Yang & Raine, 2009).

Glannon’s claim about functional magnetic resonance imaging studies fares marginally better. Motzkin, Newman, Kiehl, and Koenigs (2011) found reduced structural integrity in the pathway between right amygdala and vmPFC areas, and an attenuated correlation between their activation levels at rest. Juárez, Kiehl, and Calhoun (2013) used an auditory oddball paradigm (not a moral decision-making task) and found that Psychopathy Checklist-Revised total scores were correlated with lower activation in several frontoparietal areas, including the limbic-vmPFC regions, but also in regions (e.g., parietal and occipital) peripheral to emotional or moral processing.

Even if we accept these unreplicated findings as reliable, one problem remains: Psychopaths do not show consistent behavioral deficits in moral reasoning. Of the four functional magnetic resonance imaging studies using moral dilemma tasks (Glenn, Raine, Schug, Young, & Hauser, 2009; Harenski, Harenski, Shane, & Kiehl, 2010; Harenski, Edwards, Harenski, & Kiehl, 2014; Pujol et al., 2012), none showed behavioral differences between psychopaths and nonpsychopaths.¹ Other experiments corroborate this (for a review, see Borg & Sinnott-Armstrong, 2013). In short, psychopaths do not show consistent impairments on moral reasoning tasks, nor do they show systematic abnormalities in brain areas putatively responsible for moral reasoning. Taken together, what do these results mean? One interpretation is that psychopaths have subtle moral decision-making impairments, which our current tests cannot detect. Another interpretation is that, if psychopaths show neurobiological abnormalities, but perform similarly to nonpsychopaths on moral tasks, then those abnormalities are irrelevant to moral decision making, at least in these populations. Whatever the case, these data are too inconclusive to play a meaningful role in the responsibility debate.

If philosophers tend to oversimplify the...