To our knowledge, this study establishes for the first time the existence of a structural difference in the striatum of psychopathic individuals. This group had a 9.6% increase in the volume of the striatum compared with controls, a large effect size corresponding to d = .76. Within the subregions, the left and right lenticular nuclei were 9.4% and 10.0% larger, respectively. Dimensional, regression analyses of the subfactors of psychopathy revealed that volumes of the lenticular nuclei were associated with several aspects of psychopathy. In contrast, the caudate body was primarily associated with the interpersonal and affective features of psychopathy, and the caudate head was primarily associated with the impulsive and antisocial features. These volume differences in the striatum could not be attributed to age, sex, ethnicity, substance or alcohol abuse, whole brain volume, or socioeconomic status. Findings implicate the striatum as a region that may be partially involved in the pathophysiology of psychopathy.
The present finding of increased
volume is not likely to be attributable to environmental insults such as head injury, which tend to result in volumetric reductions, or to substance and alcohol dependence, which was held constant. This raises the important question of how increased striatal volume may develop. One possibility is that increased volume of the striatum in psychopathic individuals is due to neurodevelopmental pathology, which may include genetic or environmental (e.g., prenatal) influences. Results from recent imaging genetics studies suggest there may be a genetic basis to reward-related striatum activity that may increase the risk for impulsivity and reward dependency, such as that observed in psychopathy. Genetic polymorphisms associated with increased dopamine release in the striatum has been found to account for 9–12% of between-subject variability in reward-related striatum activity (36
). A polymorphism in the human fatty acid amid hydrolase (FAAH) gene has also been found to account for increases in reward-related ventral striatum activity (37
). Interestingly, this polymorphism was also associated with reduced threat-related amygdala activity, making it a particularly interesting mechanism to be examined in future studies of psychopathy.
An important question concerns how increased striatum volumes may predispose to psychopathy. First, results are congruent with data supporting the involvement of the striatum in reward sensitivity, which facilitates stimulation-seeking behavior, persistence in repeating actions related to rewards, and enhanced learning from rewarding signals (14
). Psychopathic individuals demonstrate increased stimulation-seeking (7
), maladaptive response perseveration (39
), and superior performance on tasks involving rewards only (40
). Second, activity in the striatum has been associated with individual differences in impulsivity, as indicated by a preference for immediate over delayed rewards (41
); psychopathic individuals are described as impulsive (7
) and have been found to have deficits in delaying rewards (40
). Third, the striatum is part of a neural circuitry involved in stimulus-reinforcement learning – the process of learning appropriate behaviors for given situations through reward and punishment information (8
). Abnormalities in the striatum may disrupt this process. For example, a recent study found reduced striatum activity during threat of electric shocks in violent individuals with antisocial personality disorder (APD) (42
). Impaired stimulus-reinforcement learning is theorized to be associated with poor decision-making, impaired socialization, and diminished empathy-based learning following cues of distress in others (43
). Empirical studies have repeatedly confirmed that psychopathic individuals have deficits in stimulus-reinforcement learning (9
). Furthermore, the striatum is densely connected to the amygdala and ventromedial prefrontal cortex (44
) which are necessary for stimulus-reinforcement learning and have been implicated in the related deficits observed in psychopathy (2
). The present results suggest that differences in the striatum may further contribute to these previously observed deficits in stimulus-reinforcement learning.
Heightened sensitivity to reward may be a source of motivation for criminal behavior; indeed, approximately 45% of psychopaths have been found to be motivated by material gain in their crimes (45
). Community individuals scoring higher in psychopathy have also indicated a willingness to violate moral principles in exchange for a lesser amount of money than low-scoring individuals (46
), demonstrating a moral flexibility in the presence of reward. Furthermore, adolescents with aggressive conduct disorder have shown increased activity in the striatum when viewing images of others in pain (47
); it is suggested that aggressive individuals may enjoy seeing their victims in pain and, because of diminished amygdala / ventromedial prefrontal cortex connectivity, may not effectively regulate positively reinforced aggressive behavior. Together, these studies suggest that reward sensitivity, in combination with disinhibition, may play a large role in motivating the antisocial behavior observed in psychopathy.
The differences in the striatum observed in the present study were not exclusively associated with impulsive, stimulation-seeking traits, but demonstrated approximately equal relationships to all of the factors of psychopathy, although relationships with Factor 4 were marginally significant. This suggests that volume increases cannot be attributed solely to impulsive, stimulation-seeking traits, but are related to the broader construct of psychopathy. Further exploratory analyses of the relationship between the factors of psychopathy and the subregions of the striatum revealed that the head and body of the caudate demonstrated differential associations with the subfactors of psychopathy. The body was primarily associated with the interpersonal and affective features of psychopathy whereas the head was primarily associated with the impulsive and antisocial features. The head and body of the caudate have been found to be involved in functionally dissociable neural circuits (48
), although further research will be needed to elucidate these differential relationships with respect to psychopathy. Activation specifically in the caudate body has been observed during deception (49
) and may be involved in inhibiting truthful responses (50
). Although speculative, increased volume of the caudate body in individuals scoring higher on the interpersonal and affective features may reflect an enhanced ability to deceive others. In contrast, the head of the caudate may be more involved in responding to rewarding feedback (48
) which may in turn lead to impulsive, stimulation-seeking traits. Increased caudate functioning has been observed in children with psychopathic traits during punished reversal errors, which is suggested to reflect impairment in processing reinforcement information (2
). It should be noted that we did not correct for multiple comparisons in the correlations involving the subregions of the striatum, so these results should be regarded as preliminary.
It is important to be cautious in interpreting the findings of increased striatal volumes observed in the present study. First, although greater volume of a brain region is commonly interpreted as an indication of better functioning (11
) and vice versa (23
), increased volume may also reflect a lack of synaptic pruning during development, a process by which unnecessary connections are eliminated to increase the efficiency of other connections; thus, it is possible that increased volume could indicate poorer functioning (11
). This interpretation would be consistent with previous findings of reduced striatum activity in criminal psychopaths (1
) and violent individuals with APD (42
). Second, it is unclear whether increased volume affects excitatory or inhibitory connections. Third, only an association has been shown between striatal volume and psychopathy, so causality cannot be assumed; structural differences in the striatum are more likely to contribute to a wider network of brain systems that motivate and regulate behavior. Finally, the volumetric difference observed in the present study is a phenotypical variable that is not, per se, a sign of abnormality.
Limitations of this study include the fact that samples were predominantly male, so findings cannot be generalized to psychopathic women. Group sizes were modest, although dimensional analyses on the larger sample indicated that the key results are reliable. Volumetric measurements were obtained from manual tracings, which may be more subjective than other methods. The putamen and globus pallidus were measured as one structure (lenticular nucleus), so differential relationships could not be assessed. Finally, we lack evidence from a behavioral measure that could potentially confirm or disconfirm the hypothesized link between striatal volumes and specific traits of psychopathy.
The present study represents a beginning step in determining whether differences in the striatum may be a contributing factor in the development of psychopathy. Important questions for future research include how the striatum may interact with other brain regions to produce psychopathy, as well as how genetic and developmental influences may alter the striatum in ways that may increase risk for psychopathic traits. Future research to place this specific structural brain difference within the functional context of more widespread frontolimbic circuits could help to confirm and extend the present findings.