The present study characterized differences in resting and task-evoked brain activity associated with different levels of moral reasoning. Significant brain activity differences were observed between individuals at high-level post-conventional moral reasoning and individuals at the pre-conventional and conventional level of moral reasoning. Specifically, using ASL fMRI to quantify regional brain function at rest, we found greater activity in the VS and vmPFC/sgACC in the post-con group compared to the pre-con + con group, and a positive correlation between the activity in these two regions and the degree of post-conventional reasoning (DIT-N2 Scores) across groups. Using BOLD fMRI during the BART to examine evoked brain activity accompanying decision making under increasing risk, we also found greater activity in the VS in the post-con compared to the pre-con + con group. Our results provide converging evidence that individuals at a higher level of moral reasoning are characterized by increased activation in the mesolimbic frontostriatal system, in particular in value-related reward-processing regions (VS and vmPFC). According to previous studies, the VS is a key region in the mesolimbic frontostriatal reward processing system and plays an important role in value-based decision making, reinforcement learning, and affective processing (i.e., processing for reward and punishment)24, 46,47,48,49. It has been demonstrated that increased VS activation is linked to reward anticipation and evaluation21, 46, 48.

The vmPFC also plays an important role in reward processing, responding to various hedonic stimuli, reinforcement, and reward preferences50,51,52. Subjective valuation of reward-related stimuli is an important factor that activates both VS and vmPFC regions. Taken together, these findings extend our prior work showing a link between moral reasoning and gray matter structural volume in vmPFC/sgACC5 by demonstrating increased neural activity in these regions, both at rest and during a decision making task. Although these results are exploratory, they provide new insights into the potential neural basis and underlying psychological processing mechanism of individual differences in moral development.

As we mentioned before, value representations and reinforcement learning are shared components of moral judgment and decision making. Cushman’s dual-system approach which emphasizes valuation of actions versus valuation of outcomes provides a useful framework to understand diverse moral judgment and decision making10. From Cushman’s dual-system framework perspective (action-based value and outcome-based value) of moral judgment, activation differences in the VS and vmPFC/sgACC between the two DIT groups may reflect the potential importance of outcome valuation in moral reasoning development because the VS and vmPFC are more involved in outcome-based rather than action-based valuation in this framework. This is in line with Kohlberg’s theory that higher levels of reasoning do tend to focus on outcomes (i.e., the greatest good for the greatest number of people) rather than actions (i.e., this action is forbidden or permitted)1.

Additionally, based on the link between prosocial behaviors and high-level moral development4, 14, differences in prosocial motivation might be another important explanation for the link between brain activations and individual differences in moral development. Functional neuroimaging studies have consistently demonstrated the involvement of the VS and the vmPFC in both moral judgment16, 22 and prosocial behaviors such as charity and philanthropic activity19, 26, 27. Therefore, the current findings of the activation differences in the VS and vmPFC between two DIT groups may raise the potential importance of positive motivations towards others in moral reasoning development, rather than inhibition or selfish motives. Again, this is in line with Kohlberg’s theory that higher levels of moral reasoning tend to be promotion/other-focused (i.e., do it because it is right) rather than prevention/self-focused (i.e., do not do it because it is wrong).

There are other possible explanations for the links we observed, given that the vmPFC is involved in not only valuation but also affect regulation and social cognition (i.e., inferring others’ mental states and empathy for others)53. Thus, increased vmPFC/sgACC activity in the post-con group may be related to the ability to regulate emotional conflicts elicited by moral dilemmas or to the ability to infer others’ mental states (e.g., theory of mind and empathy). Notably, the AIS represents one’s own and others’ emotional states54 as well as being associated with aversion and harm-avoidance55, 56. Thus, the link between moral reasoning and IFG/AIS activity also points to a possible explanation in terms of sensitivity to aversion to harming others or empathic feelings to others.

In addition, a number of clinical studies have provided evidence that impairments in the brain’s reward system are associated with poor decision making, lack of empathy, and social inappropriateness. First, individuals with antisocial personality disorder and psychopaths, who are characterized by lower levels of moral reasoning than normal controls12, show structural and functional alterations in the VS57 and the vmPFC58. Psychopaths also exhibited reduced vmPFC activity during moral decision making57. Antisocial individuals also showed impaired VS activity in response to reward57, 59. Specifically, Gatzke-Kopp and colleagues59 found that normal controls had increased VS activity only during reward trials, while antisocial individuals with externalizing behavior disorders showed increased VS activity during both reward and non-rewarding trials, suggesting that antisocial individuals may be impaired in processing the omission of reward. Second, our previous findings regarding impulse control disorders (ICDs) in Parkinson’s Disease (PD) support the notion that the change in VS activation may underlie behavioral changes29. Dopaminergic therapy to treat PD can cause ICDs such as pathological gambling, excessive spending, and hypersexuality. Compared to PD patients without ICDs, PD patients with ICDs had significantly reduced activity in the VS during resting state and the BART, suggesting that abnormal behaviors observed in patients may result from diminished VS activity following dopamine treatment29. Taken together, all of the above-mentioned results have demonstrated that dysfunction of the VS and vmPFC are often linked to impulsivity and immoral behavior, which further supports our claim that the function differences in the VS and vmPFC regions are associated with the different levels of moral reasoning development.

We did not find any differences in risk taking on the BART between individuals at high-level post-conventional moral reasoning and individuals at the pre-conventional and conventional level of moral reasoning. This is consistent with the idea that people may view risky behaviors as a matter of personal choice instead of a moral issue33, 34, and that moral reasoning does not consistently predict risky behaviors35,36,37. However, at the neural response level, we do see that the post-conventional group was associated with higher VS activation during risky decision making. As this association was observed across all risk levels, it likely reflects intrinsic VS function rather than a VS response specific to risk.

We observed considerable individual differences in moral development levels in this relatively homogeneous and well-educated MBA group of subjects. It will be of great interest for future studies to know to what extent our DIT group differences depend on learned or in-born differences and to better understand why individuals with similar education may differ in their level of moral reasoning. Although the current results do not speak to this issue, we believe that both nurture (e.g., education, parental socialization, life experience etc.) and nature (e.g., biological or evolutionary basis, the innate capacities of the mind, the genetic basis, etc.) contribute to individual differences in moral development60. However, as a result of the correlative nature of this cross-sectional study, it is unclear whether brain function differences between the two groups are the cause or the result of differential levels of moral reasoning. Our study only documents differences in brain activation patterns associated with higher and lower levels of moral reasoning, but cannot explain the origin of those differences. Future research is needed to isolate the most formative socialization and educational experiences having an impact on moral development.

This study has some limitations which need to be addressed in future work. First, although other studies have demonstrated the predictive validity of the DIT for prosocial behaviors4, 14, we did not explicitly test whether post-conventional moral reasoning is associated with moral and prosocial behavior in our study. Future studies could also investigate the link between the neural correlates identified here and actual moral/prosocial behavior using moral reasoning or moral decision making tasks. Second, although our sample is relatively homogeneous in terms of age and level of education since these variables affect moral reasoning2, 61, unfortunately, we were not able to collect further education background information from our participants. For future research, it would be beneficial to know if individuals’ previous education background has any influence on their moral development. Third, previous studies have suggested that neural responsiveness in the VS and vmPFC may be modulated by mood states62. Although we did not assess participants’ mood states, this should be examined in further research. Fourth, we used the online version of the DIT-263 to assess participants’ moral reasoning level. However, level of moral reasoning can also be defined according to other metrics, such as the prosocial moral reasoning objective measure (PROM), which is similar to the DIT-2, but centered on a caring or prosocial perspective rather than on a justice- or prohibition-oriented perspective64. Future studies could also characterize moral reasoning using this measure. In addition, although some evidence suggests that resting CBF measured by ASL fMRI primarily reflects trait-like brain function65, longitudinal studies are needed to determine the stability of CBF in the brain reward system. Finally, recent studies suggested that the VS may not be homogenous and subregions of the VS may play differential roles in reward learning and motivation66,67,68. Specifically, in the VS core, dopamine dynamics are consistent with learning-based theories (such as reward prediction error), whereas in the VS shell, dopamine dynamics are consistent with motivation-based theories (e.g., incentive salience)66. However, the limited spatial resolution of ASL and BOLD fMRI in this study does not allow us to localize the CBF and BOLD activation differences to more specific subregions of the VS, and it is unknown whether the observed differential VS activity between the two DIT groups reflects learning-based or motivation-based differences in moral reasoning development. Future imaging studies with much higher spatial resolution are needed to address this question.

In summary, we combined ASL perfusion and BOLD fMRI and examined both resting and task-induced brain activity associated with post-conventional moral reasoning in a relatively homogeneous cohort of 64 Wharton School MBA students. Individuals who reached the post-conventional moral reasoning level have increased resting CBF and task-induced activation in the VS and vmPFC/sgACC, which are key regions related to decision making and reward processing. This suggests that high-level moral reasoning is associated with increased activity in the frontostriatal system, both in task-dependent or task-independent states. These findings support the view that the development of morality is associated with measurable aspects of human brain function.