Reply to Harris and Chan: Moral judgment is more than rational deliberation

In their letter, Harris and Chan (1) challenge our conclusion that serotonin modulates moral judgment and behavior (2). Unfortunately, their argument is based on a narrow definition of moral judgment, one that is out of touch with empirical research. Harris and Chan (1) seem to subscribe to a rationalist view of moral judgment that defines moral judgment as the exclusive preserve of a conscious, deliberative reasoning process. However, an overwhelming amount of evidence shows that intuitive and emotional processes play significant roles in moral judgment (3). One challenge to the rationalist view is that people often form moral judgments without being able to articulate the reasons for their judgments (3). Moreover, patients with ventromedial prefrontal damage, whose analytical reasoning processes are intact but socioemotional processes are impaired, show altered moral judgment in the same dilemmas used in our study (4). Although our research cannot address the potential role of serotonin in conscious, moral deliberation, it clearly shows how serotonin modulates intuitive moral judgments.

Considering whether serotonin affects moral behavior, Harris and Chan (1) claim “it does so adversely by impairing moral judgment, subjugating it to emotional instinct.” This statement falsely assumes that behavior guided by reason is morally superior to behavior motivated by emotional instincts. Emotions like empathy strongly predict altruistic helping behavior (5), and nonhuman primates, who lack the capacity for language-based moral reasoning, nevertheless engage in prosocial acts such as consoling distressed peers (6).

Harris and Chan (1) also question whether enhancing serotonin increases harm aversion, stating that “saving five lives, albeit through violence, is also a moral outcome that expresses aversion to harming others.” Here, they misunderstand the definition of harm, which refers to personally and directly hurting others as opposed to general loss of lives. Although choosing the utilitarian option reduces overall lives lost, it nonetheless leaves the actor with greater personal responsibility for harming another.

Harris and Chan (1) suggest that it is unclear whether enhanced harm aversion “would promote either moral behavior in the public at large or indeed, harm reduction more generally.” We agree that the issue of what constitutes moral or prosocial behavior is ambiguous; an action can be simultaneously harmful to one individual and helpful to society. In fact, this ambiguity is reflected in our study's scenarios, where deontological and utilitarian perspectives motivate different moral decisions. As such, we deliberately avoided claiming that serotonin enhances or impairs moral judgment or that it makes people's behavior more or less moral. Rather, we suggested that serotonin modulates prosocial sentiments, which, in turn, bias moral judgment and behavior in complex ways.

Defining which mechanisms distinguish moral from other socially relevant decisions is a profound and unresolved issue. Some may define moral decisions as solely the purview of our conscious reasoning systems, but this view ignores the substantial contributions psychology and neuroscience have made to our understanding of how individuals make moral decisions, independently of whether they are the most morally sensible ones. Such knowledge has implications for how we define morality, and we hope that scholars across disciplines continue to engage one another in this debate.