Understanding human morality is of major interest across the cognitive and behavioral sciences. Empirical approaches often focus on two theories from moral philosophy — consequentialism and deontology —, explaining moral cognition by appealing to either calculation of consequences, adherence to rules, or both. By contrast, a third influential philosophical tradition — contractualism — has received little empirical investigation. According to contractualism, ethics is a matter of forming, adhering to, and enforcing (hypothetical) agreements. Drawing upon virtual bargaining — a recent psychological proposal that models social interactions in contractualist terms — we investigate moral contractualism in five preregistered online experiments (n = 3,636). We find that characteristically contractualist concerns (e.g., agreement, consent, mutual interests) heavily shape incentivized decisions in a new experimental game designed to split apart contractualism from consequentialism and deontology. Moreover, they influence moral judgments in three distinct settings. Contractualist reasoning may play a central role in human morality.

People solve a myriad of coordination problems without explicit communication every day. A recent theoretical account, virtual bargaining, proposes that, to coordinate, we often simulate a negotiation process, and act according to what we would be most likely to agree to do if we were to bargain. But very often several equivalent tacit agreements — or virtual bargains — are available, which poses the challenge of figuring out which one to follow. Here we take inspiration from virtual bargaining to develop a cognitive modeling framework for dynamic coordination problems. We assume that players recognize their common goal, identify one or more possible tacit agreements based on situational features, observe the history of their partner's choices to infer the most likely tacit agreement, and play their role in the joint plan. We test this approach in two experiments (n = 125 and n = 133) based on a dynamic coordination game designed to elicit agreement-based behavior. We fit our model at the individual level and compare its performance against alternative models. Across four different conditions, our model performs best among the set of models considered. Behavioral results are also consistent with players sustaining coordination and cooperation in the task by converging on tacitly agreed strategies or “virtual bargains”.

Contractualist moral theories hold that morality is primarily about acting according to what would be agreed by rational agents. The contractualist tradition has received little attention in empirical work compared to competing normative ethical theories that are primarily concerned with rules (deontology) and consequences (consequentialism). Recent theoretical developments and empirical results suggest that agreement-based considerations and forms of reasoning could play a central role in moral judgment and decision making. However, the computational foundations of moral contractualism have scarcely been explored. In this paper, we present a flexible experimental paradigm to study contractualist moral behavior in an incentivized setting. We then develop a simple computational model of agreement-based moral decision making that captures participants’ behavior in the task. Results suggest that agreement-based cognitive processes are required to explain the nuances of observed behavioral patterns. Contractualism could offer a fruitful framework to understand how humans coordinate their behavior in morally ambiguous contexts.