The distinction between moral monism and moral pluralism has been reflected in the early vision of moral philosophy. Moral pluralism can be traced back to moral relativism, which holds that there is no universal moral principle. And any moral value applies only within certain cultural boundaries and individual value systems. However, moral universalism, a monistic ethical position, holds that there are universal ethics that apply to all people. In recent years, the above theoretical confrontations have entered the field of moral psychology. The dispute between monism and pluralism is one of the most active theoretical controversies in the field of moral psychology in recent years. Moral monism holds that all external moral-related phenomena and internal moral structures can be explained by one factor. The representative theories are stages theory of moral development and dyadic morality theory and so on. On the other hand, moral pluralism holds that morality cannot be explained by a single factor, but there are many heterogeneous moral dimensions, which are culturally sensitive. The representative theories include the triadic moral discourse theory, the relational model theory and the moral foundations theory and so on.
Among them, the dyadic morality theory put forward by Kurt Gray et al. and the moral foundation theory put forward by Jonathon Haidt are the typical representatives of the disputes between monism and pluralism. Gray et al. argued that harm is the most powerful factor in explaining moral judgments and moral judgments about harm are more intuitive. Moreover, people with different political orientations reach a consensus that harm is the core of moral judgments. On the contrary, Haidt et al. believed that people of different political orientations, cultures and social classes is manifested with different moral foundations, and the moral foundations scale has good construct validity, discriminant validity, practical validity, etc. The disputes between the two theories mainly focus on the explanatory power of harm, the harmfulness of moral dumbfounding, modularity views and the problem of purity. Specifically, Gray et al. argued that moral dumbfounding stems from biased sampling that confounds content with weirdness and severity, rather than purity violation. They also believed that the so-called "harmless wrongs" can be explained by perceived harm. Importantly, purity cannot be regarded as an independent construct of morality. Moreover, there is few evidence to support the modular claims. Nevertheless, Haidt et al. believed that moral monism oversimplifies the connotations of morality. The different moral foundations are not " Fodorian modularity", but more flexible and overlapping "massive modularity". Furthermore, plenty of evidence supported purity as an independent moral foundation.
Future research should be carried out in the following aspects. First of all, morality must need a clearer definition. To ensure the validity of moral research, future research should try to define moral concepts more clearly and should ensure that only one construct is tested at a time. Without ensuring that the situation clearly reflects a certain moral dimension, it is difficult for researchers to pinpoint which moral dimension influences people’s moral judgments. Secondly, in addition to paying attention to the disputes between monism and pluralism, we also need to separate from the disputes, take an objective view of the different characteristics of the controversies, learn from each other and complement each other, so as to promote the development of moral psychology. Specifically, moral monism emphasizes the simplicity of moral constructs and the accuracy of measurement, while pluralism emphasizes the understanding of the nature of morality among people in different cultures. These are two different theoretical constructs and explanations of the nature of morality. Future research should combine the advantages of moral monism and moral pluralism, and try to adopt realistic situations with high ecological validity, so as to construct a more perfect integrated theoretical model. Last but not the least, most previous empirical studies have been dominated by the "WEIRD (Western, Educated, Industrialized, Rich and Democratic)” sample. Future research should urgently consider the possibility of carrying out morality research in different cultures, especially based on the Chinese culture to explore the nature of morality.