Among naturalist philosophers, both defenders and opponents of moral relativism argue that prescriptive moral theories (or normative theories) should be constrained by empirical findings about human psychology. Empiricists have asked if people are or can be moral relativists, and what effect being a moral relativist can have on an individual’s moral functioning. This research is underutilized in philosophers’ normative theories of relativism; at the same time, the empirical work, while useful, is conceptually disjointed. Our goal is to integrate philosophical and (...) empirical work on constraints on normative relativism. First, we present a working definition of moral relativism. Second, we outline naturalist versions of normative relativism, and third, we highlight the empirical constraints in this reasoning. Fourth, we discuss recent studies in moral psychology that are relevant for the philosophy of moral relativism. We assess here what conclusions for moral relativism can and cannot be drawn from experimental studies. Finally, we suggest how moral philosophers and moral psychologists can collaborate on the topic of moral relativism in the future. (shrink)