After some consideration about the recent interest in experimental philosophy, I must state some charitable features of the role of psychological data:
1. (perceived) Asymmetries: Moral theorising is often percieved and practised as an a priori excercise. A utilitarian may say that moral decisions may be made on the basis of the amount of welfare or gain or investment into one’s own ends ( which include, inter alia, happiness); but this kind of view may be too simplistic. Why?
If we were to accept a few propositions a priori we may asses moral situations with these generic principles, this seems obvious. If we consider utility as our moral desiderata, we may say that some moral situations are parallel; such as whether to forsee the death of a minority to save a majority, or to perform animal testing. We may find, through empirical studies that what moral situations are a priori (through these normative ethical principles) symmetrical are in fact, perceived as asymmetrical. To follow up on this thought, consider the Knobe effect.
The conclusion of these kinds of studies is not to say something simple like, there is empirical data to refute a normative thesis (this never will work), but it is simply not as easy to apprehend moral situations viz moral principles without considering the influence of our background psychological dispositions (c.f. priming studies [Doris 2003 et. al])
2. The Kantian appeal: This argument comes straight from my dissertation, which in itself is more or less an argument from Kant. Kant believed that human anthropology assists us in knowing about human beings. We know about human nature in various ways; through the people we meet in our lives; through media, like television, film, literature; and through empirical and ethnographic study of others. Sociological and anthropological data can tell us about the ways in which human beings do in fact behave, my favourite examples for this kind of thing is Goffman in the Presentation of Self in Everyday Life, where he spend some time analysing hotel staff in the Shetland isles.
The crux of Kant’s point was that given a moral system (in his case that was his deontology, but we don’t need to commit to any moral system for this line of argument), we still need empirical knowledge of people so as to know how to apply it. Consider the platitude of do not lie, we might be able to manipulate a social situation so as not to lie, but not to tell the truth, or not to bring the offending issue at hand, or non-participation in any situation where you may be brought to lie. Knowing how to apply moral principles is not enough to help us as agents, having the know-how and practical wisdom of the conduct of human beings would help as well.