His expertise lies at the intersection of anthropology, history of science, and the philosophy of modernity. Professor Rees is particularly interested in how categories that order thought/knowledge mutate over time (or differ across space) and in what effects these mutations have on conceptions of the human. Most of his empirical work has been about the neuronal sciences (the recent emergence of a plastic conception of the adult human brain) and global health. In addition, he has written about anthropology after ethnos, that is, about an anthropology concerned with thought and the human-in-motion, rather than with culture and/or society.