This article takes a new idea, ‘normative behaviourism’, and applies it to global political theory, in order to address at least one of the problems we might have in mind when accusing that subject of being too ‘unrealistic’. The core of this idea is that political principles can be justified, not just by patterns in our thinking, and in particular our intuitions and considered judgements, but also by patterns in our behaviour, and in particular acts of insurrection and crime. The (...) problem addressed is ‘cultural relativism’, understood here not as a meta-ethical doctrine, but as the apparent ‘fact’ that people around the world have culturally varying intuitions and judgements of a kind that lead them to affirm different political principles. This is a problem because it seems to follow (1) that global agreement on any substantial set of political principles is impossible, and (2) that any political theory in denial of this ‘fact’ would be, for that reason, deeply unrealistic. The solution argued for here is that, if domestic political principles (i.e. principles intended to regulate a single state) could be justified by normative behaviourism, and in reference to culturally invariant behaviour, then an international system supportive of such principles is justifiable by extension. (shrink)