Making policy to ensure separated parents pay child maintenance is a tricky business. It involves managing numerous competing interests: those of the state, the children and the parents both as individuals and as co-parents. On socio-economic and moral grounds, policy makers want everyone to pay something, but instead they can face high rates of non-compliance, as found in the UK’s agency-based system. This article argues that the ‘assumptive world of policy makers’ forms part of the problem of non-compliance. They believe it is founded on instrumental rational action and is indicative of selfish individualism. But this collides with the behaviour of non-resident fathers; paying maintenance is underpinned more by affective rationality and is exemplified in the norm of reciprocity. Reciprocity guides parents’ social and material exchanges with each other and explains not only how contact and maintenance are linked but how they act as expressions of parents’ affective ties. Policy that treats child maintenance as a ‘public good’ requiring rational actor solutions tends to dismiss the power of reciprocity as illegitimate; as merely a smokescreen for selfishness. This misses the point, child maintenance is not a public good but a private obligation operating between individuals. There is room to be cautiously optimistic about policy that returns child maintenance back into the hands of parents to make private agreements.