Theoretical treatments of the relationship between paternity (or certainty of paternity) and paternal care predict, under certain conditions, that individual males should invest less in paternal care if their paternity is reduced (Whittingham et al. 1992; Westneat & Sherman 1993). Several recent papers have addressed this question using information about males’ genetic paternity from unmanipulated breeding attempts together with measures of those males’ contributions to some components of parental care (Table I). The general picture that emerges from these studies is that there is no positive relationship between paternity and paternal effort (although in some studies the sample size was small, and statistical power limited). However, we argue here that it is inappropriate to use data of this kind to test models relating paternity to paternal care. We discuss the sort of data that can provide a better test. Models of optimal paternal care in response to variation in paternity are based on the effects of trade-offs between current reproductive effort and future reproductive success (Westneat & Sargent 1996). They ask, in effect, what the optimal response in terms of parental effort would be for an individual male responding to a change in paternity. Individuals may lie at different positions along a trade-off between current and future reproduction, and may also experience qualitatively different trade-offs because of (for example) differences in resource availability (van Noordwijk & de Jong 1986). As a consequence, one cannot assume that all males within a