When a male's paternity differs from one breeding attempt to another, the male's parental effort is expected to increase as paternity increases. Davies et al. (1992, Anim. Behav. 43, 729-745) and Whittingham et al. (1993, Anim. Behav., 46, 139-147) manipulated paternity in monogamous dunnocks, Prunella modularis, and tree swallows, Tachycineta bicolor, respectively and found that parental effort did not vary with paternity. Previous discussions of this topic are reviewed and various examples presented to illustrate that the optimal parental effort may not depend strongly on paternity. In contrast to Whittingham et al., it is argued that the existing data do not indicate a discontinuous relationship between effort and paternity. Reasons to be sceptical about attempts to infer the shape of the relationship between parental effort and offspring survival from the form of the relationship between paternity and parental effort are also presented.