We consider a cooperatively breeding group and find the optimal pattern of reproductive parasitism by a subordinate helper as a function of its body size, and hence the share of reproduction obtained by the subordinate. We develop the model for the social system of the cooperatively breeding cichlid fish Neolamprologus pulcher but the general framework is also applicable to other cooperative systems. In addition to behaving cooperatively by sharing tasks, sexually mature male cichlid helpers may directly parasitize the reproduction of dominant breeders in the group. We investigate the relative influence of life history and behavioural variables including growth, parasitism capacity, future reproductive fitness benefits and costs, relatedness and expulsion risk on the optimal reproductive strategy of subordinates. In a detailed analysis of the parameter space we show that a male helper should base its decision to parasitize primarily on an increase in expulsion risk resulting from reproductive parasitism (punishment), intra-group relatedness and the parasitism capacity. If expulsion risk is high then helpers should not parasitize reproduction at medium body size but should parasitize either when small or large.