The developmental mode of parasitoid wasps links social evolution and the evolution of life-history traits. Parasitoids develop in either a solitary or gregarious brood. This dichotomy is dependent upon the interaction between competitors, usually siblings, and in the case of solitary species frequently results in the evolutionary phenomena of parent-offspring conflict. Aggressive or non-aggressive larval phenotypes, (hence solitary or gregarious development) represent a divergence within the social environment, which may subsequently influence the evolution of a number of important behavioral and life-history traits. We focus upon our recent empirical studies conducted with the braconid genus Cotesia. In particular, we examine how the outcome of parent-offspring conflict/sibling rivalry may provide the overall framework in which female wasps utilize endogenous and exogenously derived information in making clutch size decisions.