Adaptive changes in reproductibve effort (RE) of parasitized hosts may account for both inverse and positive relationships between host reproductive output and incidence or degree of parasitism. This hypothesis can be made more general (adaptive changes in patterns of allocation to reproductive functions), but has been largely ignored in the fields of behavioral and evolutionary ecology. Theory on life-history trade-offs predicts that hosts may minimize the impact of parasites by altering their own RE. Such alterations may occur with or without induction of defenses against parasites. Several empirical approaches exist for studying adaptive changes in host RE; all require a firm understanding of the natural history of the parasite-host association under study. Tests of adaptive changes in host RE (and patterns of allocation) should provide a better understanding of parasite-host interactions and coevolution as well as insights into the evolution of adaptive phenotypic plasticity. -from Author