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In many mammalian species, males are selected to kill unrelated infants and/or fetuses in order to cause lactating and pregnant females to begin cycling sooner than they otherwise would. As a result, females have evolved numerous counterstrategies to prevent infanticide and feticide. One such proposed counterstrategy is the Bruce effect, an apparently costly strategy in which inseminated or pregnant females cease reproductive investment in a developing embryo or fetus following exposure to nonsire males. Here I present a quantitative model that seeks to explain under what conditions females will be selected to exhibit the Bruce effect (i.e. to block or terminate pregnancy) rather than risking future infanticide or feticide. I first present an analytical model of the costs of the Bruce effect relative to the costs of potential feticide or infanticide. I then test the resulting predictions using an individual-based model operating …