||ABSTRACT: Though marine protected areas (MPAs) have recently become a central element of many marine resource management plans, the potentially negative effects of MPAs for fisheries harvests remain the major impediment to their use. Understanding population responses to implementation of MPAs, and their consequences for harvests, is essential to assessing the value of MPAs as a management tool. Here I use a simple model to link what we know best about marine species, namely how fast they grow and how much they reproduce, with what we want to know, namely what will be the effects of spatial management efforts. Specifically, I show that whether maximum sustainable yield (MSY) or yield at fixed total recruitment increases with MPAs can be determined by an intuitive equation comparing the change in average individual biomass (i.e. harvestable material) with the change in individual lifetime reproduction due to increasing reserve area. Application of this model to different population structures demonstrates that initial MPA results indicating MSY would be unchanged with reserves for species with simple life-histories do not hold up for species for which biomass, reproduction or mortality vary with age. This explains many negative MPA results from age-structured models of optimally exploited species. On the other hand, the model predicts that species harvested later in life, and which begin reproduction after first harvest, may benefit from MPAs. Nevertheless, for these increases to occur, the age at first reproduction must in general be a considerable time after the age at first harvest.