The cost-benefit paradigm is central to functional biology and to ecological and evolutionary theory because fitness costs and benefits associated with a trait determine its equilibrium value in a population and can be used to generate testable hypotheses about the function of TPI and the fitness consequences in N. attenuata plants. The following collection of papers examines the costs and benefits of TPI expression in N. attenuata. Here I show that trypsin proteinase inhibitors (TPIs) expression in Nicotiana attenuata decrease plant fitness when plants compete with unattacked conspecifics that do not produce TPI and also decrease the performance of attacking herbivores. In order to test whether the putative benefits of TPI production outweigh its cost, we compared the lifetime seed production of N. attenuata genotypes of the same genetic background with low or no TPI to that of genotypes with high TPI levels on which M. sexta larvae were allowed to feed freely. Caterpillar attack reduced seed capsule production in all genotypes and reversed the pattern of seed capsule production among genotypes, demonstrating that the fitness benefits of TPI production outweigh their costs, when plants are attacked. We conclude that despite the ongoing evolutionary interaction between N. attenuata and M. sexta, TPI-mediated decrease in herbivore performance translates into fitness benefit for the plant.