A computer simulation was used to investigate the relative effects of changes in net fecundity and development time on population growth rate (r). Increases in net fecundity had the largest effect on r when net fecundity is small. Conversely, decreases in developmental time had the largest effect on r when net fecundity was large. The intersection point where similar changes in developmental time, and net fecundity had equivalent effects on r was also investigated in four organisms with very different life histories. Cases were considered where changes in net fecundity and developmental time are correlated and different results were obtained.