Early demographic models of life-history evolution were formulated in a density-independent framework and saw extrinsic sources of mortality, such as predation, as the primary driving force that shaped the evolution of life-history traits. The evidence for density dependence in nature motivated theoreticians to build models that incorporated population regulation. These later generations of models acknowledge that demographic mechanisms of population regulation and extrinsic mortality interact with one another and predict a wide variety of life-history responses. Such ecologically realistic models require knowledge of the demographic traits and life-stages most affected by density. Despite the vast empirical literature characterizing population regulation, and a wealth of methods to analyze it, such mechanistic understanding is rare. Ecological experiments whereby density is manipulated can be a powerful tool to disentangle the life-history determinants of population regulation. Here we review published density-manipulation experiments and highlight how they can be coupled with existing analytical tools to extract the mechanistic information needed for evolutionary models of life histories.