The two-tiered life history model: from interrogating assumptions to refining concepts and hypotheses

Abstract The two-tiered life history ( LH ) model proposes that different sources of extrinsic mortality (EM) have opposing effects that regulate development toward both slower and faster LH traits. Although the 53 commentators generally endorsed the two-tiered model and empirical conclusions of the target article, the devil is in the details. Some commentators challenged model assumptions (e.g., the mechanistic basis of the two-tiered model; whether the model is genetically confounded; the relative importance of child versus adult mortality). Other commentators proposed extensions/modifications of model concepts and hypotheses (e.g., incorporating density-dependent regulation; use of formal models to generate and test hypotheses; connection to the internal predictive adaptive response [PAR] model). In this reply, we review and address these challenges and proposed extensions/modifications. We hope that this iterative process advances our understanding of the complexity of EM, its opposing tiers, and their dualistic effects–both hierarchical and countervailing–on variation in human life histories.