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.

The two-tiered life history model remains inadequate in addressing how social factors (particularly cultural norms and religious beliefs) interact with biological determinants to shape life history strategies. This limitation significantly constrains the model’s explanatory power when applied to certain historical demographic phenomena. We recommend introducing a third explanatory framework (the cultural-institutional tier) to augment the two-tiered model.

While I applaud the effort that went into their article, I believe Ellis and colleagues’ two-tiered theory of life history has three problems: (1) relying on a straw man argument; (2) limitations in their data; and (3) alternate theoretical explanations for their correlations. Failure to attend to these issues resulted in a more confident presentation of their conclusions than is warranted.

Life history strategies are linked to reproduction. Fast life history strategies place a particular burden on women biologically, socially, and politically. From the point of view of women’s rights and gender equality, slower life history strategies are the more optimal solution. In the article, I show how feminism has an important role to play in slowing down life history strategies.

In this commentary, I highlight the challenge of incorporating the harshness–unpredictability framework into the two-tiered model. While the effects of energetic stress (first tier), which are mostly mediated through biological pathways, can be decomposed into harsh (high mean) and unpredictable (high variance) components, ambient cues to EM (second tier) blur this distinction because they often share the same psychological mediators.

The two-tiered model proposed by Ellis et al. offers a compelling framework for understanding how extrinsic mortality (EM) sources shape life history strategies, including fertility. This commentary evaluates this model in light of modern fertility trends, identifies gaps in its current formulation, and proposes modifications to enhance its explanatory power in modern contexts.

Ellis, Reid, and Kramer advance a two-tier model of extrinsic mortality and life history strategies, emphasizing that extrinsic mortality can promote higher reproduction. The article implies this second tier operates via conscious, volitional processes. I present evidence that extrinsic mortality events are not merely environmental cues but also bereavement-inducing events that can define histories principally by disrupting, not facilitating, life strategies.

The relationship between local extrinsic mortality rates and the timing of life-history milestones can be better understood when examined through food environments rather than by population “types,” like small-scale societies or high-income countries. By mapping observed life-history variation onto food environments, which mediate the fundamental relationship between mortality and fertility, the explanatory value of a 2-tiered model becomes less compelling.

I argue that the two-tier model represents a clear step forward, resolving debates and disentangling complexity in our understanding of human life history strategies. I relate the model to findings from my previous work, as well as to embodied capital and market integration, providing relevant schematics. I then discuss the timing of exposure to mortality cues and the age of the deceased, discussing what the two-tier model does and does not add to these debates.

In response to recent arguments for a one-tiered model of life-history development, Ellis and colleagues present a two-tiered model that integrates energetics with development. Early formulations of this model were, however, already two-tiered. The target article separates itself from earlier work by digging deeper into the underlying energetics assumption and how it transacts with ambient cues to impact development.

Ellis et al.’s two-tiered model compellingly integrates biological and psychosocial pathways through which extrinsic mortality shapes life history strategies. An understated, yet important, implication of their framework is the role of social inequalities in signaling mortality risk – particularly through differential exposure to bereavement – and how it may give rise to distinct life history strategies and outcomes.

After presenting their two-tier model, Ellis, Reid, and Kramer (2024) tentatively state that child mortality risk should predict offspring quantity, while adult mortality risk should accelerate reproductive timing. I test these assertions using mortality risk and fertility data from the Global Burden of Disease (GBD) project for 204 countries and territories over 32 years, finding partial support for their predictions.

Building upon the two-tiered model of life history strategies introduced by Ellis, Reid, and Kramer (2024), we discuss avenues for future research that apply the theory to sexual and gender minority individuals. We propose that minority stress and suicide exposure may serve as extrinsic ambient mortality cues among sexual and gender minority (SGM) populations. We also identify a feature of pubertal development requiring more consideration.

The field of life history “theory” is in a state of heightened entropy; the paper by Ellis, Reid, and Kramer provides a proposal for structuring future research on this topic. I find their proposal a constructive one and describe its potential benefits. Furthermore, I propose several ways in which we can additionally develop the life history framework.

Our systematic analysis of operationalizations and conceptualizations of harshness (extrinsic mortality) and unpredictability in the (psychology) life history literature highlights that (1) employment of extremely diverse measures contributes to the confusion about the effect of harshness on life history traits and (2) few measures reflect energetic stress or ambient EM, such that Ellis et al.’s two-tiered model should motivate future research.

The featured article introduces a much-needed theoretical framework for developing a dual-process model of life history calibration. This model accounts for the counterbalancing effects of individual energetic stresses and extrinsic mortality threats of the environment. This framework also reinstates resource availability – a key determinant of energetic conditions – into life history research, resolving its previous exclusion due to similar countervailing influences relative to extrinsic mortality threats.

Simulations, correlational studies, and laboratory experiments have recently shown that cues to high-resource competition may be steering individuals toward increasingly less favorable attitudes toward reproduction and slower life-history strategies. This work suggests that, alongside extrinsic mortality, it may be fruitful to examine the role of competitive stress cues in shaping life-history strategies.

Ellis and colleagues propose that plasticity in human life history strategy (pubertal timing, reproductive pacing) is governed by energetic and mortality-cue related regulatory tiers. We feel that our published interpretation of these findings – that growth (and thus maturational timing) is energetically driven, while reproductive behaviors are largely free to vary independent of energetics – provides a more parsimonious interpretation of this work. The concept of tiers is mechanistically vague and introduces unnecessary complexity to this literature.

This commentary complements Ellis et al.’s two-tiered framework by using panel data and emphasizing the role of population density in decelerating life history (LH) strategy. We show that mortality cues and energetic constraints do not operate in isolation, demonstrating how density-dependent competition shapes LH strategies over time and underscoring the need for a dynamic, multifaceted approach to LH development.

The force of selection on survival changes with age and models of reproductive effort (RE) reflect this finding that increases in infant and adult mortality have different effects on optimal effort. Using an age-structured demographic model, I show that infant mortality constrains selection on the human life cycle. Effort directed at increasing infant survival is strongly favored over efforts to increase adult survival. Complete accounts of human reproductive decisions require age-specificity of effects.