Ellis and colleagues’ target article is a response to recent critiques of the “fast/slow” paradigm as applied to individual differences in human reproductive development (e.g., Dinh & Gangestad, Reference Dinh and Gangestad2024; Sterns & Rodrigues, Reference Stearns and Rodrigues2020). Most directly, the target article responds to Volk (Reference Volk2023), who argues that energetics is the primary, if not sole, environmental regulator of life-history development and that harshness/unpredictability, when defined by energetics, leads to slower rather than faster life-history strategies. Ellis and colleagues reject this one-tiered model and propose an integrated, two-tiered model whereby energetics operates either foundationally (in the case of puberty) or independently (in the case of adult reproductive behavior) from ambient/psychosocial cues of extrinsic mortality that impact development. Importantly, the two-tiered model permits countervailing predictions from the energetics and developmental hypotheses, such as maturing slowly with high fertility, not predicted by the standard fast/slow paradigm. The target article does an excellent job of describing the model, providing background literature, and even novel analyses in support of the two-tiered structure. However, while the target article makes a vital contribution by explicating the two-tiered structure, life-history theory applied to development was arguably always a two-tiered model.

The application of life-history theory to human development was originally described in Belsky, Steinberg, and Draper (Reference Belsky, Steinberg and Draper1991) and expounded upon in a 2009 landmark paper, also by Ellis and colleagues. In the 2009 study, Ellis et al. drew from the biological literature on life-history theory to propose evolutionarily shaped psychobiological mechanisms that “detect and respond” to ambient cues of extrinsic morbidity/mortality (EM). These cues were predicted to entrain development along faster vs. slower pathways (broadly speaking) in an evolutionarily coordinated way. In describing the ecological conditions that would lead to different evolutionary and developmental outcomes, the authors used the phrase “adequate bioenergetic resources” and similarly worded phrases roughly 15 times throughout the paper. Although the authors of the 2009 paper did not explicitly hypothesize a two-tiered model, they did repeatedly refer to energetic conditions as a foundational assumption for supporting growth and reproduction that had to be satisfied before EM cues could influence development. For example, under conditions of resource scarcity/energetic stress, the 2009 model hypothesized the development of slow life-history strategies, consistent with Volk’s arguments. It was only under adequate bioenergetic resources that EM cues were hypothesized to potentially lead to faster life-history strategies (see Figure 3 from that paper), consistent with the two-tiered model. Although the specifics about this underlying assumption regarding adequate bioenergetic resources and the consequences of its violation were not as well developed in 2009, the model was, in fact, two-tiered.

The target article builds upon the 2009 paper (as well as other recent theoretical developments) in a number of ways and departs from it by better describing how energetics and EM cues intersect to impact variation in adult reproductive outcomes, and to some degree, pubertal timing, rather than conceptualizing energetics as only an essential precondition. The target article also importantly points to future directions of research, focused on behavioral mediators in the relation between EM cues and reproductive behaviors, a much-appreciated guidepost for future research in this area. In total, though the empirical literature reviewed in the target article, as well as early formulations of life-history models, and even the literature reviewed in Volk (Reference Volk2023), coalesce around a two-tiered structure of energetics and EM cues on life-history development.

As it turns out, explicating the two-tiered model is consistent with recent calls to better develop life-history models applied to development. In a recent special issue on current debates in life-history research, Stearns and Rodrigues (Reference Stearns and Rodrigues2020) implored evolutionary psychologists to employ mathematical modeling – as it is done in biology – as a mechanism for theory development noting that these models can lay bare underlying assumptions as well as provide mechanical predictions as the values of model components are tweaked. Although the target article is a narrative theoretical development rather than mathematical, better clarifying what the assumptions are in the developmental life-history model, and what the consequences of assumption violation are, is consistent with the spirit of this call. In addition, and perhaps more importantly, the target article defines how variation in energetics may moderate the influence of EM cues on adult reproductive behavior. Highlighting the fact that adult reproductive parameters are influenced by early developmental experiences as well as current energetic conditions is also consistent with calls to incorporate additional parameters, such as proximate incentives, cultural norms, and mismatch, to explain more variance in reproductive outcomes than a single-tired, unidimensional fast-slow paradigm can on its own (Stearns & Rodrigues, Reference Stearns and Rodrigues2020). Clarifying the two-tiered model is a positive step in this direction, but as Stearns and Rodrigues point out: “…there is not yet any formal theory for plastic responses of the suites of correlated life history traits to which evolutionary psychologists often refer. The reason is simple: in such cases the interactions are so complex that analytic theory has proven intractable.” (p. 478). A broader, more comprehensive life-history theory of development will likely require additional tiers and/or incorporating additional dimensions, verifying additional underlying assumptions, and clearly delineating complex interactions at these multiple levels (see Volk, Reference Volk2025).

Last, highlighting exceptions to existing theoretical models does not necessarily invalidate the model. Instead, it presses the question of whether the model needs to be re-evaluated and/or if new evidence should be incorporated into existing thinking. Scientific debate is useful in that way as it necessitates a re-thinking of ideas as new perspectives are applied and/or new data emerge (e.g., Del Guidice, in press, Reference Del Guidice2020; Ellis et al., Reference Ellis, Sheridan, Belsky and McLaughlin2022; Volk, Reference Volk2025). Such is the case in this target article. The recent discourse around life-history theory reflected in Ellis and Volk’s work (and many others) will help the field to find a better way forward as the general life-history model of developmental plasticity is improved. No doubt that these efforts will lead to a stronger theoretical orientation and empirical approach without throwing the proverbial baby out with the bath water.