To begin with, the authors falsely claim that I advocate that human life histories are determined entirely by caloric constraints as a single-tier approach. I recently published that it is “my contention that humans can and do pay attention to variables that relate to life history trade-offs” (Volk, Reference Volk2021, p. 280). I have numerous publications on aspects of life history, including links with: personality (Franklin-Luther & Volk, Reference Franklin-Luther and Volk2022), behavior (Volk et al., Reference Volk, Brazil, Dane, Vaillancourt, Al-Jbouri and Farrell2025), developmental environments (Brazil et al., Reference Brazil, Farrell, Boer and Volk2025), and nutritional, resource, and social status (Volk et al., Reference Volk2023). In addition, I co-authored a systematic examination of ancestral levels of infant and child mortality rates that I related to life history (Volk & Atkinson, Reference Volk and Atkinson2013). My paper cited by the current authors (Volk, Reference Volk2023) was an attempt to examine a specific theory of human life history originated by Belsky, Steinberg, and Draper (Reference Belsky, Steinberg and Draper1991) and later expanded upon by Ellis and colleagues (Reference Ellis, Figueredo, Brumbach and Schlomer2009). If there was any confusion, I made the authors aware of my multi-tier stance on human life history in a lengthy, explicit review of a previous version of this manuscript that was rejected (I was not invited to review this current version). Thus, their one-tier argument is unfortunately a clear straw man argument as both my past and current (Volk, Reference Volk2025) publication history shows. Instead, my 2023 paper was aimed at the next problem that I will discuss: the issue of modernity in understanding human life histories.

It is undeniable that the greatest change in human life histories is the Modern Demographic Transition (MDT). Every country on the planet is undergoing the MDT, albeit at different speeds (Volk, Reference Volk2025). Critically, even individuals in the early stages of the MDT can exist in very different conditions than in ancestral conditions. Using modern data to understand past human adaptations is like using data on polar bear mortality in zoos to understand polar bears’ life history. It is highly constrained and problematic to rely on data from vastly different ecologies with dramatically different mortality structures while ignoring data from the actual ecologies and historical periods in which humans evolved our present mental adaptations (Tooby & Cosmides, Reference Tooby and Cosmides1990; Volk, Reference Volk2023). Even poorer modernizing countries are profoundly affected by the MDT (e.g., the onset of the MDT, provision of outside food, modernity, and medicine). Thus, my 2023 argument was not directed specifically against Western Educated Industrialized Rich Democratic (WEIRD) data as the authors claimed; it was aimed at highlighting the importance of considering ancestral environmental, maturational, and behavioral data when evaluating evolutionary hypotheses, which the current study largely fails to do.

A related problematic trend with the present data is an emphasis on quantity over quality. Some of the examples provided explicitly list important caveats on their models that are not reported (e.g., McCord, Conley, & Sachs, Reference McCord, Conley and Sachs2017 note that their model assumes that malaria affects fertility only through child mortality and “suggest the reader interpret the results with due caution” p.6). Other citations omit important information, such as Kramer and Greave’s (Reference Kramer and Greaves2007) data showing lower Pumé mortality was associated with not only equal pubertal age, but a 21% increase in birth rates. Or evidence omitted from Walker et al.’s (Reference Walker, Gurven, Hill, Migliano, Chagnon, De Souza and Yamauchi2006) small societies that did not fit the patterns the authors laid out. Still other citations did not control for what the authors claim they did (e.g., Canning et al., Reference Canning, Günther, Linnemayr and Bloom2013 do not control for stage, velocity, or duration of MDT). These are a small sample of the problematic citations that did not consider alternative explanations and could generate a misleading sense of confidence in the data for readers who are not inclined to closely scrutinize the entirety of the lengthy reference list.

This data ambiguity is important because it underlies a significant theoretical ambiguity. To begin with, this paper explicitly contrasts previous work by Ellis and Belsky (e.g., Ellis et al., Reference Ellis, Sheridan, Belsky and McLaughlin2022) that claims a coherent life strategy of advanced puberty alongside other markers of a fast life history. Perhaps that should be their example of a false one-tier model? The authors are silent in critiquing their own previous models of life history as well as addressing other related critiques of their fast-slow modeling (Volk, Reference Volk2025).

Even with regard to the novelty of mortality as a salient cue, the trigger for the MDT has already been well-publicized, for decades, as lower infant mortality (Dyson, Reference Dyson2010). However, that doesn’t mean that life history needs to be silent on the MDT. Models of life history theory (LHT) that include density-dependent selection (Stearns, Reference Stearns2000), a critical life history factor or “tier” that Ellis and colleagues have explicitly minimized (Reference Ellis, Figueredo, Brumbach and Schlomer2009), can account for much of the data in the current article, as well as many other modern phenomena including (but not limited to): the onset of the MDT when fertility increases due to decreases in infant mortality, the maladaptive drop to below-replacement fertility in later stages, the major exemption from the MDT (the 20^th Century Baby Boom), the negative influence of competition on fertility, and natural experiments showing fertility gains in the face of lower mortality and increased parental leave as well as general decreases in reproduction in the face of heightened COVID-19 mortality (Volk, Reference Volk2025).

Finally, the authors have amassed an impressive array of correlations. Unfortunately, I believe their causal interpretation is wrong. Are people speeding up reproduction when threatened or slowing down when child survival and successful rearing costs increase? Causality matters, and it is the latter explanation for the MDT that has been preferred by dozens of anthropologists and demographers (see Volk, Reference Volk2025 for a partial review), including some cited in this manuscript (e.g., Canning et al., Reference Canning, Günther, Linnemayr and Bloom2013). Thus, we can all safely agree that humans don’t make decisions about puberty, children, and life strategies based solely on whether or not they had an extra candy bar for lunch. Life history is not unidimensional. But it is ironically more complex than Ellis and colleagues’ two-tier model, and it will require both the correct usage of the correct data and the astute comparison of actual theories (versus straw men) to untangle the details of human life histories.