The two-tiered life history (LH) model proposed by Ellis, Reid, & Kramer (Reference Ellis, Reid and Kramer2024) offers a compelling framework for understanding how extrinsic mortality (EM) sources—energetic stress (first tier) and ambient cues to EM (second tier)—shape life history strategies, including fertility. The model’s strength lies in explaining countervailing effects—where energetic stress delays puberty but ambient EM cues increase fertility—as seen in hunter-gatherers and LMICs. This commentary evaluates the two-tiered LH model in light of modern fertility trends, identifies gaps in its current formulation, and proposes modifications to enhance its explanatory power in modern contexts.

While the model effectively explains fertility patterns in small-scale and low-income societies, its application to modern societies—whether post-industrial (e.g., U.S., Germany, Japan) or rapidly developing (e.g., China, India, Brazil)— requires careful modification. Modern societies exhibit a universal shift from high-fertility, high-mortality regimes to low-fertility, high-investment strategies—such as delayed childbearing, below-replacement fertility (TFR < 2.1), and socioeconomic gradients in reproductive timing—suggesting that additional factors mediate the relationship between environmental cues and fertility outcomes (GBD 2021 Fertility and Forecasting Collaborators, 2024; Salonen et al., Reference Salonen, Lahdenperä, Rotkirch and Lummaa2024).

In industrialized and rapidly developing nations, energetic constraints are typically minimal due to abundant nutrition and healthcare. High resource availability minimizes nutritional barriers to puberty, leading to worldwide earlier onset of thelarche as well as menarche (Eckert-Lind et al., Reference Eckert-Lind, Busch, Petersen, Biro, Butler, Bräuner and Juul2020). However, obesity—a form of energetic surplus—further accelerates puberty, complicating the “harshness” narrative. In addition, low EM (due to healthcare, sanitation) reduces direct survival threats and the fitness benefits of high fertility, favoring slower LH strategies (e.g., delayed childbearing, lower fertility rates).

In ancestral settings, ambient EM cues (e.g., infant mortality, violence) directly signaled future instability. While in modern societies, ambient cues like socioeconomic instability, or perceived insecurity may replace direct mortality signals and dominate reproductive decision-making, functioning as “modern EM”. For instance, perceived insecurity (e.g., pandemics, climate crises, or social unrest) may indeed decrease fertility rates by amplifying future discounting, where individuals prioritize short-term security over long-term investments like raising children. The COVID-19 pandemic initially triggered widespread fertility declines globally due to economic uncertainty, health risks, and social disruptions, but some countries—particularly those with robust welfare systems—later experienced rebounds (Bailey, Currie, & Schwandt, Reference Bailey, Currie and Schwandt2023). However, in the later phase of the pandemic, many higher-income countries experienced yet another shift in birth rates, an unexpected and robust downturn from early 2022 that often persisted or even accelerated later in 2022 and 2023 (Winkler-Dworak, Zeman, & Sobotka, Reference Winkler-Dworak, Zeman and Sobotka2024).

Modern societies exhibit relatively homogeneous fertility decline despite low EM, a paradox not fully addressed by the two-tiered model. To reconcile this, the paper’s two-tiered LH model requires expansion to address why modern societies deviate from ancestral fertility patterns, and what mechanisms drive low fertility in high-resource environments. Modern low fertility could be reframed as a novel LH strategy response to high investment costs (quality-quantity tradeoff), or a mismatch where ancestral LH plasticity is maladaptive in post-industrial environments. Sociocultural mediators like education, gender equality, contraception access and economic opportunity may decouple fertility from mortality cues, emphasizing quality over quantity investments. Cultural norms (e.g., delayed parenthood) and institutional support (e.g., parental leave) may override LH tradeoffs, complicating the model’s predictions.

The two-tiered model provides a robust foundation but requires extension and refinement to address the nuanced interplay of modern “proxy cues” to mortality, energetic abundance, and sociocultural buffers in modern fertility trends—from below-replacement fertility to persistent socioeconomic disparities in reproductive timing. While ambient EM cues likely persist as drivers of reproductive behavior, their effects are moderated by sociocultural mediators. Future research should test these adaptations to industrialized contexts using cross-cultural datasets, natural experiments (e.g., economic crises), and gene-culture coevolution models.