Ellis et al. present a compelling two-tier framework to explain the countervailing effects that two components of a major environmental factor – extrinsic mortality – have on life-history strategy. Their model is consistent with global life-history patterns since the 20th century – increasingly earlier age of menarche (Papadimitriou, Reference Papadimitriou2016; Wang et al., Reference Wang, Asokan, Onnela, Baird, Jukic, Wilcox and Mahalingaiah2024) accompanied by increasingly delayed reproduction (Frejka & Sardon, Reference Frejka and Sardon2006; Beets, Reference Beets2010; Beaujouan, Reference Beaujouan2020). Indeed, fertility is down to ultra-low levels in many countries, led by East Asia, where rates have recently dropped far below replacement level to an average of 1.03 (database.earth, 2024). However, given that mortality rates – and hence, cues – have not been significantly decreasing in recent decades (World Health Organization, 2024), extrinsic mortality alone is likely unable to account for reproduction being increasingly delayed or, for many individuals, altogether forgone. As such, it makes sense to ask if there are any ambient cues apart from extrinsic mortality that may be inducing people toward the ultra-slow life-history strategies that characterize many modern societies.

Recent research suggests yes – people’s reproductive timing psychology may be responsive to cues of resource/status competition and population density. A resource-competition perspective of life-history theory posits that environments with low-resource competition select for faster life-history strategies to facilitate quicker exploitation of the available resources, whereas environments with fierce competition select for slower life-history strategies to allow more time for accumulating competitive ability (MacArthur & Wilson, Reference MacArthur and Wilson2001).

For various species including humans, competition for status goes hand-in-hand with competition for resources. High-status individuals are more formidable and capable of success across various life domains than low-status individuals (Marmot, Reference Marmot2004). This affords high-status individuals greater access to resources and social allies (van Vugt and Tybur, Reference van Vugt, Tybur and Buss2015; von Rueden, Gurven, Kaplan, Reference Von Rueden, Gurven and Kaplan2011) and, for men, better access to mates (Buss & Schmitt, Reference Buss and Schmitt1993).

Resource availability and competition are largely affected by population density (Ellis et al., Reference Ellis, Figueredo, Brumbach and Schlomer2009; Sng et al., Reference Sng, Neuberg, Varnum and Kenrick2017). In dense populations, resources tend to be more scarce and competition more intense. In the modern world, population densities can be thousands of times larger than in the ancestral past. Further factoring in the conspicuous presence of ultra-wealthy individuals in digital media and social media, modern environments are inundating individuals with supernormal cues to resource and status competition. As with the case of extrinsic mortality, reproductive timing mechanisms may be sensitive to ambient cues to status/resource competition and population density, such that stronger and more prevalent cues – at evolutionarily novel levels in the modern world – incline individuals toward a slower strategy of greater status attainment at the expense of reproduction.

Consistent with this line of reasoning, simulations of theoretical models have shown that intense status competition accompanies steep declines in fertility (Shenk, Kaplan, & Hooper, Reference Shenk, Kaplan and Hooper2016). Two experimental studies found that priming people with a higher desire for social status induced them to want children later and to prefer offspring quality (investing more in one child) over quantity (having multiple children but investing less in each) (Lim et al., Reference Lim, Li, Manesi, Neuberg, van Vugt, Meltzer and Tan2023). Likewise, population density has been shown to correlate with a greater long-term orientation, later marriage age, lower fertility, and higher parental investment (Sng et al., Reference Sng, Neuberg, Varnum and Kenrick2017). Moreover, experiments have demonstrated that compared to perceptions of low population density, perceptions of high density lead to a preference for fewer relationship partners and children (Sng et al., Reference Sng, Neuberg, Varnum and Kenrick2017) and less favorable attitudes toward reproduction (Lim & Li, Reference Lim and Li2023). Further demonstrating the link between resource/status competition and population density, perceptions of crowdedness were found to be positively related to perceptions of a competitive environment and social status uncertainty (Lim & Li, Reference Lim and Liin preparation). Moreover, other studies found that for individuals who grew up with abundant resources, economic uncertainty induced them to want to further their education and career and to reproduce later, compared to individuals low in childhood socioeconomic status, who tended to react in the opposite, faster direction (Tan et al., Reference Tan, Li, Meltzer, Chin, Tan, Lim and van Vugt2022).

Taken together, these studies suggest that ambient cues to competitive stress may be contributing to slower reproductive intentions and preferences. Whereas extrinsic mortality cues signal a limited time horizon and hence, a need to reproduce faster, competitive stress may indicate that more work is needed to build competitive abilities, especially to slow life-history individuals who grew up with very little energetic stress (Griskevicius et al., Reference Griskevicius, Ackerman, Cantú, Delton, Robertson, Simpson and Tybur2013). Hence, findings from this line of work, considered along with decreasing extrinsic mortality rates in the modern world, suggest that reproductive timing mechanisms may have evolved to process various cues at the second tier, and that investigating these processes may be instrumental in understanding the increasing prevalence of ultra-low fertility. Moreover, they are consistent with the idea that evolutionary mismatch – in this case, a life-history mismatch (Li & Manesi, Reference Li and Manesi2017) – may underlie many modern peculiarities and problems (Li, van Vugt, & Colarelli, Reference Li, van Vugt and Colarelli2018; Li, Yong, & Van Vugt, Reference Li, Yong and Van Vugt2020).