Ellis, Reid, and Kramer (this issue) present an elegant and powerfully explanatory two-tiered model for the seemingly disparate effects of extrinsic mortality (EM) on reproductive patterns constituting life history strategies. By distinguishing between sources of EM that reflect a lack of survival/energetic resources versus those that reflect the presence of cues of threat, the authors make sense of findings that EM can predict both faster life history strategies – i.e., sooner and more reproduction – and slower life history strategies – i.e., delayed sexual maturation. That is, energetic stress (tier one) is related to later puberty, whereas ambient cues to EM are related to accelerated reproduction and more numerous offspring only once energetic conditions allow for puberty to occur. Due to its failure to clearly distinguish between conceptualizations and operationalizations of extrinsic mortality, or harshness, the life history literature, particularly in psychology, has necessitated Ellis and colleagues’ (this issue) clarifying model.

Indeed, we (Maranges, Timbs, et al., Reference Maranges, Timbs, Psihogios and Haddad2025) conducted a systematic (PRISMA) review of the life history in psychology literature in order to classify and assess the frequency of use of extant conceptualizations and operationalizations of childhood harshness (as well as of unpredictability), detail the designs and samples of the studies, create a database with all the measures, reflect on the strengths and weaknesses of the various measures, and discuss theoretical implications. Our review included 250 studies within 156 articles, which included 199 measures of harshness and 78 measures of unpredictability. Although a deep dive into that analysis is beyond the scope of this commentary, our basic findings provide some insight on why there is much confusion about whether and when EM predicts faster versus slower life history strategies. In short, the literature often discusses harshness, and therefore its effects, as a monotonal factor theorized to reflect morbidity-mortality. Yet, the literature measures it in so many different ways, including in terms of modern survival resources, psychological deprivation, and physical threat (Ellis et al., Reference Ellis, Sheridan, Belsky and McLaughlin2022). As Ellis et al. (this issue) demonstrate, the effects of these sources of EM will differ. Accordingly, it is difficult to compare across studies and draw clear conclusions about the effects of harshness, profoundly limiting theory-updating and -building.

Our systematic review found that the most frequent operationalization of harshness was socioeconomic status (SES; 41.6%, e.g., Belsky, Schlomer, & Ellis, Reference Belsky, Schlomer and Ellis2012; Maranges & Strickhouser, Reference Maranges and Strickhouser2020), followed by household/family harshness (18.1%, e.g., Byrd-Craven et al., Reference Byrd-Craven, Geary, Vigil and Hoard2007; Chua et al., Reference Chua, Lukaszewski, Grant and Sng2016), which included parenting and family functioning variables. A smaller proportion of work operationalized harshness in other ways, such as safety, danger, and violence (15.7%; e.g., Amir, Jordan, & Bribiescas, Reference Amir, Jordan and Bribiescas2016; Chua et al., Reference Chua, Lukaszewski, Grant and Sng2016; Simmons et al., Reference Simmons, Rowan, Knowles, Steinberg, Frick and Cauffman2019); morbidity/mortality (5.3%; e.g., Pepper & Nettle, Reference Pepper and Nettle2013; Szepsenwol et al., Reference Szepsenwol, Shai, Zamir and Simpson2021); general harshness (4.3%; Lettinga et al., Reference Lettinga, Mell, Algan, Jacquet and Chevallier2021; Mell et al., Reference Mell, Safra, Algan, Baumard and Chevallier2018); and exposure to stress (2.5%; e.g., Valderrama, Macrynikola, & Miranda, Reference Valderrama, Macrynikola and Miranda2020; Wuth et al., Reference Wuth, Mishra, Beshai and Feeney2021). Each representing less than 2% of measures were harshness operationalizations of sexual abuse, general stress, household/family unpredictability, and exposure to prenatal/birth stress. Manipulations of harshness (8.1%; e.g., Amir et al., Reference Amir, Jordan and Bribiescas2016; Griskevicius et al., Reference Griskevicius, Delton, Robertson and Tybur2011) predominately reflected economic or resource scarcity or cues of EM.

In the literature we reviewed, relatively few operationalizations capture energetic stress, though more frequently, ambient cues of EM (e.g., violence) are operationalized, making it difficult to apply the two-tiered model to reproductive and broader life history strategies. Notably, Ellis and colleagues (this issue) do not focus on other sources of EM, such as SES, parental psychopathology, or father absence. This highlights an important future avenue of theory and research linking variegated sources of harshness and puberty and reproduction. To the extent that SES, the most common operationalization for harshness, is associated with energetic stress – in that very low levels of SES may entail energetic deprivation – we would expect SES to be part of tier one in predicting sexual maturation. Once SES is high enough to afford energy resources, then the presence of those other EM cues should predict faster life history strategies in the form of increased reproductive effort and output.

Ellis and colleagues (this issue) also do not examine the effects of unpredictability (i.e., random variation in EM) on puberty and reproduction. Adding more confusion to understanding how EM is linked to life history outcomes, our analysis finds that the most frequent conceptualizations and operationalizations are far afield from stochastic changes in morbidity-mortality. Instead, the plurality (38.5%; e.g., Brumbach, Figueredo, & Ellis, Reference Brumbach, Figueredo and Ellis2009; Kim-Spoon et al., Reference Kim-Spoon, Maciejewski, Lee, Deater-Deckard and King-Casas2017) of studies measure unpredictability with changes in aspects of the household and family. The second most frequent conceptualization captures multiple sources of unpredictability, including, e.g., the family/household, economic resources, and family structure (16.6%; e.g., Hartman et al., Reference Hartman, Li, Nettle and Belsky2017; Maranges & Strickhouser, Reference Maranges and Strickhouser2020). This is followed by parental structure (12.7%; e.g., Ellis & Garber, Reference Ellis and Garber2000; Hehman & Salmon, Reference Hehman and Salmon2021), stress and stressful experiences (4.9%; Dickerson & Quas, Reference Dickerson and Quas2021; Zhu, Hawk, & Chang, Reference Zhu, Hawk and Chang2018), and economic instability (4.4%; e.g., Belsky et al., Reference Belsky, Schlomer and Ellis2012; Hartman et al., Reference Hartman, Sung, Simpson, Schlomer and Belsky2018). Rarely, but still to the detriment of clear conceptualizations, what is said to be unpredictability actually reflects harshness, including safety, danger, and violence (4.38%; e.g., Kogan et al., Reference Kogan, Cho, Simons, Allen, Beach, Simons and Gibbons2015; Maranges et al., Reference Maranges, Hasty, Maner and Conway2021); SES (.98%; i.e., Hill et al., Reference Hill, Rodeheffer, DelPriore and Butterfield2013); and perceived life expectancy (.49%; i.e., Dickerson & Quas, Reference Dickerson and Quas2021). Manipulations of unpredictability (16.6%; e.g., Gregory et al., Reference Gregory, Quoidbach, Haase and Piff2021; Mittal & Griskevicius, Reference Mittal and Griskevicius2014) largely reflect economic uncertainty and sometimes general unpredictability or cues of the presence of EM. Whether changes in energetic stress and ambient EM shape puberty and reproductive patterns according to the two-tiered model is an open question, and the extant literature appears to lack operationalizations of those constructs.

The implication of the two-tiered model and our analysis is that measuring and explicitly delineating between energetic stress and ambient cues of morbidity are key to understanding how ontogenetic environmental challenges shape life history traits. Our analysis underscores that future work should also consider how unpredictability, very often centered in life history research, shapes sexual maturation and reproduction. Without clarity on those concepts and operationalizations, it is impossible to build and revise life history theory as applied to humans.