In their target article, Benenson, Webb, and Wrangham (Reference Benenson, Webb and Wrangham2021) expand upon Campbell's (Reference Campbell1999) “staying alive” hypothesis, which reasons that because offspring survival is more tightly linked to maternal than paternal care, women should place a higher value on survival, and therefore exhibit stronger self-protective responses to threat. Core to this argument is the idea that, “survival is more fitness enhancing for females than males” (Benenson et al., Reference Benenson, Webb and Wrangham2021, p. 5). However, the original Campbell hypothesis, and in turn Benenson et al.'s use of it here, is not supported by our current understanding of human life histories. We therefore argue that the staying alive hypothesis is an insufficient explanation for their findings.

The staying alive hypothesis assumes that a reduction in lifespan is more damaging to women's fitness than to men's. Importantly, this requires not only (1) a differential effect of women's and men's mortality on the survival of existing offspring, but also (2) that this difference is not outweighed by a sex difference in the effect of longer lifespan on having additional offspring. Only when both of these assumptions hold is it possible that a gender-difference in the fitness value of longevity could drive differences between women and men in survival-promoting behaviors. Campbell's hypothesis focuses exclusively on the first assumption, ignoring the second. Here we use demographic data to examine the effects of survival on both the well-being of existing offspring and the prospects for future offspring.

First, we agree with Benenson et al. (Reference Benenson, Webb and Wrangham2021) that mothers' care is more strongly correlated with offspring survival than is fathers'. Cross-culturally, maternal death has a much stronger negative effect on child survival than paternal death (Hill & Hurtado, Reference Hill and Hurtado2009; Sear & Mace, Reference Sear and Mace2008). The involvement of fathers in their children's upbringing varies across populations and paternal care can more easily be replaced by the care of another alloparent, such as a grandparent or other kin, than maternal care (Boyette, Lew-Levy, Sarma, Valchy, & Gettler, Reference Boyette, Lew-Levy, Sarma, Valchy and Gettler2019; Meehan, Helfrecht, & Quinlan, Reference Meehan, Helfrecht and Quinlan2014; Sear & Mace, Reference Sear and Mace2008). Combined with the uncertainty men have of their genetic relatedness to alleged offspring, which can be a disincentive for investment, it follows that women's survival has greater effects on offspring well-being than men's, especially for young children. However, this fact alone is not enough to conclude that women's fitness is more strongly dependent on their survival than men's. For this, we need to assess the second assumption by considering sex differences in fertility across the life course.

In most human populations, men's fertility peaks later and declines more slowly with age than women's. Demographic data collected by Blurton Jones (Reference Blurton Jones2016) and further analyzed by Muller et al. (Reference Muller, Blurton Jones, Colchero, Thompson, Enigk, Feldblum and Pusey2020) indicate that among Hadza hunter–gatherers in Tanzania, men's age-specific fertility (ASF) is highest at 33, compared to 26 for women. Women's reproductive careers start early but halt with the onset of menopause around age 50, while men's begin later but are characterized by an extended fertile period lasting into their seventies (Muller et al., Reference Muller, Blurton Jones, Colchero, Thompson, Enigk, Feldblum and Pusey2020). Demographic samples across a variety of subsistence modes show the same pattern, with men's ASF peaking later and declining more slowly than women's (Marlowe, Reference Marlowe2000; Nisén, Martikainen, Silventoinen, & Myrskylä, Reference Nisén, Martikainen, Silventoinen and Myrskylä2014; Tuljapurkar, Puleston, & Gurven, Reference Tuljapurkar, Puleston and Gurven2007). This amounts to a substantial effect of later-life fertility on men's fitness. Furthermore, Tuljapurkar et al. (Reference Tuljapurkar, Puleston and Gurven2007) show that these differences in men's and women's fertility curves impact senescence. Their model shows the human aging pattern reflects selection for survival for as long as men reproduce.

These differences in ASF curves reflect a variety of cultural and demographic processes: Men tend to be older than women at marriage, men are more likely to remarry than women, and in some societies polygyny allows older men to monopolize women of reproductive age. Furthermore, the same processes of sexual selection that Benenson et al. (Reference Benenson, Webb and Wrangham2021) emphasize, where male competition for resources drives mating access, can lead to these age differences. It takes time to accrue wealth and status, shifting mens' reproductive careers later and enhancing prospects for later-life fertility when men are often in the best position to compete.

These data show that survival has different but important consequences for men's reproductive success. The potential for older-age fertility strongly increases the value of staying alive for men. How this weighs against women's payoffs for continued reproduction and childcare is an empirical question that cannot be answered easily with the currently available data. However, we posit that while for women longevity is critical for helping existing offspring to survive, for men staying alive longer can have important effects on the total number of offspring they father. Both factors need to be considered when determining whether survival is more fitness-enhancing for one sex than the other.

We encourage serious consideration of the alternative explanations offered by Benenson et al. (Reference Benenson, Webb and Wrangham2021) when aiming to understand the drivers of the observed sex differences in self-protective behavior, including women's increased vulnerability to physical threats due to their smaller body size and lower status. Additional factors that we expect to play a role are cultural notions of masculinity and femininity that may shape how women and men respond to threatening situations as well as how they self-report behavioral and emotional responses to threats.