R1. Additional evidence and future directions

Both Hawkes and Veit and Browning expand SAT by using life-history theory to understand whether females approach the trade-off between survival and reproduction differently than males do. Much of Campbell's SAT and our extension of it is derived from or consistent with Hawkes' research demonstrating that women enhance the reproductive success of both their offspring and their grandchildren. While a former idea that only males can benefit by extending their reproductive careers into old age is therefore wrong, Hawkes nevertheless emphasizes that precisely how the reproductive strategies of each sex are expected to differ remains a complex problem. Her analyses of the trade-offs between survival and reproductive success for each sex provide avenues for future research, including in terms of understanding residual reproductive value, mating sex ratios, birth spacing, and grandmothering.

We appreciate Veit and Browning's focus on the importance of carefully weighing whether commonly occurring, cross-culturally present patterns of behavior, such as those we have identified in girls and women, should be dubbed “pathological.” What looks pathological from one perspective may be adaptive from another. Veit and Browning suggest that further research is needed to investigate whether compared to males, females face more threats, find modern life more difficult, or need to live longer to be reproductively successful. We have focused on the latter as a reason for females' greater reactions to threat. Nonetheless, we fully concur that more focus on the details of females' and males' life histories and the components that may maximize reproductive success (RS) for each sex are necessary in order to distinguish pathology from normality.

Bleske-Rechek and Deaner add the important insight that cultures often prioritize the survival of females over males. Their evidence is compelling that in times of disasters, in the military, or in dangerous jobs more generally, females' lives are prized over males' lives at least in many western, educated, industrialized, rich, and democratic (WEIRD) societies. Furthermore, women are more likely than men to be perceived as victims and female victims generate more sympathy than male victims. Their evidence added to the SAT suggests that a biological basis for females' greater self-protectiveness is complemented by widespread social norms stipulating that females merit greater protection than males. Further investigation across non-WEIRD cultures could help verify the universality of this norm.

The coronavirus disease-2019 (COVID-19) data presented by Troisi perfectly fit the predictions of SAT. Referring to the importance of the behavioral immune system, Troisi presents evidence that women are more fearful than men of contracting COVID-19 and exhibit a larger increase than men in disgust sensitivity to pathogens. Nevertheless, he argues, and we agree, that many other factors besides sex are important in determining how the behavioral immune system functions. Two factors in particular, slow versus fast life histories and fearful versus not fearful attachment patterns, were shown by Troisi to affect the behavioral immune system's reactions to COVID-19. Although those factors were unrelated to sex, it would be interesting to investigate whether more males than females opt for a faster life history (Del Giudice, Reference Del Giudice2009) or begin life with disorganized/fearful attachments (Carlson, Cicchetti, Barnett, & Braunwald, Reference Carlson, Cicchetti, Barnett and Braunwald1989) as some have suggested, which could influence the interaction between sex and these variables.

We appreciate the evidence that Tice, Baumeister, and Sedikides (Tice et al.) provide from a re-analysis of their data that elegantly fits predictions from SAT. Their results show that females are much less enthusiastic than males about potentially dangerous hypothetical new inventions, such as a tree-chopping robot or a pet cloner. They concur that females are more self-protective than males, but they add that this can entail costs. Specifically, they hypothesize that greater self-protectiveness reduces self-enhancement, which is associated with greater life satisfaction, higher self-esteem, more positive affect, and lower levels of anxiety and depression. Thus, they postulate that anxiety and depression in particular can be multi-determined, caused by both extreme forms of self-protection and/or reduced levels of self-enhancement. Worldwide however, no evidence shows that females exhibit lower life-satisfaction or less positive affect than males, and if anything females appear more positive (Bem, Reference Bem1974; Lucas & Gohm, Reference Lucas, Gohm, Diener and Suh2000). Furthermore, cross-culturally sex differences in anxiety and depression along with many of the other self-protective characteristics we have described, such as insomnia, smiling, or neuroticism, emerge or are accentuated in puberty. It seems unlikely that sex differences in self-enhancement suddenly emerge in puberty. Nonetheless, we concur that self-protection likely inflicts costs as Tice et al. propose, which may be related to reduced self-enhancement depending on its definition in a particular culture. SAT posits that for females the costs of self-protection are outweighed by the benefits of enhanced survival of themselves and their descendants.

March and Gaertner distinguish between detection of and reaction to visual threat. We appreciate their re-analysis of their data to show that females exhibit stronger reactions than males specifically to threatening stimuli as would be predicted by SAT. What females do not exhibit however is faster detection of threat which also would be expected by SAT. This disparity is intriguing and warrants further investigation. One possibility is that it depends on sex differences in visual acuity. Although the cause is not genetic, myopia is twice as prevalent in white and East Asian females as males by late adolescence according to a large meta-analysis (Rudnicka et al., Reference Rudnicka, Kapetanakis, Wathern, Logan, Gilmartin, Whincup and Owen2016). In contrast, much evidence supports the lower threshold of females than males for detecting varied types of auditory, olfactory, and tactile stimuli (Velle, Reference Velle1987). Further research is necessary to examine whether females would detect threatening stimuli more rapidly than males using other sensory modalities.

Cassar adds to our conceptualization of the social relations that would be predicted by SAT. She suggests that reducing competition benefits females not only through harm avoidance and protection of relationships but also by facilitating future cooperation. Her thesis is that cross-culturally females tend to display more egalitarian behavior than males in order to buttress reciprocal relationships with unrelated females and asymmetrical relations with higher-ranked mates. By insisting on equality, females signal that they are not planning to compete directly against another female or a mate. Nonetheless, as Cassar has shown, females will compete under some contexts, such as in support of their own children or on others' behalf, ways of competition that may be acceptable to unrelated females and a mate. Emphasizing equality or competition for acceptable outcomes therefore reduces threat, which would be in line with SAT.

Crosby, Hahnel-Peeters, and Buss (Crosby et al.) and McDonald and James note that we omitted processes directly related to females' and males' unique concerns about reproduction. Thus, morbidity or mortality related to childbirth, gestation, and lactation; female or male reproductive organs; and sexuality were excluded. Our purpose was to compare processes that could threaten females and males to the same extent.

As Crosby et al. highlight, sex differences in self-protective behavior would be even greater if some of these were included. The examples they provide demonstrate this convincingly. For instance, one of the largest sex differences occurs for sexual disgust (see also Sparks, Fessler, Chan, Ashokkumar, & Holbrook, Reference Sparks, Fessler, Chan, Ashokkumar and Holbrook2018). Crosby et al. make the compelling argument that this protects women from sub-optimal mates and sexually transmitted diseases. Likewise, both McDonald and James and Crosby et al. emphasize that fear of rape is one major reason for women's self-protective behaviors. McDonald and James go even further and suggest that threats to survival and to reproductive choice are conflated for women and should be distinguished, especially for the social behaviors we describe. We are unsure how they could be disentangled as they are not necessarily mutually exclusive. For instance, rape can co-occur with murder. Furthermore, if fear of rape were the sole reason for the greater self-protective social behaviors exhibited by females than males, then self-protective social behaviors and neuroticism would be unnecessary with other women. This is not the case. Several of the self-protective behaviors we described, including smiling, politeness, or identification of emotions, are even stronger when girls and women interact with same-sex peers than with their male counterparts. Nevertheless, had we included all sex-biased concerns about reproduction and sexuality, then some unique aspects of male reproductive success, such as public displays of wealth, degree of sexual motivation, or erectile function, would be more threatening to males than females, unless these threatened females' lives personally.

Archer and Fuentes suggest that SAT would be improved by examining how self-protective characteristics co-vary and whether they form a package. As Archer recommends, using multivariate statistical analyses such as Mahalanobis's D permits examination of covariation between the characteristics we identify. We expect that covariance will be strong in human females, as many studies already demonstrate positive associations between two or more of the self-protective behaviors we have described including among immune functioning, pain, sleep, social behaviors, neuroticism, and emotional reactions to threat. If the disparate self-protective reactions form a single package, this could motivate useful ideas about underlying mechanisms.

Archer further asks how sexual selection theory and SAT fit together. We accept that sexual selection theory, which typically refers to male mate competition, is usefully redefined as natural selection that includes each sex's reproductive strategies (Carranza, Reference Carranza2009). This permits male mating competition, female self-protective behaviors, as well as many other sex-differentiated strategies for survival and reproductive success to be categorized under one overarching rubric (Clutton-Brock & Huchard, Reference Clutton-Brock and Huchard2013; Stockley & Bro-Jørgensen, Reference Stockley and Bro-Jørgensen2011). As Aung, Baek, and Puts (Aung et al.), Wallentin, and others recommend, understanding of the ultimate and proximate mechanisms that produce specific traits will be better illuminated after the differential forces that shape particular adaptations or groups of adaptations are identified. Some of these will differ by sex.

R2. Is longevity more important to females' than males' reproductive success?

Several commentators question our postulation that longevity is more important to reproductive success for females than males, including Brooks and Blake, Hagen, Knorr, Li, Mensing, and Scelza (Hagen et al.), and Wallentin. Because theoretically only males can reproduce until death, common wisdom holds that survival at late ages is less important for females than males. SAT is constructed on the opposite premise, that extended survival is more important for the reproductive success of females than males. This analysis relies heavily on Hawkes and others' research on the importance of grandmothering (Campbell, Reference Campbell1999; Hawkes et al., Reference Hawkes, O'Connell, Jones, Gurven, Hill, Hames and Churchill1997; Sear & Mace, Reference Sear and Mace2008). Lacking the input for a mathematical or game theoretic model, we present several assumptions that support our premise.

First, all women benefit reproductively from staying alive long enough to invest in both children (especially daughters) and grandchildren (Daly & Perry, Reference Daly and Perry2021; Hawkes et al., Reference Hawkes, O'Connell, Jones, Gurven, Hill, Hames and Churchill1997; Sear & Mace, Reference Sear and Mace2008). Second, men's fighting power (Daly & Wilson, Reference Daly and Wilson1988), fluid cognitive abilities (Salthouse, Reference Salthouse1996), and fertility decline with age (Matsumoto, Reference Matsumoto2002; Santiago, Silva, Alves, Oliveira, & Fardilha, Reference Santiago, Silva, Alves, Oliveira and Fardilha2019), thereby reducing many elements that contribute to male mating success. Thus, in a contemporary study of over 330 million births across 17 rich European nations, where men live longer than in poorer areas, “age-specific fertility rates of men quickly decline to very low levels above age 45” (Dudel & Klüsener, Reference Dudel and Klüsener2021, p. 424). Ages 20–45 years were men's maximum reproductive years and at most 0.2% of men became fathers after age 59 years. Therefore, despite the extended reproductive success of a few famous men (Betzig, Reference Betzig1986), relatively few older men can amass enough resources to both outcompete younger, stronger men and be attractive to younger, fertile women. In contrast, despite large variation across 25 contemporary Western nations in when grandparenthood begins (ages 47–60), a first grandchild is born when a grandmother is on average 51 years and a grandfather is 54 years (Leopold & Skopek, Reference Leopold and Skopek2015). Mortality in this sample is 83 years for women and 78 years for men, so that grandparenting can in theory extend for decades following the end of fertility. In a separate analysis of 12 contemporary European nations with over 36,000 grandparents older than 50 years and a grandchild younger than 13 years, grandmothers were more than twice as likely as grandfathers to provide frequent care to their biological grandchildren (Daly & Perry, Reference Daly and Perry2021). Furthermore, when they helped, grandfathers invested as much in non-biological children as biological children. The mean age of grandmothers was 63 years (range 50–96 years) and grandfathers 65 years (range 50–100 years). Thus, while a few older men, particularly in polygynous societies, greatly increase their reproductive success until the end of their lives at the expense of younger men's reproductive success, most men in monogamous societies follow a similar reproductive age trajectory as women albeit beginning and ending a few years later. Furthermore, it seems unlikely that this should differ in subsistence societies where lifespan tends to be shorter.

Brooks and Blake provide several important theoretical reasons why more research is necessary to understand the relation between survival and reproductive success, especially for human females and males. They cite the aphorism that applies to many taxa that males “live fast and die young” and elaborate on how greater extrinsic mortality can act then to enhance mortality from intrinsic causes. They then make an exception for humans by suggesting that the greater survival of women over men may be incidental. As examples, because women's survival is traded off against childbirth and childcare, which increase mortality, reduced fertility in contemporary society may artificially increase women's survival. Alternatively, because amassing resources and status takes time and increases reproductive success for men as Hagen et al. also argue, selection for longevity may have increased for males: Increased longevity in females would then be a by-product. These are important questions, and we await further evidence. Nonetheless, worldwide across natural fertility populations in small scale societies, females live longer than males (Ember & Ember, Reference Ember and Ember2003). Precise supporting data exist from Sweden from the 1700s (Allman, Rosin, Kumar, & Hasenstaub, Reference Allman, Rosin, Kumar and Hasenstaub1998) quantifying the sex difference as Wallentin partially depicts in the graph he includes.

Hagen et al. concur that a mother is more important than a father to a child's survival. This might well explain what Bleske-Rechek and Deaner emphasize: Cultures routinely prioritize survival of women over men. Nonetheless, paternal investment can enhance RS especially when women live with their husbands' families or lack the support of female kin. Therefore, women should select husbands who will provide extended assistance or resources. Hagen et al. and Brooks and Blake argue that men need time to acquire lots of resources to attract women and outcompete same-age and younger men. Whether this occurs routinely in primarily monogamous societies however seems unlikely based on contemporary evidence, where women prefer mates 2–3 years older than they are (Walter et al., Reference Walter, Conroy-Beam, Buss, Asao, Sorokowska, Sorokowski and Alm2020). Thus, even though the average age of first reproduction has always been higher for males than females across primate species (Bogin, Reference Bogin1999) and human females undergo menopause, we postulate that on average, longevity is more important for females' than males' reproductive success.

Nevertheless, we fully acknowledge that more empirical research is required to examine the relation between longevity and reproductive success for each sex as Brooks and Blake recommend. A life history approach as endorsed by Hawkes, Brooks and Blake, and Veit and Browning will help clarify the trade-offs between reproduction and survival for females and mating competition and survival for males. All other things being equal, both women and men gain reproductive success from living longer, but we suggest that the marginal value is higher for women than men because of women's greater investment in children and grandchildren.

R3. Females' greater longevity: Selection for survival or incidental to males' mate competition?

Relatedly, another group of commentators argue that the sex difference in self-protectiveness results solely from a trade-off with risk-taking. Aung et al. and Wallentin are explicit that females are more self-protective than males only because males' greater mate competition prevents males from being maximally self-protective, not because of any specialized evolutionary adaptations unique to or extended in females. Likewise, in a paper of which we were formerly unaware that presents a theory and evidence that resembles our target article in many ways by Sparks et al. (Reference Sparks, Fessler, Chan, Ashokkumar and Holbrook2018), males' risk-taking is presented as the reason why females are more self-protective.

Campbell argued specifically against this proposition, and we quoted her statement in our target article “that lower rates of aggression by women reflect not just the absence of male risk-taking but are part of a positive female adaptation driven by the critical importance of the mother's survival for her own reproductive success” (p. 204). We extended Campbell's (Reference Campbell1999) SAT to examine further evidence that females exhibit many adaptations that protect them from threats. Our primary point is that women who have stronger immune systems, faster withdrawal responses to potentially injurious stimuli, more awareness of nighttime and general environmental adverse forces, greater avoidance of social threats, and a healthy degree of worry and sense of vulnerability would live longer than those women who did not have these reactions, and that this would be beneficial in terms of fitness. As we described, evidence exists for female nonhumans and humans that self-protectiveness, including immunological, pain threshold, sleep cycles, and behavioral conflict avoidance, is regulated by estrogens. More generally, those individuals who respond more self-protectively to any threat will generally survive longer, and females do so more than males. If these reactions become too extreme, they become maladaptive, and they may carry costs as Tice et al. point out, but on balance, self-protective reactions prolong life. Critically, SAT emphasizes that females' self-protectiveness does not arise as an incidental result of males' risky mating strategies. Instead, females have evolved mechanisms that have allowed them to prolong their lives. As previously emphasized, sex differences in mortality at all ages are well-supported by the graph Wallentin includes from the Human Mortality Database. Infant deaths perfectly track deaths that do not include infants. Sex differences also clearly vary over time, but they are always present even in Sweden in the 1700s (Allman et al., Reference Allman, Rosin, Kumar and Hasenstaub1998).

Wallentin further wonders what evidence indicates that it is caregiving that accounts for the benefits of greater self-protectiveness in females. Across eight species of nonhuman primates, the ratio of female to male care of offspring is positively associated with the ratio of female:male lifespan to the point that in the callitrichid Callimico, which has particularly extensive care by males, lifespans show no detectable sex difference (Allman et al., Reference Allman, Rosin, Kumar and Hasenstaub1998). This example illustrates the wider point about sex differences in adaptation that we take from Campbell (Reference Campbell1999). In species in which male–male competition is relatively worthwhile, investment in adaptations for competition is more beneficial for males than investment in longevity. Males' longevity is therefore reduced. For the females of those species, however, investment in longevity yields positive adaptive benefits. The relatively extended lifespans of females thus reflect both the absence of the trade-off faced by males and the high value of long-term survival for females.

R4. Sex is not a useful construct for understanding self-protectiveness

A final group of commentators do not believe sex contributes to understanding self-protective behaviors. In the first sub-group, Lin, Cuijpers, and Li (Lin et al.), Humeny, and Eagly suggest that in some cases women are not more self-protective than men, thereby challenging the validity of SAT.

Lin et al. raise two important examples of women's appearing less self-protective than men: Burnout in response to difficulties with work–life balance and depression surrounding the birth of a child. We suggest that these challenges are more specific to or pronounced in women than in men, as with concerns raised by Crosby et al. and McDonald and James. Consequently, comparisons with men are not equivalent. Lin et al.'s examples depend on women's taking primary responsibility for raising children and thus being more affected by burnout and childbirth. For example, in a meta-analysis of reasons for female and male physicians' burnout, amount of workload, number of hours required, nighttime shifts, schedule inflexibility, and lack of supportive relationships were some of the primary reasons for burnout (Azam, Khan, & Alam, Reference Azam, Khan and Alam2017). Too many work demands obviously are more detrimental to primary than secondary caregivers. In turn, the causes of perinatal depression have not been well identified, with both physiological responses to pregnancy and lack of social support commonly cited (Eastwood, Kemp, & Jalaludin, Reference Eastwood, Kemp and Jalaludin2015). Nonetheless, bearing primary responsibility for the survival of oneself and a newborn and possibly other children and family members, especially when difficulties arise, would seem to necessitate strong responses. Newer theories view these types of strong responses as ways of social bargaining (Hagen & Thornhill, Reference Hagen and Thornhill2017; Syme, Garfield, & Hagen, Reference Syme, Garfield and Hagen2016). Recent evidence suggests that when single fathers assume full responsibility for childcare, they are three times more likely to die than partnered fathers or single mothers (Chiu et al., Reference Chiu, Rahman, Vigod, Lau, Cairney and Kurdyak2018). SAT would predict that women would respond more rapidly than men to reduce threats whether by leaving an overwhelming job faster or procuring assistance with child-rearing under difficult circumstances.

Humeny describes women who are victims of intimate partner violence as not being self-protective. Again, this predicament is not equivalent for the two sexes. Being a victim of intimate partner violence is more common and more lethal for women because they are less physically strong as we noted in our target article. As Humeny adds however, being isolated from others and alone with an abusive partner can make it very difficult for a woman to be self-protective. In particular, attempts to leave a violent partner may be less self-protective than remaining with him. Thus, women may be choosing the most self-protective behavior available by staying with an abusive partner, especially if no one else will protect them. We find it difficult to view severe victimization from which there is little escape as evidence against women's motivation to protect themselves, despite the fact that abuse inflicts serious physical and emotional damage.

Eagly provides three further important examples in which women are less likely than men to protect themselves. First, women more than men donate their kidneys, which incurs risk. Second, more single German women than single men protected Jews during the holocaust. Third, more women than men join the Peace Corps which often entails traveling to and living in areas with greater health risks. We agree that these examples raise important questions that merit consideration. For instance, how many of the organ donations are to kin? And when women take explicit risks, what are their male counterparts doing? As Eagly notes, women have always been less likely than men to enter life-threatening professions from fighting fires, to policing, to responding to natural disasters, as well as engaging in social conflicts including the military, showing off, driving recklessly, and so forth. Thus, it is plausible that during the holocaust, for example, men engaged in even more dangerous enterprises than women. Accepting Eagly's data however, all else being equal, SAT predicts that these women who purposefully place themselves in danger by definition would have a lower probability of survival and reproductive success than women who choose not to engage in dangerous activities and thus would represent exceptions. Without knowing the specific circumstances of each woman who purposefully endangered her life however, it is also possible that the costs she experienced were outweighed by the benefits she accrued, in terms of establishing or maintaining relationships, or self-enhancement as Tice et al. propose.

O'Mara Kunz, Goodnight, and Wilson (O'Mara Kunz et al.) are concerned that the evidence we presented on autoimmune diseases (ADs) is misleading. According to O'Mara Kunz et al., outside of industrialized societies ADs rarely occur and are not more prevalent in females than males. Consequently, they believe that ADs can be better explained by the pregnancy compensation hypothesis than as maladaptive overly strong self-protective immune responses. O'Mara Kunz et al. believe that ADs become more common in industrialized societies because women spend less time gestating and lactating and more time menstruating, thereby changing their sex hormone profiles. This is an intriguing proposition. However, even pre-pubertally more girls than boys develop ADs although the sex bias is less pronounced than in adults (Cattalini, Soliani, Caparello, & Cimaz, Reference Cattalini, Soliani, Caparello and Cimaz2019). While O'Mara Kunz et al.'s theory therefore can explain the large cross-cultural differences in the prevalence of ADs, the generally accepted understanding of ADs is that they constitute an overreaction of the immune system and worldwide are more common in women. Because threats differ across cultures, ADs may be a reaction to only some types of immunological threats which may be less present in non-industrialized societies.

Another sub-group, including Fuentes, Neuhoff, Huntsinger and Raoul, and Szocik, do not believe sufficient evidence exists to provide an evolutionary/biological explanation for the self-protective processes we have identified. Specifically, Fuentes is concerned that we neglect the nuances that occur across demographics and contexts. We acknowledge that the variability is great and do not mean to downplay individual differences. Troisi too was concerned that we neglected many individual differences which also contribute to self-protective reactions. Nevertheless, we do not agree with Fuentes that SAT is predicated on the nuclear family as a basal human type. Rather, SAT is predicated on the mother–child unit being universal, with female kin and sometimes fathers and sons also participating in childcare (Kramer, Reference Kramer2005; Wood & Eagly, Reference Wood and Eagly2002).

Neuhoff believes sex/gender is a continuous variable and that considering it to be binary constitutes poor science and public policy. We acknowledge in our target article that sex/gender is a multidimensional construct and that the studies we found are unfortunately limited by emphasizing binary comparisons. However, we do not believe the complexity of the phenomenon should preclude systematic investigations into biological and evolutionary bases or explanations for sex differences.

Many characteristics associated with sex typically covary with chromosomal sex, from the presence of internal and external reproductive structures to sex-typed interests and activities, diseases, gender roles, gender identity, sexual identity, and socialization received. Levels of sex hormones and physical strength however are two characteristics strongly associated with sex chromosomes and not simply by-products as Neuhoff asserts. Within-sex variation in levels of sex hormones or physical strength that correlate with degrees of self-protectiveness demonstrates that different components of sex are related to self-protectiveness. It does not negate the value of biological sex in understanding self-protectiveness.

We consider it dangerous for scientific and practical reasons not to acknowledge biological sex differences. Until recently, sex has often not been considered to be an important variable in medical research involving human or nonhuman species. The assumption was that the results from male animals, including men, would apply to female animals, including women. This has led to inferior diagnostic and treatment tools for women, causing untold numbers of girls' and women's deaths (McGregor, Reference McGregor2020). Until girls' and women's lives are examined objectively, and they are considered important in their own right, this will not be remedied.

Nevertheless, our analysis in no way minimizes the importance of understanding sex as a continuous variable. We fully acknowledge that all individuals have some traits that are associated more commonly with the other sex. Therefore, individuals should be understood holistically and not categorized based on any single factor.

Huntsinger and Raoul, similar to Neuhoff and Fuentes, do not believe sufficient evidence exists to provide an evolutionary/biological explanation for the diverse processes we have identified. Instead, they believe the findings we present are based on stereotypes. While we do not discount the potential role of this and other social and environmental factors, we question whether sex-biased stereotypes exist for the types of evidence we have described. For example, women are less likely to die than men at any age, but more likely to experience some illnesses and chronic conditions. Does this make women less or more healthy than men? Likewise regarding pain, women are typically celebrated for their endurance of pain given how difficult childbirth can be, whereas men in turn are expected to withstand other sorts of pain. What then are sex stereotypes regarding pain? We are not aware of stereotypes regarding sleep, concern about the environment, or fears of disease transmission. Along the same lines, we think it premature to conclude that women categorically “face more chronic strains and stressors” than men, and this is the cause of sex differences in depression or anxiety. By what measures do women suffer more chronic stress than men? If they do however, how can Huntsinger and Raoul then conclude that “the emotional lives of women and men are nearly identical,” when evidence points to sex differences in prevalence rates of depression, anxiety, and many other concerns from sexual dysfunctions and paraphilias to eating disorders (Hartung & Lefler, Reference Hartung and Lefler2019)?

Moreover, there is biological evidence for many of the sex differences we describe. Although females frequently have been excluded from human (McGregor, Reference McGregor2020) and nonhuman (Clayton, Reference Clayton2016) studies, several studies of sex differences with rodents indicate that compared with males, females exhibit stronger immune responses to various pathogens and vaccines (Klein & Flanagan, Reference Klein and Flanagan2016); exhibit lower pain thresholds (Smith, Reference Smith2019); and experience more nighttime awakenings (Paul, Dugovic, Turek, & Laposky, Reference Paul, Dugovic, Turek and Laposky2006; Swift et al., Reference Swift, Keus, Echeverria, Cabrera, Jimenez, Holloway and Poe2020). Likewise, across primate species, females more than males avoid serious social conflicts (Fedigan, Reference Fedigan1982; Smuts, Reference Smuts, Smuts, Cheney, Seyfarth, Wrangham and Struhsaker1987).

Nevertheless, we believe that if a behavior is adaptive, then socialization and biology should work together to reinforce it, not counter one another, as Bleske-Rechek and Deaner suggest for self-protectiveness. Thus, disentangling the effects of evolved behaviors and environmental inputs constitutes a difficult task that requires as much evidence as possible.

Finally, Szocik believes that our analysis could be “used to justify the exploitation and subordination of women in their traditional roles as mothers and caregivers.” Obviously, we did not intend that outcome, but it is probably true that any evidence of group differences can be used by one group to disparage another. One response could be to abandon any research on sex differences. We prefer the alternative strategy of understanding the issue, and in this case showing how the evolutionarily significant roles of women as mothers and caregivers illuminate the ultimate and proximate forces that shape women's lives. Harmful gender stereotypes get perpetuated by discounting the role not only of social factors but also of biological ones – leaving us to compare female phenotypes to male reference points rather than considering them as adaptive responses in their own right.

Szocik also believes that we have implied that “that women inherently have lives less worth living than men.” This was not our intention, and we strongly disagree with this conclusion. Our paper showed that human females have a series of adaptations that tend to differ from those of men in a consistent pattern. We did not focus on men's lives but had we done so, we would have underscored the difficulties men confront. As examples, boys and men die at every age from disease, accidents, interpersonal conflicts, and other traumatic events more than girls and women do. Men are more likely than women to suffer from addictions, sexual dysfunctions and paraphilias, conduct disorders, attention-deficit hyperactivity disorder, autism, learning disorders (Hartung & Lefler, Reference Hartung and Lefler2019), and to commit suicide or take such poor care of themselves that they end up dying (Case & Deaton, Reference Case and Deaton2020). Those facts mean we are not surprised by the finding that we cited earlier, that life satisfaction is at least as high among women as among men (Lucas & Gohm, Reference Lucas, Gohm, Diener and Suh2000). But whether women or men can be said to have lives less worth living is a judgment that we see as another matter entirely, one that our paper does not address.