Grossmann's fearful ape hypothesis provides a novel and compelling account for the supposed link between heightened fearfulness and enhanced cooperation in humans. We appreciate the author's re-conceptualization of fear as a potentially adaptive response in some contexts, which in previous clinical works has largely been viewed as maladaptive. Despite its novelty, however, we argue that, at present, there is insufficient evidence to conclude that: (1) it is specifically heightened fearfulness (and not other affective traits or emotions in general) that enhances cooperation and (2) that heightened fearfulness is related to enhanced, “uniquely human,” cooperation.

Grossmann singles out fearfulness as the key affective trait that evolved to support “human-unique” levels of cooperation. He suggests that fearful infants evoke more (allo)parental care and, by receiving more care, become more cooperative children, adults, and parents themselves. We argue, however, that the influence of affective states on cooperative behaviour may not be bound to fearfulness alone. Other affective states may also be associated with – or even evoke more – cooperation.

Similarly to fear, from early in human ontogeny, infants experience and display positive affect (Messinger, Reference Messinger2002; Messinger & Fogel, Reference Messinger, Fogel and Kail2007) and can distinguish positive from other facial expressions (de Haan & Nelson, Reference de Haan, Nelson and A.1998). Furthermore, although results are mixed, there is some evidence that humans also display a perceptual sensitivity bias towards happy faces (Wirth & Wentura, Reference Wirth and Wentura2020; Zsido et al., Reference Zsido, Arato, Ihasz, Basler, Matuz-Budai, Inhof and Coelho2021). Crucially, although Grossmann and others failed to find an association between sensitivity to happy faces and infants' prosociality (Grossmann, Missana, & Krol, Reference Grossmann, Missana and Krol2018; Rajhans, Altvater-Mackensen, Vaish, & Grossmann, Reference Rajhans, Altvater-Mackensen, Vaish and Grossmann2016), there is robust evidence that experienced and displayed positive affect results in more cooperation in child and adult actors (e.g., Aknin, Van de Vondervoort, & Hamlin, Reference Aknin, Van de Vondervoort and Hamlin2018; Centorrino, Djemai, Hopfensitz, Milinski, & Seabright, Reference Centorrino, Djemai, Hopfensitz, Milinski and Seabright2015; Isen & Levin, Reference Isen and Levin1972; Kushlev, Radosic, & Diener, Reference Kushlev, Radosic and Diener2022; Moore, Underwood, & Rosenhan, Reference Moore, Underwood and Rosenhan1973; Rosenhan, Salovey, & Hargis, Reference Rosenhan, Salovey and Hargis1981), as well as observers (including in cooperative parental-care contexts) (e.g., Centorrino et al., Reference Centorrino, Djemai, Hopfensitz, Milinski and Seabright2015; Danvers & Shiota, Reference Danvers and Shiota2018; Lengua & Kovacs, Reference Lengua and Kovacs2005; Scharlemann, Eckel, Kacelnik, & Wilson, Reference Scharlemann, Eckel, Kacelnik and Wilson2001). Therefore, it is not only fearful infants, but also happy infants, who may evoke cooperation in others, and may, by receiving more care, themselves become more cooperative adults. The putative effect of fearfulness on cooperation is therefore not necessarily unique in this regard – and it may also be that happiness, or emotional expressivity more generally, foster cooperation.

Grossmann also argues that humans display and perceive more fearfulness in comparison to other great ape species, and that this heightened fearfulness evokes increased levels of “human-unique” cooperation (i.e., alloparental care). However, there is not enough evidence that: (1) fearfulness is heightened in humans compared to other great apes; and that (2) heightened fearfulness is associated with enhanced “human-unique” cooperation.

Although some studies found that great apes do not show attentional bias to fear specifically (Kret, Jaasma, Bionda, & Wijnen, Reference Kret, Jaasma, Bionda and Wijnen2016; Kret, Muramatsu, & Matsuzawa, Reference Kret, Muramatsu and Matsuzawa2018), other work has found heightened attention towards fear (Pritsch, Telkemeyer, Mühlenbeck, & Liebal, Reference Pritsch, Telkemeyer, Mühlenbeck and Liebal2017), and towards emotions in general (Wilson & Tomonaga, Reference Wilson and Tomonaga2018, Reference Wilson and Tomonaga2021; for a review, see Kret, Prochazkova, Sterck, & Clay, Reference Kret, Prochazkova, Sterck and Clay2020). Furthermore, the claim that human infants express more fearfulness than other great apes is based on one empirical study (Herrmann, Hare, Cissewski, & Tomasello, Reference Herrmann, Hare, Cissewski and Tomasello2011). This study, however, used a human-centred paradigm (sensitivity to novel humans and objects) to contrast the reactions of human children to adult great apes. Herrmann et al. (Reference Herrmann, Hare, Cissewski and Tomasello2011) assumed that showing an unfamiliar human is similarly relevant and meaningful for a captive adult ape (who is likely exposed to unfamiliar humans many times on a daily basis) and a human child (for whom this context may be drastically different). As such, this study might have or might have not captured the full extent of the fear response in other primates, calling to question Grossmann's claims about uniquely human heightened fear production.

Even if displaying and perceiving fear is, on average, heightened in humans, this does not mean that it necessarily evolved for “human-unique” cooperative care purposes (i.e., alloparental care). First, whether alloparental care is uniquely human is questionable considering some evidence for such forms of cooperation in other primate species (Boesch, Bole, Eckhardt, & Boesch, Reference Boesch, Bole, Eckhardt and Boesch2010; Fairbanks, Reference Fairbanks1990; Samuni, Wittig, & Crockford, Reference Samuni, Wittig and Crockford2019; Tokuyama et al., Reference Tokuyama, Toda, Poiret, Iyokango, Bakaa and Ishizuka2021). Second, to support the association between fearfulness and “human-unique” cooperation, Grossmann cites research showing associations between enhanced fearfulness sensitivity and cooperation in non-alloparental care contexts, such as parental care of own children (e.g., Kiel & Buss, Reference Kiel and Buss2011), and helping/sharing in infants, children, and adults (e.g., Grossmann et al., Reference Grossmann, Missana and Krol2018; Marsh & Ambady, Reference Marsh and Ambady2007; Rajhans et al., Reference Rajhans, Altvater-Mackensen, Vaish and Grossmann2016) that are, importantly, not unique to humans. Given that such forms of cooperation exist also in other species (for a review, see de Waal & Suchak, Reference de Waal and Suchak2010), evidence is lacking that heightened fearfulness is associated with “human-unique” cooperation.

Grossmann further argues that, next to the fight-or-flight response seen in many animals, heightened displayed and detected fearfulness elicits approach behaviours (tend and befriend) in humans in particular. However, the groundwork for a link between fearfulness and cooperation also exists in nonhuman animals. For example, newborn chimpanzees and bonobos are known to express fearfulness and distress through pout moans and whimpers in the context of maternal care (De Waal, Reference de Waal1988), and adult caregivers comfort offspring who show such signs by embracing and kissing them, similarly to humans (de Waal & Preston, Reference de Waal and Preston2017). Furthermore, offspring displaying distress provoke parental cooperative care in other species, including rodents and birds (de Waal & Preston, Reference de Waal and Preston2017). Even outside of infant–parent relationships, nonhuman animals, such as rodents, react to conspecifics displaying fear by engaging in prosocial behaviours (Keysers, Knapska, Moita, & Gazzola, Reference Keysers, Knapska, Moita and Gazzola2022). Therefore, it seems highly unlikely that heightened fearfulness elicits approach behaviours exclusively in humans.

In sum, singling out fearfulness as the affective trait that evolved specifically for cooperative purposes in humans seems premature. Additionally, there is insufficient evidence to conclude that human fear perception and production is significantly increased compared to other nonhuman animals. Finally, the link between heightened fearfulness and alloparental care awaits first empirical support. To truly examine these facets, it will be critical for future work to take the sensitivities and milieu of both human and nonhuman animals into account.