Grossmann's fearful ape hypothesis stipulates that heightened fearfulness is an adaptation, first seen in infancy, to solve the problem of cooperation in Homo sapiens, mostly mediated by cooperative care. Grossmann documents powerful links between heightened fearfulness traits and enhanced levels of cooperation in humans, based on a comprehensive approach to fearfulness and grounded in the four ethological questions proposed by Tinbergen. Grossmann's approach also demonstrates the merit of an evolutionary-developmental framework to better understand both the potential adaptive and maladaptive effects of a trait (here, heightened fearfulness), depending on the type of human society (small-scale vs. large-scale), culture (Western-individualistic vs. Eastern-collectivist), and rearing environments (harsh and unpredictable vs. easy and predictable), where both child and adult life takes place. In our commentary, we focus on two issues: (1) The conceptualization of the adaptive nature of heightened fearfulness from an evolutionary-developmental psychological perspective, and (2) the assumption that cooperative breeding is a precondition (or “breeding ground”) for the adaptive role of heightened fearfulness in infancy.

Grossmann essentially conceptualizes heightened infant fearfulness as an isolated adaptation associated with enhanced cooperation. We argue instead that heightened fearfulness is better viewed as part of a suite of evolved features that serve to signal, in a reflex-like way, adults' caregiving motivations and actions toward helpless babies, increasing the chances that infants will be attended to, cared for, form attachments, and survive. Other infantile adaptations that foster infant attention and attachment, including neotenous facial features (Lorenz's kindchenschema, or baby schema), smiles, laughs, and vocal behaviors such as coos and cries (e.g., Bründl et al., Reference Bründl, Tkaczynski, Nohon Kohou, Boesch, Wittig and Crockford2021; Hrdy & Burkart, Reference Hrdy, Burkart, Hart and Bjorklund2022), decline in influence with time. In contrast, Grossmann's view of humans' heightened fearfulness is best conceptualized as a deferred adaptation (an evolutionary solution to a problem beginning early in life and serving the same or similar function later in development, Hernández Blasi & Bjorklund, Reference Hernández Blasi and Bjorklund2003), having both an immediate function in infancy (increasing the chances of adult caregiving) and a deferred one later in life (contributing to older humans' enhanced ability to cooperate, including in childcare). Although this is a reasonable proposal, an alternative, and we argue a more likely possibility, is that heightened fearfulness first stabilized as an ontogenetic adaptation (Bjorklund, Reference Bjorklund1997; Oppenheim, Reference Oppenheim, Connolly and Prechtl1981), serving to promote infant attachment and survival at a particular time in development, and that its subsequent role in promoting cooperation can be better thought of as an exaptation, or spandrel (Gould, Reference Gould1991), a feature whose final function is quite different to the one originally shaped by natural selection. Rather than heightened fearfulness being directly selected because of its benefits to cooperation, cooperation was, in part, a gradually evolving byproduct of a developmentally stable fearful temperament, with ancestral fearful infants growing up to become cooperative (and cooperative-caring) adults.

Central to Grossmann's thesis is that cooperative breeding (Hrdy, Reference Hrdy1999; Kramer, Reference Kramer2019) was a precondition for fearfulness to develop and stabilize as an adaptation over the last 1.8 million years or so of Homo phylogeny. We concur with theorists who argue that cognitive and affective changes in Homo sapiens resulted in increased abilities to cooperate and are substantially responsible for humans' current ecological dominance (e.g., Hare, Reference Hare2017; Tomasello, Reference Tomasello2019), and that cooperative childcare was an important component in this evolution. However, we question whether cooperative childcare was a precondition for the emergence of an adaptation of infant fearfulness and its later impact on adult cooperation. It is more parsimonious, we believe, that increased infant fearfulness arose over humans' biological history along with other “psychological weapons” (Trivers, Reference Trivers1974) infants evolved to compete with their parents (mainly their mothers) for investment, independent of any cooperative-caregiving practices. Although infants' fearful responses might be signals of helplessness and dependence to unrelated adults, infants' wariness of strangers might actually make it less likely (not more so) for a nonparent to form attachments to them. All primate infants display fear of loud noises, abandonment, and falling, and human babies' greater fearfulness relative to chimpanzees might instead be a neotenous feature, maintaining and extending fearfulness into later development. Rather than being the “breeding ground” for enhanced infant fearfulness, we argue that cooperative care is more likely the evolved product of enhanced fearfulness, along with other adaptations (e.g., increased inhibition, tolerance for conspecifics, friendliness, Hare, Reference Hare2017) that led to Homo sapiens' remarkable degree of cooperation, including cooperative care.

The fearful ape hypothesis has much to offer. Regardless of whether greater human fearfulness is an adaptation selected primarily to make babies more attractive to adults and used as a byproduct for adult cooperation, or a full-fledged adaptation derived from cooperative care, it identifies heightened fearfulness as an evolved feature of Homo sapiens, adds to the list of infantile features that promote caregiving, and generates discussion about the entwined relations between evolution, development, and culture.