In this commentary, we synthesize existing models of social fear that address within-species variability, a topic that we believe is underacknowledged in Grossmann's model. These developmental and evolutionary models address (1) why there are individual differences in social fear, and (2) the varying, adaptive outcomes of social fear, which can include cooperative behavior as Grossmann proposes, but also withdrawal.

Grossmann posits that the species-unique fearfulness of human infants is adaptive because it promotes a cascade of parental and alloparental cooperative responses that, in turn, seemingly promote and sustain cooperative behavior across generations. But does this model fully address the “paradox” of human fearfulness?

Grossmann's arguments are built primarily from between-species comparisons of fear, with minimal incorporation of within-species variability. The cost of this approach is the omission of developmental consideration regarding (1) why there are individual differences in fearful profiles within our species, and (2) the variety of adaptive outcomes of certain fear-related behaviors, which can include cooperation, as Grossmann proposes, but also withdrawal, as we will argue.

We believe that Grossmann's arguments can be improved by considering existing developmental models that have addressed a similar paradox in relation to social fear (also referred to as shyness or social anxiety), which can have detrimental health effects and yet is still common in our species.

If there is indeed an “optimal” amount of fear as Grossmann argues, why then, is there such a range of fear within our species? How do infants come to display the local optima of fear for their environment? Similar questions have been asked in relation to social anxiety. Karasewich and Kuhlmeier (Reference Karasewich and Kuhlmeier2020) proposed that trait social anxiety is a conditional adaptation, such that individual variation results from a developmental system that is predisposed to process cues of social threat within the environment during infancy. For individuals with inhibited temperaments, experiencing cues of social threat, such as controlling parenting, rejection, or neglect in early development, can entrain development on a path to social anxiety, in preparation for a socially threatening future. In contrast, inhibited children who are exposed to supportive parenting and general feelings of social and physical safety are less likely to be socially anxious as an adolescent or adult. Thus, social anxiety can be viewed as an adaptive response to the local social environment, which explains why we observe individual differences in this trait within the human species.

Further, several groups have proposed diverse evolutionary timelines and functions of various shyness subtypes (Hassan, MacGowan, Poole, & Schmidt, Reference Hassan, MacGowan, Poole, Schmidt, Coplan, Bowker and Nelson2021; Schmidt & Poole, Reference Schmidt and Poole2019) and shy expressions (Colonnesi, Nikolić, & Bögels, Reference Colonnesi, Nikolić, Bögels, L. A. and K. L.2020). For example, Schmidt and Poole (Reference Schmidt and Poole2019) draw from Buss's (Reference Buss, Jones, Cheek and Briggs1986) argument that some shy individuals predominantly experience fear in novel social situations. This temperamental profile is thought to have been adaptive in our ancestral past because it limited exposure to potentially dangerous individuals. They further argue that this “fearful shyness,” which is conserved across many primate and non-primate species, is evolutionarily older than a primarily self-conscious shyness subtype specific to humans. “Self-conscious shyness” is purportedly more adaptive than “fearful shyness” in our contemporary world, in which ensuring one's safety through appeasement or avoidance of social blunders is arguably more pertinent than concerns about potential physical threat.

Similarly, there is evidence for heterogeneity in the expression of shyness. Shy behaviors can be accompanied by either positive (approach-motivated) or non-positive (avoidance-motivated) facial expressions (Colonnesi, Nikolić, de Vente, & Bögels, Reference Colonnesi, Nikolić, de Vente and Bögels2017, Reference Colonnesi, Nikolić, Bögels, L. A. and K. L.2020; Susa-Erdogan, Benga, & Colonnesi, Reference Susa-Erdogan, Benga and Colonnesi2022).

Indeed, some expressions of shyness lead to approach, and associated cooperative behaviors or sociocognitive correlates like theory of mind (MacGowan, Colonnesi, Nikolic, & Schmidt, Reference MacGowan, Colonnesi, Nikolic and Schmidt2022a), but other manifestations of shyness can lead to avoidance, and therefore reduced cooperative behavior (Karasewich, Kuhlmeier, Beier, & Dunfield, Reference Karasewich, Kuhlmeier, Beier and Dunfield2019; MacGowan & Schmidt, Reference MacGowan and Schmidt2021a, Reference MacGowan and Schmidt2021b). In fact, both temperamental shyness and fearfulness have been associated with lower instances of helping and comforting from toddlerhood to middle childhood (Beier, Terrizzi, Woodward, & Larson, Reference Beier, Terrizzi, Woodward and Larson2017; Eisenberg, Spinrad, Taylor, & Liew, Reference Eisenberg, Spinrad, Taylor and Liew2017; Findlay, Girardi, & Coplan, Reference Findlay, Girardi and Coplan2006; Karasewich et al., Reference Karasewich, Kuhlmeier, Beier and Dunfield2019; MacGowan & Schmidt, Reference MacGowan and Schmidt2021a, Reference MacGowan and Schmidt2021b; Young, Fox, & Zahn-Waxler, Reference Young, Fox and Zahn-Waxler1999).

Although avoidant shy behaviors can be costly (e.g., reduced cooperation), withdrawing can be adaptive in some social situations and for some individuals. For example, for some people, it may be adaptive to engage in avoidance of certain cooperative opportunities to protect their physiological vulnerability (e.g., low physiological regulatory capacity, high physiological reactivity; MacGowan & Schmidt, Reference MacGowan and Schmidt2021a; Thompson & Calkins, Reference Thompson and Calkins1996) or to avoid the possibility of performing a risky social blunder in the presence of high-status group members. In support of this notion, there has been some evidence to suggest that withdrawn behavior in early childhood and adulthood can result in social-cognitive strengths that appear to have evolved for the purpose of remaining vigilant of possible social threat (Brunet, Heisz, Mondloch, Shore, & Schmidt, Reference Brunet, Heisz, Mondloch, Shore and Schmidt2009; LoBue & Perez-Edgar, Reference LoBue and Perez-Edgar2014; MacGowan, Mirabelli, Obhi, & Schmidt, Reference MacGowan, Mirabelli, Obhi and Schmidt2022b; Matsuda, Okanoya, & Myowa-Yamakoshi, Reference Matsuda, Okanoya and Myowa-Yamakoshi2013) and for blending in with the social environment (MacGowan et al., Reference MacGowan, Mirabelli, Obhi and Schmidt2022b). Of course, we acknowledge that some expressions and manifestations of shyness may not be adaptive at all, and result in rejection and exclusion, such as when an adult's environment does not match what they experienced in childhood (Karasewich & Kuhlmeier, Reference Karasewich and Kuhlmeier2020).

In sum, we believe that existing lines of developmental research – and related evolutionary arguments – that exist in the context of social fear can be used to incorporate within-species nuance into Grossmann's model. Specifically, we presume that delineating social from nonsocial fear may be warranted. In addition, considering the many subtypes and expressions of social fear that have been well-documented in the developmental and evolutionary literature (Colonnesi et al., Reference Colonnesi, Nikolić, de Vente and Bögels2017, Reference Colonnesi, Nikolić, Bögels, L. A. and K. L.2020; Hassan et al., Reference Hassan, MacGowan, Poole, Schmidt, Coplan, Bowker and Nelson2021; Schmidt & Poole, Reference Schmidt and Poole2019) would allow for more nuanced interpretations of when and how these behavioral profiles have been adaptive for our ancestors. Finally, we propose that, based on reasoning from Karasewich and Kuhlmeier (Reference Karasewich and Kuhlmeier2020), fearfulness, more broadly than social anxiety, may also be considered a conditional adaptation such that existing individual differences in temperamental fearfulness would likely interact with early information from the child's environment, resulting in behavioral outcomes that are suited to that environment.