We agree with many of the author's interesting and intriguing ideas, but in this commentary we suggest that in order to substantiate the fearful ape hypothesis, it should directly address alternative candidate explanations which emerge from the uniquely human abilities of social learning and interaction. In stating our case, we present a broad literature indicating that our ability to express and perceive fearfulness is adaptive in the context of social fear learning, which could potentially account, at least in part, for the emergence of heightened fearfulness.

Social learning is a fundamental and crucial ability that enables us to acquire behaviors, skills, and knowledge efficiently (Boyd & Richerson, Reference Boyd and Richerson1985; Hoppitt & Laland, Reference Hoppitt and Laland2013). Although social learning is ubiquitous among human and nonhuman animals, humans have unique abilities in this domain, for example, in mechanisms such as imitation and teaching (Kendal et al., Reference Kendal, Boogert, Rendell, Laland, Webster and Jones2018; Kline, Reference Kline2015). Empirical and theoretical work on social learning has revealed processes that adaptively guide learning behavior and facilitate cultural transmission and development (Rendell et al., Reference Rendell, Fogarty, Hoppitt, Morgan, Webster and Laland2011). Such processes are proposed to be key to understanding the sophisticated forms of group coordination and culture found in humans.

Social learning is especially important within the aversive domain, where learning (asocially) from one's own experiences can be dangerous (Lindström, Selbing, & Olsson, Reference Lindström, Selbing and Olsson2016). Social pathways are believed to contribute greatly to the acquisition of fear reactions and avoidance behavior (Debiec & Olsson, Reference Debiec and Olsson2017; Rachman, Reference Rachman1977). Social fear learning depends on human transmission of threat-related information, for instance through facial expressions, body posture, or verbal communication (Lebowitz, Shic, Campbell, MacLeod, & Silverman, Reference Lebowitz, Shic, Campbell, MacLeod and Silverman2015; Mertens, Boddez, Sevenster, Engelhard, & De Houwer, Reference Mertens, Boddez, Sevenster, Engelhard and De Houwer2018), making our ability to express and perceive fear crucial for social fear learning.

At its simplest level, perceiving fear reactions in others notifies us of the (possible) presence of a threat, making us more alert and attentive (Azarian, Esser, & Peterson, Reference Azarian, Esser and Peterson2016; Lee, Susskind, & Anderson, Reference Lee, Susskind and Anderson2013). By observing others' aversive reactions, we can acquire fear vicariously (Olsson & Phelps, Reference Olsson and Phelps2007). Observational fear learning has also been demonstrated in nonhuman animals (Mineka, Davidsson, Cook, & Keir, Reference Mineka, Davidsson, Cook and Keir1984), and is likely a common pathway through which humans learn fear (Askew & Field, Reference Askew and Field2008). It is generally believed that observational learning hinges on similar associative learning mechanisms as nonsocial learning (Heyes, Reference Heyes2016; Olsson & Phelps, Reference Olsson and Phelps2007). Further, anxious individuals have stronger fear learning responses, possibly driven by an increased sensitivity and attention toward threat-related stimuli such as facial expressions of fear (Reynolds, Field, & Askew, Reference Reynolds, Field and Askew2014).

Research also shows that humans can, unlike nonhuman animals, acquire fear through verbal instructions (Mertens et al., Reference Mertens, Boddez, Sevenster, Engelhard and De Houwer2018). Instructed fear learning appears to be similar in nature to observational or direct learning (Mertens et al., Reference Mertens, Boddez, Sevenster, Engelhard and De Houwer2018) and possibly similarly modulated by trait anxiety (Grillon, Ameli, Merikangas, Woods, & Davis, Reference Grillon, Ameli, Merikangas, Woods and Davis1993). The importance of verbal transmission of threat-related information is also evident in the negativity bias seen in social transmission of information, for example, in storytelling and social media (Bebbington, MacLeod, Ellison, & Fay, Reference Bebbington, MacLeod, Ellison and Fay2017; Tsugawa & Ohsaki, Reference Tsugawa and Ohsaki2017). Along the same line we also see that anxious individuals pay more attention to threatening or negative information such as news stories (Gadarian & Albertson, Reference Gadarian and Albertson2014)

Moreover, it is relevant to note that expressing fear is also believed to be a way to show submissiveness and accept the dominance of someone else (Öhman, Reference Öhman1985) which means that we can learn something about a group's social structure by observing others' expressions of fearfulness (Jones, DeBruine, Little, Watkins, & Feinberg, Reference Jones, DeBruine, Little, Watkins and Feinberg2011). Learning about social structures around us can help us predict and avoid social threats, either direct threats such as attacks, or more indirect threats such as loss of support, status, or one's social group.

However, some aspects of social fear learning are likely non-adaptive. Research indicates that social forms of fear learning are important pathways in the development of clinical fears such as post-traumatic stress disorder (PTSD), phobias, or anxiety disorders (Askew & Field, Reference Askew and Field2008; Öst & Hugdahl, Reference Öst and Hugdahl1981), for example, through observation of violence or accidents. There are also studies demonstrating transmission of anxiety between individuals, typically from parent to child (Aktar, Nikolić, & Bögels, Reference Aktar, Nikolić and Bögels2022; Eley et al., Reference Eley, McAdams, Rijsdijk, Lichtenstein, Narusyte, Reiss and Neiderhiser2015). This is possibly driven by transmission of beliefs that the world is unsafe or through transmission of coping strategies such as avoidance and worrying, although the exact mechanisms are unknown. It further appears as if anxious parents are worse at responding caringly and supportive to negative emotional reactions in their children (Creswell, Apetroaia, Murray, & Cooper, Reference Creswell, Apetroaia, Murray and Cooper2012) which would, at least partly, contradict Grossmann's arguments. However, typically anxious behavior does not appear to be transmitted to healthy adults (Selbing & Olsson, Reference Selbing and Olsson2019).

Taken together, a large body of research from the perspective of social learning suggests that our ability to express and perceive fearfulness is adaptive, as it helps us predict and handle potential threats while decreasing our own exposure to danger. In the fearful ape hypothesis, Grossmann builds his argument for the adaptiveness of heightened fear around the adaptiveness of our ability to express and perceive fearfulness, given that it promotes helping behavior and increases cooperation. We agree that observation of fearfulness in another individual can elicit helping or caring, even though helping also can be triggered by perceiving a threat toward other individuals without those individuals expressing fearfulness (Vieira, Schellhaas, Enström, & Olsson, Reference Vieira, Schellhaas, Enström and Olsson2020). However, given the fundamental and ubiquitous role of adaptive social learning across the human lifespan, we suggest that the fearful ape hypothesis needs to consider social learning as an alternative candidate explanation for why the expression and perception of fearfulness is adaptive.

According to Grossmann, cooperative care is the primary precondition which enables the emergence of heightened fearfulness. However, if we consider social learning as the fundamental precondition and primary mechanism, then care-based cooperation can be seen as an adaptive byproduct of more domain-general social learning capacities. In either case, we argue that any theory that builds on the adaptiveness of a (human) social signal should, for theoretical and explanatory adequacy, consider the competing or contributing role of social learning.