As reviewed in Grossmann's target article, research indicates that humans experience fear more often and intensely, and are more attuned to conspecifics' fear expressions, compared to other primates. To account for this between-species difference, Grossmann proposes (and marshals a compelling body of evidence in support of) the fearful ape hypothesis, positing that humans' heightened fearfulness emerged out of its adaptive benefits, namely: (1) Eliciting support from others in response to specific, momentary threats, and (2) promoting and maintaining long-term cooperative relationships that are crucial for survival and reproduction. We agree with Grossmann's synopsis, but contend that its scope may be too narrow. Emotions beyond fear – notably, supplication emotions (i.e., a group of emotions that includes fear but also sadness, disappointment, distress, and worry) and appeasement emotions (i.e., shame, guilt, regret, and embarrassment; van Kleef & Côté, Reference van Kleef and Côté2022) – likely facilitate the same adaptive ends as those proposed for fear. As a result, several additional distinct emotions may also underlie and shape humans' unique capacity for cooperation (see Keltner & Shiota [Reference Keltner, Shiota, John and Robins2021] for a broad review of the cooperative functions of emotions).

According to van Kleef and Côté (Reference van Kleef and Côté2022), supplication emotions are those that occur when one's expectations or desires go unfulfilled. Similar to fear, the experience and expression of other supplication emotions elicit support from others. For example, like nonverbal expressions of fear (e.g., Hammer & Marsh, Reference Hammer and Marsh2015; Marsh & Ambady, Reference Marsh and Ambady2007), nonverbal expressions of sadness and disappointment elicit emotional and financial support from observers (Hendriks & Vingerhoets, Reference Hendriks and Vingerhoets2006; Small & Verrochi, Reference Small and Verrochi2009; van Doorn, van Kleef, & van der Pligt, Reference van Doorn, van Kleef and van der Pligt2015). Furthermore, just as heightened fearfulness promotes cooperative tendencies across contexts (per the target article), individuals who are generally prone to sadness and distress are also generally prone to sympathy (Eisenberg, Reference Eisenberg2000), which increases caring behavioral tendencies toward others (Weidman & Tracy, Reference Weidman and Tracy2020). Thus, stable tendencies to feel multiple supplication emotions may underpin the same set of cooperative behaviors as does fearfulness.

Expressing supplication emotions also has been shown to facilitate relationship formation and maintenance. One study found that individuals were more willing to help a person expressing sadness than they were a person expressing no emotion, but only when they believed the expressor was open to meeting new people (Clark, Oullette, Powell, & Milberg, Reference Clark, Oullette, Powell and Milberg1987). Similarly, a willingness to express emotions like distress, nervousness, and anxiety is associated with the formation of intimate friendships (Graham, Huang, Clark, & Helgeson, Reference Graham, Huang, Clark and Helgeson2008). Further supporting the role of supplication emotions in maintaining close relationships, Parkinson, Simons, and Niven (Reference Parkinson, Simons and Niven2016) found that expressions of worry lead expressors' romantic partners to attempt to calm them, suggesting that these emotions elicit needed emotional support within existing relationships.

Unsurprisingly, given the positive relationship effects of expressing supplication emotions, suppressing these emotions can, conversely, impede relationship formation and damage existing relationships. In one study, suppressing sadness led unacquainted conversation partners to feel less rapport and a weaker desire to affiliate with the suppressor (Butler et al., Reference Butler, Egloff, Wlhelm, Smith, Erickson and Gross2003). Suppression of these emotions in romantic relationships is associated with greater conflict, as well as suppressors' partners feeling greater negative emotions and lower relationship satisfaction (Impett et al., Reference Impett, Kogan, English, John, Oveis, Gordon and Keltner2012). Overall, research on supplication emotions complements research consistent with the fearful ape hypothesis, suggesting that the hypothesis might be fruitfully broadened to include this entire family of emotions.

Appeasement emotions occur when a person who has committed a social transgression or failed to meet others' expectations anticipates reactive physical or relational aggression. These emotions serve similar social functions to supplication emotions, but they are more specialized to facilitate exchange relationships (i.e., relationships based on the reciprocal exchange of resources instead of mutual care), rather than the communal close relationships that seem to benefit from supplication emotions (Batson, Reference Batson1993; van Kleef & Côté, Reference van Kleef and Côté2022). More specifically, appeasement emotions are particularly functional by virtue of their impact on individuals' tendency to follow social norms and meet obligations (Beall & Tracy, Reference Beall, Tracy, Barkow, Workman and Reader2020). For example, when individuals commit a social transgression, they risk being perceived as untrustworthy, selfish, and unfit for future relationships (Gilbert, Reference Gilbert, Tracy, Robins and Tangney2007). Displaying shame or embarrassment in these situations can appease onlookers by demonstrating awareness of social norms and acknowledgment that they were violated, thereby helping the transgressor maintain their relationships by conveying one's motivation to behave prosocially moving forward (Feinberg, Willer, & Keltner, Reference Feinberg, Willer and Keltner2012; Keltner & Buswell, Reference Keltner and Buswell1997; Martens, Tracy, & Shariff, Reference Martens, Tracy and Shariff2012).

In fact, this appeasement function extends beyond existing relationships; studies have found that individuals choose more lenient penalties for unacquainted transgressors (e.g., CEOs making public apologies) who express shame and embarrassment, compared to those who express no emotion (Giner-Sorolla, Castano, Espinosa, & Brown, Reference Giner-Sorolla, Castano, Espinosa and Brown2008; Keltner & Buswell, Reference Keltner and Buswell1997). Appeasement emotions thus help individuals form and maintain cooperative exchange relationships by reducing the consequences of social transgressions and preventing relationship dissolution, which benefit both parties' long-term fitness, and may explain the evolution of these emotions (Barkow, Reference Barkow1989; Gilbert, Reference Gilbert1997; Leary & Baumeister, Reference Leary, Baumeister and Zanna2000). In this way, appeasement emotions serve a function very similar to that of fear, proposed in the fearful ape hypothesis, but more narrowly in exchange relationships.

In closing, we commend Grossmann's review and strong support for the fearful ape hypothesis, while also recommending that it be expanded to include supplication and appeasement emotions. Both classes of emotions are associated with signals and behaviors that underlie humans' desire and ability to cooperate in specific moments, as well as to form and maintain long-standing cooperative relationships. Nonetheless, questions remain for future research. To our knowledge, the primate research that Grossmann describes has yet to be conducted for other supplication emotions like sadness, leaving it unclear whether humans are uniquely prone to experiencing sadness and recognizing it in others in the way they appear to be for fear. We look forward to research in this vein, and thank Grossmann for bringing these important questions to light with his novel and thought-provoking hypothesis.