Grossmann proposes that heightened fearfulness could be evolutionarily adaptive in the context of human cooperative care, which explains why humans display enhanced fearfulness compared to other apes. I would like to propose that the central mechanisms stipulated may also be potentially applied to explain the evolution of enhanced happiness expression among humans, which may shed light on questions about the scope and boundary of the framework.

The target article first presents evidence showing that humans are “fearful apes” who display heightened fearfulness compared to chimpanzees. In fact, humans may also experience and display more enhanced happiness than chimpanzees do. Humans are a particularly happy species, evolved to experience positive feelings not only to positive stimuli but also to even neutral stimuli (Diener, Kanazawa, Suh, & Oishi, Reference Diener, Kanazawa, Suh and Oishi2015). People around the world have been found to be generally in positive moods most of the time, even including those living under difficult circumstances (Diener & Diener, Reference Diener and Diener1996). In terms of happiness expressions, chimpanzees laugh in response to playful stimuli (e.g., a good tickle) similar to how humans do (Kret, Prochazkova, Sterck, & Clay, Reference Kret, Prochazkova, Sterck and Clay2020). But only humans display smiles when experiencing inner enjoyment. The “Duchenne smile” – the authentic smile that engages muscles around the eyes – enables human happiness to be more obviously and frequently observed, which has no known correspondence among nonhuman primates (Parr & Waller, Reference Parr and Waller2006). It has been theorized that ubiquitous happiness among humans may have evolved to motivate behaviors (e.g., sociality, creativity, and planning) that increase one's own survival and reproductive fitness (Diener et al., Reference Diener, Kanazawa, Suh and Oishi2015). But is it possible that human cooperative care also plays a role in the evolution of happiness expression among humans?

Grossmann has provided evidence that human children and adults are highly sensitive to fearful faces, which is necessary to support the view that enhanced fearfulness is an adaptive trait linked to cooperative cognition. In the same vein, it has been shown that humans are highly sensitive to happy faces. Happy faces are vivid to humans: They are more discriminable at a distance than other emotional expressions, and they automatically engage processing even at the earliest stages of perception (Becker & Srinivasan, Reference Becker and Srinivasan2014). The ability to perceive happiness expressions is so deeply rooted among humans that even 3-month-old infants are able to discriminate happiness expressions from other expressions (e.g., Barrera & Maurer, Reference Barrera and Maurer1981; Kuchuk, Vibbert, & Bornstein, Reference Kuchuk, Vibbert and Bornstein1986; Young-Browne, Rosenfeld, & Horowitz, Reference Young-Browne, Rosenfeld and Horowitz1977). Directly relevant to caregiving behaviors, human adults are found to be able to detect even subtle features and changes in happiness expressions of infants and children (Bolzani Dinehart et al., Reference Bolzani Dinehart, Messinger, Acosta, Cassel, Ambadar and Cohn2005; Lobmaier, Sprengelmeyer, Wiffen, & Perrett, Reference Lobmaier, Sprengelmeyer, Wiffen and Perrett2010).

Crucial to establishing an evolutionary link between cooperative care and happiness expressions, human adults are known to display positive and caring responses when perceiving happy expressions of infants and children (see review by Franklin & Volk, Reference Franklin and Volk2018). Human adults respond to happy infant and child faces with strong interests and nurturing motivation (Aradhye, Vonk, & Arida, Reference Aradhye, Vonk and Arida2015; Parsons et al., Reference Parsons, Young, Bhandari, van Ijzendoorn, Bakermans-Kranenburg, Stein and Kringelbach2014). Happy (but not sad or neutral) infant faces activate reward-related brain regions in mothers (Strathearn, Li, Fonagy, & Montague, Reference Strathearn, Li, Fonagy and Montague2008). Happier infants also receive higher levels of parental care and play behaviors (Braungart-Rieker, Garwood, Powers, & Wang, Reference Braungart-Rieker, Garwood, Powers and Wang2001). Therefore, similar to the possibility that fearful expression stimulates care and approach responses, happiness expression also seems to elicit positive and caring responses from others.

To establish the existence of a virtuous caring cycle, Grossmann also presents evidence that fearfulness is associated with cooperative tendencies among fearful children and adults, supporting the possibility that fearful children may grow up to be cooperative people who are caring toward younger generations. Similar (and even stronger) associations have been found between happiness and cooperation. Happy adults and adolescents are more motivated to perform prosocial and kind behaviors (e.g., Aknin, Dunn, & Norton, Reference Aknin, Dunn and Norton2012; Erreygers, Vandebosch, Vranjes, Baillien, & De Witte, Reference Erreygers, Vandebosch, Vranjes, Baillien and De Witte2019; Isen & Levin, Reference Isen and Levin1972; Otake, Shimai, Tanaka-Matsumi, Otsui, & Fredrickson, Reference Otake, Shimai, Tanaka-Matsumi, Otsui and Fredrickson2006). Children who display higher levels of happiness are rated by their peers as being more prosocial and cooperative even 1 year later (Yu, Chen, Li, Liu, & Yang, Reference Yu, Chen, Li, Liu and Yang2022). Contributing to the virtuous caring cycle, happiness early in life is predictive of happiness levels across the life span (e.g., Coffey, Warren, & Gottfried, Reference Coffey, Warren and Gottfried2015), and it has been shown that happier adults are more likely to be nurturing parents and raise happy children (Douglas, Reference Douglas2019).

Therefore, the above analysis shows that just as cooperative care could lead to enhanced fearfulness among humans, cooperative care may also promote enhanced happiness expressions among humans. Two interesting questions remain unanswered: (1) The virtuous cycle involving cooperative care might predict that humans would be selected to engage in increasingly higher levels of fear (and happiness), which may not be optimal or evolutionarily adaptive. High intensity of happiness has been shown to lead to overwhelmingness and negative adjustment outcomes (Aragón, Clark, Dyer, & Bargh, Reference Aragón, Clark, Dyer and Bargh2015; Gruber, Mauss, & Tamir, Reference Gruber, Mauss and Tamir2011). Might there be some built-in mechanism for cooperative care to promote optimal rather than excessive levels of emotions? (2) Relatedly, the framework is currently silent about whether cooperative care may play a role in the evolution of some emotional expressions (e.g., fear and happiness) but not others. Nonhuman primates and humans share many emotions (De Waal, Reference De Waal2019), but some emotional expressions (such as those associated with surprise) appear to be visibly present only in our own species (Kret et al., Reference Kret, Prochazkova, Sterck and Clay2020). Might cooperative care also have influenced the evolution of these emotional expressions? To answer these questions, the framework may have to go beyond explaining how cooperative care could amplify a displayed emotion during evolution, but it also has to explain why and how the virtuous cycle is established in the first place.

In conclusion, I propose that similar to its role in promoting enhanced fearfulness among humans, cooperative care may also be a key mechanism leading to enhanced happiness expressions among humans. It will be important to further theorize why certain emotional expressions are more likely to elicit caring responses from others, which may help illuminate the scope of the theory as well as the origins of our emotional nature, such as the fearful ape or happy ape.