Grossmann argues that humans display unusually high levels of fear compared to other apes, and that we do so because heightened fear allowed our ancestors to acquire extra help from others, especially within the small-scale cooperative societies in which humans evolved. This “fearful ape” hypothesis is intriguing, but is currently underspecified. There are a number of issues that need to be addressed to flesh out the hypothesis and to increase its usefulness and its ability to generate novel predictions.

First, it is unclear whether the hypothesis should apply only to fear. There are many emotions or affective responses that elicit caring responses from parents or alloparents, including fear, hunger, discomfort, cold, sickness, and so on. If humans are more fearful because this elicits help from others, then all emotions that garner sympathy and assistance (henceforth “care-eliciting emotions”) should be heightened in humans as well. To say that there's something special about fear, we would need much more research comparing parental responses to infant fear with other affective states, and how the heightened levels of fear in humans compare with other care-eliciting emotions.

Furthermore, if one's ability to detect fear is linked to increased helpfulness, then people's ability to detect other care-eliciting emotions (i.e., not just fear) should also lead to increased helpfulness. For example, prosocial behavior has been robustly linked to trait empathy, including the ability to accurately detect all emotions in others (Kim & Kou, Reference Kim and Kou2014; Smith, Norman, & Decety, Reference Smith, Norman and Decety2020). Although there are several studies showing enhanced cooperative behavior among children and adults who had heightened sensitivity to fearful faces (e.g., Kiel & Buss, Reference Kiel and Buss2011; Marsh & Ambady, Reference Marsh and Ambady2007; Marsh & Blair, Reference Marsh and Blair2008; Marsh et al., Reference Marsh, Stoycos, Brethel-Haurwitz, Robinson, VanMeter and Cardinale2014; Rajhans, Altvater-Mackensen, Vaish, & Grossmann, Reference Rajhans, Altvater-Mackensen, Vaish and Grossmann2016), those studies either did not compare fearful faces with faces displaying other care-eliciting emotions, or found little difference. Other papers argue for the link between perceiving general emotions of distress and cooperation, rather than a specific link with fear (e.g., Warneken, Reference Warneken2015). To make the “fearful ape hypothesis” stronger and more specific, we need stronger evidence of fear being unique to other aversive states, and why. Without such evidence, it seems that instead of the “fearful ape hypothesis,” a better term might be the “emotional ape hypothesis.”

Second, it is unclear whether this hypothesis is specific to humans. Cooperative breeding is rare in primates, but is not unique to humans (reviewed by Hrdy, Reference Hrdy2011). If the “fearful ape hypothesis” is true, then we should observe higher levels of fearfulness in callitrichid monkeys (e.g., marmosets, tamarins), as well as other cooperatively or communally breeding mammals (e.g., meerkats, naked mole rats, wild dogs, spotted hyenas, lions) and birds (e.g., scrub jays, groove-billed anis). If this heightened fearfulness is not observed in these species (relative to similar non-cooperatively breeding species), then this would rebut the “fearful ape hypothesis.” There is evidence that both human children and (non-cooperatively breeding) orangutans attend preferentially to fearful faces (Pritsch, Telkemeyer, Mühlenbeck, & Liebal, Reference Pritsch, Telkemeyer, Mühlenbeck and Liebal2017), which suggests that attention to fear is not limited to humans or other cooperative breeders.

Third, the target article characterizes fearfulness as an enhanced reactivity to, and avoidance of, novelty. It is unclear why this avoidance would elicit aid from others. Dangerous stimuli – whether novel or not – should trigger fear and elicit emergency aid to overcome that danger. Novel stimuli should not trigger emergency aid in the same way, although it is possible that they should trigger teaching. Framed in this manner, avoidance of novelty may be more indicative of anxiety or uncertainty rather than fear. Research on chimpanzees indicates that fear and anxiety result in different facial expressions (Parr, Waller, Vick, & Bard, Reference Parr, Waller, Vick and Bard2007). However, anxious facial expressions in other great apes and their responsiveness to them are currently an underexplored topic (Kret, Prochazkova, Sterck, & Clay, Reference Kret, Prochazkova, Sterck and Clay2020). Investigating whether there are cross-species differences in the detection of anxious faces rather than fearful ones may provide more relevant results to support the hypothesis.

Fourth, it is currently unclear why one's own expressed fear should be positively related to one's ability to detect fear in others (and respond with help). One might even predict the opposite relationship, given the potential arms race between individuals exaggerating their fear to extract more aid than they need versus audience members lowering their response threshold to avoid such exploitation. Young in other species do exaggerate their need (reviewed by Caro, West, & Griffin, Reference Caro, West and Griffin2016a), especially when other dependent siblings are present or when those siblings are only half-siblings because of remarriage, both of which occur regularly in humans (e.g., Daly & Wilson, Reference Daly and Wilson1999; Hrdy, Reference Hrdy2011). These exaggerated reactions should then lead to lowered response thresholds. In fact, parents in poor and unpredictable environments respond less to signals of need, and rely more on signals of offspring quality (Caro, Griffin, Hinde, & West, Reference Caro, Griffin, Hinde and West2016b). It is currently unclear how such arms races and lowering response thresholds would apply to cooperatively breeding species like humans, and whether this would cause an escalation of care-eliciting emotions. However, it does mean this part of the “fearful ape hypothesis” requires further specification, to understand how one's own fear levels are related to the ability to detect fear in others and respond with the appropriate help.

Overall, the “fearful ape hypothesis” is an intriguing idea, but several aspects are currently underspecified. By highlighting these aspects, we hope to advance the “fearful ape hypothesis” into a testable idea that can be supported or rebutted.