Chimpanzees
Our nearest primate relatives live in large, multi-male, multi-female social groups and as a consequence display a variety of highly complex social behaviours. Most relevant for current purposes, in response to deeply hierarchical social structures chimpanzees form long-term and stable cooperative social relationships, which are aimed at outcompeting conspecific rivals. Within these relationships, chimpanzees display and engage in a series of morally-relevant behaviours, such as mutual grooming and food sharing. Observational evidence from the wild indicates that chimpanzeesâ prosocial behaviours are heavily skewed toward cooperative partners: male chimpanzees extend as much as 66â81 per cent of their grooming toward their top three partners, and the sharing of food follows a similar pattern (reviewed in Muller & Mitani, 2005; see also Engelmann & Herrmann, 2016). The crucial question is whether these behaviours qualify as altruistic in the sense that they proximately enhance partner welfare and at the same time entail nothing but costs for the donor.
While natural observations alone cannot answer this question, as benefits for the partner cannot be systematically manipulated (e.g. grooming might prove to be beneficial not only to the groomee but also to the groomer who might gain access to fleas in the process), carefully controlled experiments can distinguish between different motivations underlying the same behaviour. In fact, a number of recent studies provide evidence that chimpanzee helping is indeed the real thing, and not motivated by hidden benefits to donors. Specifically, in a series of experiments captive chimpanzees have been shown to help a conspecific by fetching an out-of-reach tool, opening a door, and making food available (Melis et al., 2010; Warneken, Hare, Melis, Hanus, & Tomasello, 2007; Yamamoto, Humle, & Tanaka, 2012). All of these studies involve control conditions, ruling out the possibility that chimpanzees act only for personal or self-serving goals. Furthermore, chimpanzeesâ behaviour cannot be interpreted in terms of immediate strategic goals, like the improvement of their reputation as cooperators. Recent work suggests that chimpanzees do not show concern for reputation, and do not selectively help more when they are observed by a conspecific compared to when they are alone (Engelmann, Herrmann, & Tomasello, 2012; Engelmann, Herrmann, & Tomasello, 2016a).
One hypothesis is that chimpanzee behaviour in helping contexts is motivated by the underlying emotion of sympathy. Indeed, recent work on the mammalian bonding hormone oxytocin and its facilitation of instances of bonding and cooperation in chimpanzees provides support for this explanation. Crockford et al. (2013) and Wittig et al. (2014) found that oxytocin is involved in grooming and food sharing in wild chimpanzees, and that the individual initiating these behaviours (as well as the recipient) experiences an increase in this social bonding hormone.
Close social relationships are defined in terms of attitudes and intentions to trust, help, support, and share preferentially with friends. A second, no less important part of interpersonal relationships consists of forming and holding each other to certain expectations (Scanlon, 2008; Wallace, 2013). That is to say, morality is not only expressed in what I do for others, but also in what I expect them to do for me. Thus, one further source of evidence for chimpanzeesâ moral sense comes from their reactions to these social expectations being unmet.
Little work has directly addressed the question of whether great apes form and hold special expectations of their friends and experience reactive attitudes when such expectations are disappointed. However, a re-interpretation of two studies by Brosnan and colleagues (Brosnan, Schiff, & deWaal, 2005; Brosnan, Talbot, Ahlgren, Lambeth, & Schapiro, 2010) using the inequity aversion task suggests that they do. The basic result is that chimpanzees reject food given to them by a human experimenter (food they would otherwise readily accept) if a conspecific gets better food for the same or even less effort. The authors interpret this finding in terms of social comparison, and thus ultimately as a burgeoning sense of fairness. However, a different interpretation, suggested by Roughley (in press) and Tomasello (2016), is that chimpanzeesâ reactions in those studies are not based on a comparison of how they have been treated compared to a conspecific, but rather on how they are being treated by the human experimenter with whom they share a cooperative relationship. A recent study lends support to this alternative interpretation. Engelmann, Clift, Herrmann and Tomasello (2017) contrasted two conditions in which food is distributed by either a machine or a cooperative partner, and found that chimpanzees indeed react negatively only in the latter context. Furthermore, chimpanzees show negative emotional reactions to their food-distributing partner independent of whether a conspecific was present or not, further supporting the hypothesis that the inequity aversion task reveals special expectations of cooperative partners and not fairness considerations. The social anger displayed by chimpanzees in the inequity aversion task is thus distinctively interpersonal, and, in Tomaselloâs (2016) words, might take the form: âI am angry that you are treating me without sympathyâ.
As a whole, multiple lines of evidence suggest that chimpanzee prosociality toward their friends is the real thing in that it is, proximately speaking, driven by genuine altruistic motivations. Chimpanzee helping is flexibly tailored to their friendsâ needs and connected to the prosocial emotion of sympathy. The flip side of such prosociality is that chimpanzees form special expectations that their friends will treat them with sympathy in turn. We have reviewed evidence suggesting that chimpanzees respond to failures to meet these expectations with a distinct reactive attitude, namely social anger.
Children
Whether they encounter an adult needing a door to be opened, or reaching for an out-of-reach object, or missing an object needed to continue an activity, human infants from as young as 14 months reliably react to these situations by helping (Svetlova, Nichols, & Brownell, 2010; Warneken & Tomasello, 2006, 2007). Several experimental paradigms confirm that childrenâs helping behaviour at this age emerges spontaneously and naturally, with no need for external incentives. Having their mother watch passively or even actively encourage them to help does not increase helping levels (Warneken & Tomasello, 2013), and children will even help adults in situations in which the helpee doesnât know that he needs help (Warneken, 2013). Perhaps most convincingly, rewarding a child for helping actually decreases levels of helping over time once the reward is taken away (Warneken & Tomasello, 2008). Indeed, childrenâs sensitivity to potential external rewards for cooperative behaviour does not seem to emerge until 5 years of age, when they show the first signs of more strategic forms of helping and sharing â for example, to improve their reputation (Engelmann et al., 2012; Engelmann, Over, Herrmann, & Tomasello, 2013) or to benefit from acts of reciprocity (Warneken & SebastiĂĄn-Enesco, 2015).
Childrenâs early forms of helping are rooted in sympathetic concern for the plight of others. Support for this view comes from studies suggesting that children preferentially help, even at a high cost to themselves, individuals displaying signs of justified emotional distress (Hepach, Vaish, & Tomasello, 2012a; Nichols, Svetlova, & Brownell, 2009), and that childrenâs own level of distress in response to a harmed individual is positively correlated with subsequent helping behaviour (Vaish, Carpenter, & Tomasello, 2009). Moreover, using direct physiological measures of emotional arousal, pupil dilation and body posture, Hepach, Vaish and Tomasello (2012b, in press) found that young children are equally satisfied both when they help someone in need and when they see that person being helped by a third party â and more satisfied in both of these cases than when the person is not being helped at all â suggesting that their motivation is not to provide help but only to see that the other person is helped.
Unlike their great ape cousins, by around their second birthday young children show an expanded sympathetic concern that includes non-kin and non-friends in the circle of recipients. It is only around 3 years of age that children begin to selectively help their friends more than their non-friends, similar to chimpanzees (Engelmann, Haux, & Herrmann, 2017). To our knowledge, no previous study has specifically addressed whether, and to what extent, young children this age form special expectations of their peers and caretakers. However, our analysis of chimpanzeesâ emotional reaction to being treated without sympathy by closely bonded individuals suggests that this form of social anger is a familiar experience for young children also.