Be Nice When You Can, But Never If It Costs You Too Much
In all of the previous work discussed, the capuchins’ behavior could easily be explained by mutualism, which provides an immediate benefit to the self, or reciprocity, which provides a benefit in the future. However, sometimes animals do things that help their partners for no obvious benefit. There has been a recent interest in prosocial behavior, or the willingness to benefit another at either no or a very small cost to the self. Initial studies in nonhumans (Jensen, Hare, Call, & Tomasello, 2006; Silk, Brosnan, Vonk, Henrich, Povinelli, Richardson et al., 2005; Vonk, Sarah, Brosnan, Silk, Henrich, Richardson et al., 2008) found that chimpanzees did not bring food to conspecifics, even when it did not cost them anything. In these studies, the chimpanzees were given a choice between an option that brought food only to themselves or an option that brought food to both themselves and a partner. The subject received the same food no matter which choice they made, so there was no cost to behaving prosocially. To control for the possibility that they preferred the option with more food items, the subjects’ behaviors were compared when paired with another chimp versus when alone. In none of these studies did chimpanzees choose to bring food to their partners more often when their partner was present than when they were next to an empty cage.
A series of similar studies have been run with capuchin monkeys with very different results. The capuchins behaved prosocially, choosing outcomes that benefit their partners over those that did not (de Waal, Leimgruber, & Greenberg, 2008; Lakshminarayanan & Santos, 2008; Takimoto, Kuroshima, & Fujita, 2009). Additional research indicates that there are many factors that affect prosocial behavior. Chimpanzees, despite failing to bring food to their partners, do help them in nonfood related tasks (Warneken, Hare, Melis, Hanus, & Tomasello, 2007; Warneken & Tomasello, 2006). Moreover, callitrichids vary, with some species behaving prosocially (Burkart, Fehr, Efferson, & van Schaik, 2007) and some not (Cronin, Schroeder, Rothwell, Silk, & Snowdon, 2009). The fact that some cooperative breeders are prosocial, including humans, has led to the hypothesis that cooperative breeding creates the interdependence that selects for prosocial behavior (Burkart, Fehr, Efferson, & van Schaik, 2007). However, additional data are required before firm conclusions can be drawn.
Note that capuchins monkeys’ behavior seems on the surface contradictory. These monkeys make choices that are prosocial, choosing to bring food to their partners when it doesn’t cost them anything to do so, yet they also dislike inequity, refusing to participate when outcomes are unequal or when their partners do not share better rewards. Thus, what happens when capuchins must decide whether or not to be prosocial when doing so will also create inequity?
To test this we recently ran a study in which monkeys had to decide whether to bring a set reward to themselves and their partner (Brosnan, Houser et al., 2010). Rewards varied such that they were either equal, somewhat unequal (no versus less-preferred reward or less-preferred versus more-preferred reward), or very unequal (no versus more-preferred reward). Capuchins were prosocial—that is, they chose to pull in the rewards more often when a partner was present than absent—when rewards were equal or moderately unequal. However, when rewards were very unequal, the subjects were no longer prosocial. This cannot be explained by the presence or absence of a reward for the subject, since the subject received no reward in one of the low-inequity conditions. Thus, it appears that capuchins are willing to be prosocial even when it results in inequity, as long as the inequity is not too great.