Helpful versus non-helpful persons

For Experiment 1, we tested seven capuchin monkeys. In front of each monkey, two actors performed in ways that portrayed a helpful or a non-helpful disposition. In six ‘helper’ sessions, actor A tried to get a toy out of a container, requested help from actor B, and the latter helped. In six ‘non-helper’ sessions B refused to help (Fig. 1a), leaving A to continue in vain. Both actors then offered a piece of food to the monkey, who accepted one offer. In helper sessions, monkeys readily accepted food from both actors, showing no significant bias (Table 1). In contrast, in non-helper sessions they accepted food more frequently from A (attempter) than B (non-helper) (Student’s t-test, t=2.71, df=6, P=0.035). There was no significant correlation between session number and frequency of selecting either actor (Spearman’s rho=0.09, P=0.841), ruling out learning across sessions. Comparing frequency of selecting B in the two types of session confirmed the bias against the non-helper (Student’s t-test, t=−2.51, P=0.046) (Fig. 2). In non-helper sessions, A and B were chosen significantly more than and less than expected by chance, respectively (one-sample t-test, t=2.71, df=6, P=0.035).

Figure 1: Human interactions observed by monkeys. (a) A requests help; B refuses by turning away (Experiment 1). (b) A requests help; B refuses by turning away (Experiment 2); she goes on to manipulate the container in front of her (as in c). (c) A requests help; B does not acknowledge the request but continues to manipulate her container (Experiment 3). (d) Both actors briefly stop manipulating their containers, while B turns away from A; this is not a refusal to help (Experiment 4). Full size image

Table 1 Individual monkeys’ food acceptance results with the actors playing different roles. Full size table

Figure 2: Mean frequency of accepting food offered by the helper and the non-helper. Both Experiments 1 and 2 involved the monkeys accepting food from helpers and non-helpers. In Experiment 1, N=7; in Experiment 2, N=6 (see Results). *P<0.05; †P>0.05<0.10. Error bars represent mean±s.e.m. Full size image

Equalized competence and activity

Experiment 2 followed the same general procedure as previously, but now each actor had a container with a small toy inside. At the start of and half-way through each session, we presented ‘success demonstrations’ to show that both actors could open the containers: each actor manipulated a container for 5 s, opened it and emptied the contents into one hand. In helper sessions, B again responded positively to A’s request for help, but their joint manipulation of the container did not result in success. In non-helper sessions, not only did B again refuse to help (Fig. 1b), she also began to manipulate her own container, thus equalizing post-request manipulatory activity.

In helper sessions, the monkeys again expressed no overall bias towards either actor. In non-helper sessions, the tendency to accept food more often from the attempter than the non-helper was marginally significant (Student’s t-test, t=1.48, df=6, P=0.095); there was again no significant correlation between this measure and session number (Spearman’s rho=0.37, P=0.40). Because one monkey accepted more food from the non-helper in these sessions (Binomial test, P<0.05), we re-ran the analysis excluding this individual; this revealed a significant bias for accepting food from the attempter compared with the non-helper (Student’s t-test, t=2.18, df=5, P=0.046). Direct comparison of frequency of accepting food from B in both conditions confirmed a significant bias against the non-helper (Student’s t-test, t=−2.83, df=5, P=0.019) (Fig. 2). In non-helper sessions, choices of A and B were marginally significantly above and below chance, respectively (one-sample t-test, t=2.14, df=5, P=0.09). Thus, the same tendency to accept food less frequently from the non-helper was observed, albeit less strongly than in Experiment 1.

Explicit non-helper versus occupied non-helper

In Experiment 3, B did not explicitly refuse to help A, but simply failed to acknowledge A’s request. In these sessions, each actor manipulated a container from the start of the trial. Otherwise, in non-helper sessions B behaved as in Experiment 2 (now called ‘explicit non-helper’ sessions). Helper sessions were replaced by ‘occupied non-helper’ sessions, in which B failed to acknowledge A’s request due to focussing on trying to open her container (Fig. 1c). In occupied non-helper sessions, monkeys accepted food non-differentially from the two actors (Student’s t-test, t=−1.64, df=6, P=0.151). In explicit non-helper sessions, there was again a strong bias against the non-helper (Student’s t-test, t=4.60, df=6, P=0.002); the correlation with session number was not significant (Spearman’s rho=0.49, P=0.263). Direct comparison showed that the explicit non-helper was chosen significantly less often than the occupied non-helper (Student’s t-test, t=3.14, df=6, P=0.011) (Fig. 3). In explicit non-helper sessions, A and B were selected more than and less than chance, respectively (one-sample t-test, t=3.14, df=6, P=0.020).

Figure 3: Mean frequency of accepting food by different types of non-helpers and non role-playing actors. Experiment 3 involved the monkeys accepting food from an explicit non-helper and an occupied non-helper. Experiment 4 involved accepting food from the actors not playing roles. In both experiments, N=7. *P<0.05. NS, not significant. Error bars represent mean±s.e.m. Full size image

Controlling for head-turn

In a fourth experiment, we examined whether the rejecting non-helper’s gesture of turning away, rather than refusal to help per se, might have led to the expressed discrimination. The materials were as in Experiments 2 and 3. Six sessions were run, each consisting of 2 pairs of success demonstrations and 12 experimental trials as follows: after 5 s of manipulation, A stopped manipulating. While discreetly monitoring A’s activity, B also stopped manipulating and turned away from A for 1–2 s. This movement replicated that shown by B in previous experiments, but now it was not a refusal to help (Fig. 1d). As soon as B turned to face forward again, both actors resumed manipulating their containers for 5 s.

In these sessions, the mean frequency of accepting offers from A and B was exactly 36 in each case (A range: 33–40) (B range: 32–39), respectively (Student’s t-test, t=0.00, P=1.00 (Fig. 3); indeed, no individual monkey showed a significant bias for either actor. This result rules out any general learning effect across experiments, and the possibility that the simple act of turning away induced a bias in earlier experiments. We conclude that the monkeys’ discriminatory responses required an explicit refusal to help by the non-helper.