Proactive, that is, spontaneous, prosociality reflects a psychological interest in the welfare of others and has been reported in callitrichid monkeys, capuchin monkeys (Cebus apella), and humans, but not in chimpanzees. One explanation for the co-occurrence of proactive prosociality in these species is that it is linked to shared infant care (cooperative breeding); alternatively, it might merely reflect unusually high social tolerance or be mediated by advanced cognitive abilities. To date, distinguishing between these alternative explanations is difficult, partly because available evidence is restricted to only a handful of species and partly because methodological differences thwart comparisons across studies. Here, we present an experimental paradigm called group service, which allows estimation of both social tolerance and proactive prosociality in group settings. Its simplicity makes it intuitively plausible to subjects and allows testing a broad variety of species, including in zoos. We applied the test to independently breeding Japanese macaques (Macaca fuscata), cooperatively breeding common marmosets (Callithrix jacchus), and capuchin monkeys with an intermediate breeding system. Social tolerance was slightly higher in marmosets than capuchins and much higher in both compared to macaques, but only marmosets provided a service to other group members. Furthermore, we validated the group service paradigm in the common marmosets by comparing their performance to earlier data. Although our results are consistent with the cooperative breeding hypothesis, a comprehensive evaluation requires adding data from additional groups and species, which should be facilitated by the group service approach.