Human societies function by virtue of cooperation. Peer punishment seems an important regulatory factor for the functioning of human societies, as various studies showed that the opportunity to punish peers can deter free-riding and sustain high levels of cooperation. However, here we show experimentally that peer punishment fails to deter free-riding in diversified groups with pluriform populations. This insight is important because – although a common assumption in the peer punishment literature is that free-riding takes place in demarcated groups with rather uniform populations – diversity is an inherent feature of human societies. Humans have various social ties and belong to a wide range of collectives within their local community and society. In such diversified groups with pluriform populations, peer punishment opportunities allow and may invite individuals to apply double standards of cooperation and be psychologically reactant to punishments by members with whom they are less affiliated. We argue and demonstrate that this undermines the effectiveness of peer punishment in promoting cooperation.

In our experiment, a total of 144 students participated in an iterated public goods game with real monetary stakes. In groups of four members, participants played 20 rounds of the public goods game with and 20 rounds without peer punishment opportunities. There were two treatment conditions: A uniform condition and a pluriform condition. In the uniform condition, the group consisted of four players all sharing the same real social affiliation. In the pluriform condition, the group consisted of two players sharing a real social affiliation and two players sharing another real social affiliation. In both conditions, participants were not informed about the identity of the others in their group, only about each other’s social affiliation.

Our results show that more antisocial punishment takes place in diversified groups than in demarcated groups, as well as that pluriform populations invite individuals to apply double cooperation standards and be psychological reactant to ‘outgroup’ punishment. More importantly, we show that, although peer punishment opportunities are effective in deterring free-riding in demarcated groups, the opportunity to punish peers does not deter free-riding in diversified groups and is ineffective in stabilizing cooperation. Our results question the notion that peer punishment is an important regulatory factor for the functioning of human societies, as peer punishment opportunities only seem effective under very specific and rather artificial conditions.