Coerced Cooperation in Insects, Vertebrates, and Humans
Excerpt from Ratnieks & Wenseleers (2008):
Coercion and enforced cooperation are not restricted to insect societies but also occur in vertebrate [12] and [50] including human [51] and [52] societies and multispecies mutualisms [53], [54] and [55] (Figure 1). In contrast to insect societies, however, in which coercion typically promotes inequality in a group composed of related individuals [5], in human society and in mutualisms, coercion often promotes fairness and reduces exploitation or cheating in groups with low or zero relatedness [51] and [54]. In other words, coercion is used to promote cooperation rather than altruism. Another difference is that, whereas coercion in insect societies usually has indirect inclusive-fitness benefits, in social vertebrates and mutualisms it usually provides direct benefits [53] and [54]. For example, in mutualisms, individuals can terminate interactions with uncooperative partners, thereby focusing interactions onto more cooperative partners [53], [54] and [56]. Similarly, in social vertebrates, punishment can have direct benefits to the punishing individual, such as by inducing the victim to behave more cooperatively in the future [50]. In such cases, punishment and stable social behaviour can evolve even at zero relatedness.
In vertebrate societies, enforcement can take varied forms: in cooperatively breeding cichlid fish, subordinates that do not help are evicted and thereby prevented from inheriting the natal territory [57]; monkeys punish individuals who do not share food [58], police disruptive individuals in the group [59] and enforce fairness by not cooperating with others perceived as unfair [60]; naked mole rat queens shove pups to stimulate them to work [61]. A particularly ruthless example of enforcement is found in meerkats, in which the dominant female suppresses the reproduction of subordinates [62]. If a subordinate female becomes pregnant when the dominant female is also pregnant, the dominant female subjects the subordinate female to aggressive attacks and temporarily evicts her from the group. This behaviour usually leads to abortion of the subordinate’s brood [62].
Enforcement can also explain cooperation between species [53] and [54]. For example, the cleaner fish Labroides dimidiatus eats ectoparasites on its client reef fish but could also cheat by eating client tissue or mucus. Clients use three different mechanisms to enforce cooperation: (i) they avoid cleaners that have been observed cheating (partner choice); (ii) they switch to other cleaners (partner switching); and (iii) they aggressively chase uncooperative cleaners (punishment) [63] and [64]. After such punishment, cleaner fish act more cooperatively and are less likely to feed on mucus [63] and [64]. Similar examples occur in the mutualisms between yucca plants and their yucca moth pollinators [53] and [54] and between legume plants and nitrogen-fixing bacteria, in which plants sanction bacteria that fail to fix nitrogen by cutting resources to root nodules containing such bacteria [55].
Finally, recent studies in experimental economics show that humans have a strong tendency to punish individuals who do not reciprocate or act fairly [51] and [52]. For example, in the ultimatum game a resource is divided between two individuals. The first player decides how to divide the resource. The second player can then choose whether to accept his share or to reject it, in which case both players receive nothing. The rational strategy is for the first to offer a mimimal share and for the second player to accept whatever is offered. However, first players often offer an equal share, and second players frequently reject less [51]. The apparently altruistic behaviour of the first player and the spiteful behaviour of the second player, if offered a small share, makes no sense in the context of a single interaction with an anonymous partner. However, it could be explained on the basis of the presence of ’relatedness’ owing to nongenetic, cultural causes (cultural relatedness) caused by the inheritance of identical cultural variants from common cultural ancestors [65]. Alternatively, and more likely, the results suggest that, in humans, single interactions with anonymous partners are uncommon and have not shaped our general social responses [66].
Other games also show that humans are willing to expend resources to punish selfish individuals [51]. Here, the paradox is to explain why punishment occurs given that it is costly to punish selfish individuals [51] and [65]. The benefits probably come from the repeated nature of human interactions. Punishment can be costly in the short term but beneficial in the long term [52]. Overall it is clear, however, that in humans, punishment forms a powerful mechanism for promoting cooperation. In an experimental setting, the presence of punishment increased levels of cooperation [67]. In a comparative study of 15 small-scale societies, levels of cooperation were correlated with the degree to which uncooperative individuals were punished [68]. This finding parallels results from insect societies showing that more stringent policing promotes greater worker altruism (Figure 3).