Emergence of group rationality from irrational individuals

Evolutionary theory predicts that animal decision makers should be rational, meaning that they consistently choose fitness-maximizing options. Despite this, violations of rationality have been found repeatedly in humans and other animals. The significance of these violations remains controversial, but many explanations point to cognitive limitations that prevent animals from adequately processing the information needed for fully rational choice. Instead, they rely on heuristics that usually work well but yield systematic errors in specific contexts. Although past research on rationality has focused on individuals, many highly integrated groups, such as ant colonies, regularly make consensus choices among food sources, nest sites, or other options. These collective choices emerge from local interactions among many group members, none of whom take on the whole burden of decision making. We hypothesized that groups may evade the irrational consequences of individual limitations by distributing their decision making across many group members. We tested this in the well-studied case of collective nest-site selection by Temnothorax ants. We found that individual ants, but not colonies, strongly violated rationality when presented with a challenging nest-site choice. Specifically, isolated individuals irrationally switched their preference between 2 alternative nest sites based on their experience of an unattractive decoy. Given the same choice, intact colonies maintained consistent preferences regardless of the decoy's presence. Previous studies have stressed how distributed decision making can filter out random errors made by group members. Our results show that collectives can also suppress systematic errors that emerge from the decision heuristics of cognitively limited individuals. Copyright 2011, Oxford University Press.