Violation of rationality in shoaling decision making in zebrafish (Danio rerio)

Several organisms, from slime molds to humans, are known to violate normative principles of economic rationality in decision making. In animals, the neural circuitry underlying behaviors that violate or conform to normative rationality is relatively poorly understood. We investigated whether zebrafish, a model organism with a strong suite of functional neuroimaging and genetic manipulation tools, showed rational behavior with respect to the principle of the Independence of Irrelevant Alternatives (IIA). We examined IIA in social decision making by measuring revealed preferences from spatial trajectories of freely swimming individual zebrafish in an arena where they could view and perform shoaling behavior near conspecific zebrafish in adjacent display tanks. IIA was tested in terms of the invariance of shoaling choices between binary and ternary sets of various display fish group sizes. We provide the first report of evidence for violation of IIA in zebrafish, both in terms of the constant ratio rule and in terms of the principle of regularity. This also is a rare example of violation of rationality in a single attribute dimension, and it opens up a range of possibilities to study the neural basis of context dependent decision making.