Logical and illogical behavior in animals

Not only humans but also nonhuman animals show logical and illogical behavior. There are two typical areas of evidence for this: transitive inference and the formation of equivalence relations. Transitive inference has been observed in a broad range of subjects, suggesting that they share the evolutionary reasons that support transitive inference. On the other hand, while the formation of equivalence relations is typically found in human subjects, it appears to occur rarely in nonhuman subjects. In the case of pigeons, even if they are trained explicitly in equivalence, they do not subsequently exhibit equivalence relations. The performance of the sea lion represents the most reliable evidence of equivalence relations in nonhumans, suggesting that species-specific ecological properties support this particular cognition. These evidences suggest that in searching for the origin of logic, we should examine not only ontogenetic history, but also the phylogenetic history of the subjects.