In this paper, I briefly summarize the research history on the (non-) emergence of symmetry in the chimpanzees of the Primate Research Institute, Kyoto University. The chimpanzee Ai, who had learned the visual artificial symbols (lexigrams), did not exhibited the spontaneous emergence of symmetry between name and objects, but showed gradual improvement during the repeated exposure of training-testing cycles. This repeated exposure effect on the emergence of symmetry was also observed in the other individual who were not “language-trained”. This chimpanzee also exhibited the control by exclusion, which is considered as another type of illogical biases in humans. Chimpanzees also showed significant effect of differential outcome and differential responding on symmetry. Conclusion so far is that chimpanzees show no emergence of symmetry immediately after the training of conditional discriminations. Repeated exposure of training-testing contexts, however, has critical effect on the emergence of symmetry. For the future direction, it is suggested to promote studying the symmetry bias during the causal reasoning not using traditional conditional discrimination paradigm from the comparative-cognitive perspective.
