1. Biro, D., & Matsuzawa, T. (2001). Chimpanzee numerical competence: Cardinal and ordinal skills. In T. Matsuzawa (Ed.), Primate origins of human cognition and behavior (pp. 199–225). Tokyo: Springer.
2. Boysen, S. T. (1993). Counting in chimpanzees: Nonhuman principles and emergent properties of number. In S. T. Boysen & E. J. Capaldi (Eds.), The development of numerical competence: Animal and human models (pp. 39–59). Hillsdale: Erlbaum.
3. Boysen, S. T., & Berntson, G. G. (1989). Numerical competence in a chimpanzee (Pan troglodytes). Journal of Comparative Psychology, 103, 23–31.
4. Boysen, S. T., Berntson, G. G., Shreyer, T. A., & Quigley, K. S. (1993). Processing of ordinality and transitivity by chimpanzees (Pan troglodytes). Journal of Comparative Psychology, 107, 208–215.
5. Carey, S. (2009). The origin of concepts. New York: Oxford University Press.