References
- Havighurst, Robert J. Developmental Tasks and Education. New York: McKay, 1953.
- Piaget, Jean. The Child and Reality. New York: Grossman, 1973.
- Piaget, Jean. Piaget's theory. In P.H. Mussen, ed., Carmichael's Manual of Child Psychology, Vol. 1. 3rd. ed. New York: Wiley, 1970, pp. 703–732.