p28.
R.M. Yerkes and J.D. Dodson, “The Relation of Strength of Stimulus to Rapidity of Habit-Formation”, Journal of Comparative Neurology and Psychology, vol 18, 1908, pp 459–482, available at http://psychclassics.yorku.ca/Yerkes/Law/. See also summary in Jerome S. Bruner, Alison Jolly and Kathy Sylva, eds. (1976), Play: Its Role in Development and Evolution, p 15; and Martyn Long (2000), The Psychology of Education.