Considerable evidence has accumulated to show that human operant behaviour differs qualitatively from that of animals, and that adult schedule performance is a function of (i) instructions, (ii) the subject's conceptualisation of
the contingency, and (iii) prior schedule performance. Lowe (1979) has argued that these differences can be accounted for by reference to the human capacity for self-directed speech and has predicted, on the basis of Vygotsky's theory,
that infants who lack speech should behave like animals.
Three studies of children's fixed-interval schedule performance are reported in this thesis. In the first the fixed-interval performance of infants and children was
investigated using a variety of schedule values. In the second the effects of instructions and self-instructions on the fixed-interval responding of children between two and a half and nine years of age was investigated. In the final
experiment, the effects on fixed-interval responding of prior experience of either a fixed-ratio or a differential - reinforcement - of - low - rates schedule was investigated with infants and older children. It was found that, in agreement with Vygotsky's theory, infant subjects behave in all respects like animals, that a transitional stage in which verbal control of operant behaviour is weak exists
approximately between the ages of two and five years, and that older children behave like adults on schedules of reinforcement.