The learning theory of digit sucking was proposed by
The learning theory: Davidson (1967)
This theory advocates that non-nutritive sucking stems from an adaptive response.
The infant associates sucking with such pleasurable feelings as hunger. These events are recalled by sucking the suitable objects available, mainly thumb or finger.
Davidson, 1967
Sears and Wise, 1982
Johnson and Larson, 1993
Sigmond Freud, 1905