Proponents of behaviorism argued that language may be learned through a form of operant conditioning. In B. F. Skinner's Verbal Behaviour (1957), he suggested that the successful use of a sign, such as a word or lexical unit, given a certain stimulus, reinforces its "momentary" or contextual probability. Since operant conditioning is contingent on reinforcement by rewards, a child would learn that a specific combination of sounds stands for a specific thing through repeated successful associations made between the two. A "successful" use of a sign would be one in which the child is understood (for example, a child saying "up" when he or she wants to be picked up) and rewarded with the desired response from another person, thereby reinforcing the child's understanding of the meaning of that word and making it more likely that he or she will use that word in a similar situation in the future. Some empiricist theories of language acquisition include the statistical learning theory. Charles F. Hockett of language acquisition, relational frame theory, functionalist linguistics, social interactionist theory, and usage-based language acquisition.