Contingengy-specifying stimuli (CSSs) can function differently than discriminative stimuli. Rather than evoking behavior due to a history of discrimination training, they alter the function of other stimuli and, therefore, the behavioral relations involving those stimuli. CSSs can alter the evocative function of discriminative stimuli, establishing operations, and conditional stimuli, as well as the efficacy of reinforcing and punishing stimuli and of stimuli that can function in second-order respondent conditioning. The concept of function-altering CSSs has implications for such areas of interest as stimulus equivalence, the terminology involving "rules" and "rule-governed behavior," and the way in which behavior analysts view the effects of such basic processes as reinforcement and punishment.

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Ever since Skinner's first discussion of rule-governed behavior, behavior analysts have continued to define rules, either explicitly or implicitly, as verbal discriminative stimuli. Consequently, it is not difficult to find, in the literature on rule-governed behavior, references to stimulus control, antecedent control, or to rules occasioning behavior. However, some verbal stimuli have effects on behavior that are not easily described as discriminative. Such stimuli don't evoke behavior as discriminative stimuli, but rather alter the functions of other stimuli in a manner analogous to operant and respondent conditioning. Hence, this type of control has been called function altering. Any known stimulus function (e.g., evocative, or [conditioned] reinforcing or punishing functions) can apparently be altered by such function-altering stimuli. Describing these stimuli as discriminative stimuli obscures their possible function-altering effects and consequently may retard inquiry into them. This paper encourages behavior analysts to begin separating the discriminative and function-altering effects of verbal stimuli and suggests that by doing so, behavior analysts may better understand what may be most unique about these stimuli. Results from several experiments, especially those in which children served as subjects, are analyzed. Finally, some speculations are offered concerning the genesis of function-altering stimuli.