In Slavin (2009): "Motivation: The influence of needs and desires on the intensity and direction of behavior" (p. 297).

"The concept of motivation is closely tied to the principle that behaviors that have been reinforced in the past are more likely to be repeated than are behaviors that have not been reinforced or that have been punished" (p. 297).

Problem for behaviorists: "we can't be sure what will be a reinforcer and what will not because the reinforcing value of most potential reinforcers is largely determined by personal or situational factors" (p. 298).