The concept was first proposed by Clark Leonard Hull. It basically refers to the role of primary and secondary reinforcement in shaping behavior. Primary reinforcers are also called unlearned reinforcers like food water etc. they reduce a bodily drive. Whereas secondary reinforcers are learned reinforcers, such as praise, which only acquire reinforcing properties after being associated with primary reinforcers, for e.g. a child's response is initially reinforced with candy and praising the child, with the association of candy and praise over time, praise acquires a reinforcement quality of its own, hence it is termed as a secondary reinforcer.