Another behaviorist explanation of attachment is a model based on Skinner's principle of operant conditioning. Skinner rejected Hull's idea that primary drive reduction is the only way to get children to learn. According to Skinner, a child's behavior can be increased by following the behavior with a wide variety of reinforcers besides food, such as praise or a new toy. Certain behaviors can also be decreased through punishment, such as withdrawal of privileges, scolding, or taking away a new toy. According to the theory of operant conditioning, which emphasizes reciprocal responsiveness between the caregiver and baby, babies are reinforced to smile and look at their mothers, because in return they receive social interaction. The greater the number of infant behaviors that have been reinforced consistently, the stronger the attachment relation is (Gewirtz, 1969).