Hard to Break: Why Our Brains Make Habits Stick

by Russell A. Poldrack

In the 1970s the psychologists Robert Rescorla and Allan Wagner were particularly interested in understanding a phenomenon that occurs during learning known as blocking. Previous theories had proposed that animals learn which events go with which others in the world by simply registering their co-occurrence. This would suggest that anytime a reward occurs in association with an action, the animal should learn to perform that action more frequently. However, in 1968 the psychologist Leon Kamin showed that the association between a stimulus and a reward could be blocked if the reward was already associated with another stimulus. For example, in grade school we might come to associate the sound of a particular bell with lunchtime, such that the occurrence of the bell would make us start to salivate. What Kamin showed was that if another stimulus was later added—for example, a flashing light along with the bell—the relationship between that second stimulus and the outcome was blocked, such that if the light occurred later on its own, it would not elicit the same response as the bell.