Edward Thorndike

Thorndike placed hungry cats in boxes that opened when they performed an action such as pulling a loop or pressing a lever. Their escape times gradually decreased, which he interpreted as trial-and-error learning rather than sudden insight.

The law of effect says that responses followed by satisfying consequences become more likely to recur, while responses followed by discomfort become less likely. Thorndike formulated this principle through animal-learning research conducted around the turn of the 20th century.

Pavlov studied how dogs associated one stimulus with another, as when a signal predicted food. Thorndike focused on how consequences strengthen or weaken an animal's voluntary actions, anticipating what B. F. Skinner later called operant conditioning.

Thorndike applied experimental measurement to learning, testing, and individual differences in schools. His books Educational Psychology and The Psychology of Learning helped establish education as a field that could be studied quantitatively.