The law of effect states that behaviors followed by favorable consequences become more likely, and behaviors followed by unfavorable consequences become less likely. Edward Thorndike formulated the principle on the basis of his puzzle-box experiments with cats.

A cat placed in a box learned, through trial and error, to perform the action that opened the door and led to food. Over successive trials the ineffective behaviors dropped out and the successful response occurred more quickly. Thorndike described the process as the stamping in of stimulus-response connections by satisfying outcomes and the stamping out of connections by annoying outcomes.

The law of effect became a foundation of operant conditioning. B.F. Skinner refined the concepts of reinforcement and punishment while retaining the core idea that consequences select behavior. The principle remains central to behavior modification, education, and animal training. It also explains why habits form and why they can be difficult to break when the consequences that maintain them are intermittent or delayed.

Actions that pay off tend to be repeated. That simple regularity is the law of effect.