The law of effect is a psychology principle advanced by Edward Thorndike in 1898 on the matter of behavioral conditioning (not then formulated as such) which states that "responses that produce a satisfying effect in a particular situation become more likely to occur again in that situation, and responses that produce a discomforting effect become less likely to occur again in that situation."

This notion is very similar to that of the evolutionary theory, if a certain character trait provides an advantage for reproduction then that trait will persist. The terms "satisfying" and "dissatisfying" appearing in the definition of the law of effect were eventually replaced by the terms "reinforcing" and "punishing," when operant conditioning became known. "Satisfying" and "dissatisfying" conditions are determined behaviorally, and they cannot be accurately predicted, because each animal has a different idea of these two terms than another animal. The new terms, "reinforcing" and "punishing" are used differently in psychology than they are colloquially. Something that reinforces a behavior makes it more likely that that behavior will occur again, and something that punishes a behavior makes it less likely that behavior will occur again.

Thorndike's law of effect refutes the ideas George Romanes' book Animal Intelligence, stating that anecdotal evidence is weak and is typically not useful. The book stated that animals, like humans, think things through when dealing with a new environment or situation. Instead, Thorndike hypothesized that animals, to understand their physical environment, must physically interact with it using trial and error, until a successful result is obtained. This is illustrated in his cat experiment, in which a cat is placed in a shuttlebox and eventually learns, by interacting with the environment of the box, how to escape.