Stimulus–response (S–R) compatibility is the degree to which a person's perception of the world is compatible with the required action. S–R compatibility has been described as the "naturalness" of the association between a stimulus and its response, such as a left-oriented stimulus requiring a response from the left side of the body. A high level of S–R compatibility is typically associated with a shorter reaction time, whereas a low level of S-R compatibility tends to result in a longer reaction time, a phenomenon known as the Simon effect.