Simon Effect


Many people find driving a trailer in reverse difficult, and most driving instructors will tell you the concept that is so hard for many people to learn, is that in order to make a trailer go left in reverse you must turn the steering wheel to the right (and vice versa).... This simply stumps us. So in a day and age when most of us can pick up a new cell phone and figure out how to use it in a manner of minutes, why does it take people such a long time to learn how to a turn a trailer in reverse? The delay is probably related to a psychological phenomenon known as 'The Simon Effect' which is the tendency to associate responses on the right with movements on the right, and responses on the left with movements on the left. When driving a trailer forwards this works perfectly well, but in reverse we have to unlearn this pattern and do the opposite of what our instincts tell us to do. In the following experiment demonstrating The Simon Effect you will see the relationship between spatial location and response selection.


The results from a typical Simon paradigm experiment demonstrate what is known as 'The Simon Effect' first described in Simon (1969); participants respond fastest when the stimulus is located in the same spatial location as the required response. So when the stimulus is on the left and a response with the left hand is required participants are faster than when the stimulus is on the left and a response with the right hand is required. Participants tend to be equally accurate at responding to stimuli of each colour, and with each hand, but tend to make more mistakes in the 'incongruent' than the 'congruent' condition.

Results from experiments using the Simon paradigm are explained using an information processing model that suggests three stages of processing; stimulus identification, response selection, and response execution. It is argued that the Simon paradigm manipulates the duration of the response selection stage of processing such that processing for incongruent trials is slower than processing for congruent trials. This claim is supported by several aspects of the data. The fact that error rates are consistently similar for different stimulus types suggests that the Simon Effect is not the result of a delay in perceptual processing on incongruent trials. The fact that response times are consistently similar for the right and left hand suggest that the Simon Effect is not the result of a delay in response execution on incongruent trials; that leaves the response selection stage as the source of the Simon Effect. The tendency for individuals to pair responses to the visual field that the stimulus was presented on has important implications for engineering; asking a machine operator to reach to their left to respond to something on the right, or vice versa, could lead to errors and accidents. The Simon Effect has also impacted the design of virtual training scenarios for driving and piloting; students are deliberately presented with stimuli in their path and trained to turn AWAY from the stimuli to avoid collision. This training is essential since, as the results of Simon paradigm experiments have shown us, the natural tendency is to turn toward a stimulus instead.


Simon, J. R. (1969). Reactions towards the source of stimulation. Journal of experimental psychology, 81, 174-176.