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The Yerkes–Dodson law is an empirical relationship between pressure and performance, originally developed by psychologists Robert M. Yerkes and John Dillingham Dodson in 1908. The law dictates that performance increases with physiological or mental arousal, but only up to a point. When levels of arousal become too high, performance decreases. The process is often illustrated graphically as a bell-shaped curve which increases and then decreases with higher levels of arousal. The original paper (a study of Japanese dancing mice) was only referenced ten times over the next half century, yet in four of the citing articles, these findings were described as a psychological "law". Researchers have found that different tasks require different levels of arousal for optimal performance. For example, difficult or intellectually demanding tasks may require a lower level of arousal (to facilitate concentration), whereas tasks demanding stamina or persistence may be performed better with higher levels of arousal (to increase motivation). Because of task differences, the shape of the curve can be highly variable. For simple or well-learned tasks, the relationship is monotonic, and performance improves as arousal increases. For complex, unfamiliar, or difficult tasks, the relationship between arousal and performance reverses after a point, and performance thereafter declines as arousal increases. The effect of task difficulty led to the hypothesis that the Yerkes–Dodson Law can be decomposed into two distinct factors as in a bathtub curve. The upward part of the inverted U can be thought of as the energizing effect of arousal. The downward part is caused by negative effects of arousal (or stress) on cognitive processes like attention (e.g., "tunnel vision"), memory, and problem-solving.