Research has 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 can be considered linear with improvements in performance as arousal increases. For complex, unfamiliar, or difficult tasks, the relationship between arousal and performance becomes inverse, with declines in performance as arousal increases.
The effect of task difficulty led to the hypothesis that the Yerkes-Dodson Law can be decomposed into two distinct factors. The upward part of the converted 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.
There has been research indicating that the correlation suggested by Yerkes and Dodson exists (such as that of Broadhurst, 1959; Duffy, 1962; Anderson, 1988), but a cause of the correlation has not yet successfully been established (Anderson, Revelle, & Lynch, 1989).
The Yerkes-Dodson law predicts that overlearning can improve performance in states of high arousal.