From Slavin (2009): "Expectancy theory is a theory of motivation based on the belief that people's efforts to achieve depend on their expectations of reward" (p. 304).

Atkinson (1962) developed an expectancy model, sometimes called the "expectancy-valence model", which implies "that people's motivation to achieve something depends on the product of their estimation of their chance of success and the value they place on success" (p. 304).

"Atkinson added an important aspect to expectancy theory in pointing out that under certain circumstances an overly high probability of success can be detrimental to motivation" (p. 304). Because "success in an easy task is not as valued as it is in a difficult task" (p. 305). "Confirming Atkinson's theory, more recent research has shown that a person's motivation increases as task difficulty increases up to a point at which the person decides that success is very unlikely or that the goal isn't worth the effort" (p. 305).