The goal difficulty effect--the assertion that difficult specific goals lead to higher performance than vague or easy goals--has received a great deal of empirical support. Little research, however, has been directed toward discovering why this effect is obtained. This study reports an experiment designed to examine the extent to which the Valence-Instrumentality-Expectancy (VIE) and Naylor-Pritchard-Ilgen (NPI) theories of motivation can explain processes underlying the goal difficulty effect. Questions designed to elicit motivational force, as defined by NPI and VIE theories, were answered by 121 subjects who participated in a four (goal difficulty level) by two (experimental session) factorial experiment. Goal difficulty is not related to performance in this study. Although subjects in the hard goal condition achieved the highest performance in Session 2, subjects given easy goals increased their performance on the puzzles over experimental session slightly more than subjects given hard goals. Very hard goals failed to motivate high increases in performance and subjects given moderately hard goals exhibited a mean decrease in performance. NPI motivational force accurately predicted the direction of performance change across sessions--positive force values are associated only with increases in performance whereas negative force values are associated only with performance decrements. VIE motivational force is not significantly related to performance. Though much work is needed to standardize and streamline the collection of NPI motivational components, NPI is likely to contribute to the understanding of the goal difficulty effect and the development of a goal process model.