In a classic experiment in the 1930s, behaviorist Clark Hull observed that rats on a straight runway ran faster as they moved closer to the food box. Knowing the reward was almost at hand presumably motivated the rats to work harder, a phenomenon that Hull called the "goal-gradient" hypothesis.
[This article has been reproduced with permission from Capital Ideas, the research journal of University of Chicago's Booth School of Business http://www.chicagobooth.edu/capideas/ ]