A group of workers was discussing the previous day's snowstorm when very few came into work. One commented "I knew it would be almost impossible to get there from my home and, if I did, no one here would thank me for it anyway, so why bother?" Another said, "Well, I can walk here so, although it was no fun, I knew I could make it and if I got that report done I could take a long weekend." The theory which best explains their differing levels of motivation in coming to work is