Larry Ferlazzo has blogged on a study, which found that people were unlikely to donate to a charity for a second time if the said charity threw in a sweetener for their first donation. The post can be found here.
I think that this behavior is definitely reflected in the classroom. Once last year I gave away a tote-bag as a prize, and during my CELTA, I tested a little bit of bribery with a chocolate fish. Both of these lessons worked just fine, and the students were very motivated. Although the tote-bag lesson was a one-off, I had the interesting experience of sitting in on some lessons with the class that had been competing for the chocolate fish. They were disappointed with other teachers that weren’t offering prizes, and were even more disappointed when I arrived the next day to teach them empty-handed.
So, in my experience, bringing one chocolate fish to class means setting a precedent, and making a commitment to bringing one every lesson from then on. It would be interesting to hear if any other teachers have had any different experiences.