I never offer extra credit in my classes any more, but a lot of my colleagues do. And we've all noticed that students will sell their grandmothers for tiny amounts of extra credit. Amounts that almost never impact on their final letter grade.
All in the Mind has an interview this week with author Dan Ariely, who wrote Predictably Irrational. He talks at some length about the persuasive power of “Free!” People will make bad decisions when something is free. A $1,000 discount on a car may not means as much as a few free oil changes – even though that may only total a hundred dollars in value or so.
I suspect that “extra” does much the same thing for students. Somehow, something “extra” has more weight than regular points, even if their tiny.
I’ll let you students in on a secret. You will never, ever be able to talk a professor into changing your final grade is he or she offers extra credit. If you were close to the dividing line and you didn’t take the extra credit, the professor will say, “Your fate was in your hands – you could have got the better grade if only you'd taken the extra credit.” If you did take the extra credit, the professor will say, “Look, you didn't make the grade even with all that extra credit you took.”
Heads I win, tails you lose.