This is about motivation, but I want to look specifically here at the over justification hypothesis. Lots of work around this, but a nice short read on it here.
The link is a short explanation of work by Lepper and Greene from 1975. What it tells us is that for children who already enjoy a specific activity an expected reward is actually a negative on their motivation. Further than that, it also shows that there is no statistically significant difference between a surprise reward and none at all.
One of the little projects I'm working on at the moment is related to how we could use Warcraft to teach particular skills. The hypothesis above causes some concerns here, and an idea that can be more generally applied to using other games in education.
In our early years we learn through play, and it is only once we arrive at school that learning appears to turn into work. Something we have to do in order to achieve a specific goal rather than purely for the sake of learning itself.
In a general sense we see this in secondary with the games branded up as 'educational'. To make a sweeping generalisation about those we usually see that they aren't hugely popular with students, just another task set in the classroom. Yes, they are often more popular than achieving the same outcome using pen and paper but I wouldn't be going too far out on a limb to suggest this is more about the novelty of the activity rather than the specific game mechanics at play.
The challenge for me with Warcraft, and to educators using other games for an educational purpose is to avoid this over justification. I have a group of students who already enjoy MMORPGs, they spend hours of their own time already doing it. If it becomes a school task with associated reward for completing certain things is it too much?
Like the students who enjoyed drawing in the example- once it becomes a task they must complete in order to achieve X certificate we are in danger of damaging the intrinsic motivation (and any learning that was going along with it) they had to participate in the activity to start with. Image sources- MMOsymposium.com, perspicuity.com