[XaulZan11]

Giving a stipend (an extrinsic motivation) may be a bad idea for those mentors who are instrinsically motivated and do it because they enjoy it and like the feeling of helping out. After a while they may think they are doing it for the stipend and not for their previous reasons and not enjoy it as much and be less likely to help; also known as cognitive dissonance.

I'm almost positive this subject has been already discussed.