Dan Ariely has written extensively on psychological research that involves paying people for various activity.
Paying immediately removes intrinsic benefit and self-motivation. If before you were (and considered yourself) a great sport for coaching a local kids' baseball league and volunteering at a library over the weekend, once the money enters the equation, you're just another chump trying to pull money out of the system, grinding at it before you can go home and relax.
That's an important point, and your general sketch is certainly correct, though I think you are overstating and overgeneralizing the research a little bit. (That said, I grant that it's certainly possible your understanding is more correct than mine.)
Paying immediately removes intrinsic benefit and self-motivation. If before you were (and considered yourself) a great sport for coaching a local kids' baseball league and volunteering at a library over the weekend, once the money enters the equation, you're just another chump trying to pull money out of the system, grinding at it before you can go home and relax.