Well, I think the research per the article tried to compensate for this by letting the children choose what would be the reward before hand, so theoretically, t

Well, I think the research per the article tried to compensate for this by letting the children choose what would be the reward before hand, so theoretically, they're playing for something they actually want, not just party favors. I'm not clever enough to know exactly how this could introduce a bias into this particular experiment (that is, how such an operation would affect the response of depressed children), but I suppose it could. Frankly speaking I feel like the general premise is probably true, though the how on it is still unclear. I know extremely low self-esteeem and disbelief in adulation/rewards was pretty common for me when I was much deeper into depression. Even now I tend to be pretty dismissive of acknowledgements and rewards.