People who do small goods are less inclined to do subsequent goods.

Oh really? What about the FITD effect?

On the other hand, see Doing your good deed for the day (presumably what wedrifid was referring to). Figuring out which effect dominates under which circumstances seems like an important open problem. (My first, simple, guess would be that altruism is depleted in the short term and strengthened in the long term by use, like willpower or muscular strength.)