"We don't gain a greater capacity for self-control, though, do we? It appears to me that what happens is that at some point, a thing that once required an exercise of self-control to resist no longer does."
I find that I do gain ability in self-control. As you mention, the same task becomes effortless, but also, new ones become easier to confront. Practicing self-control has, for me, led to it becoming easier in general.
I'm wondering if most of that is attributable to conscious/rational factors, though, and not the ego depletion phenomenon examined in the study. How much of it is learning to recognize patterns that presage tests of will and dealing with them by some optimized heuristic that doesn't tax willpower much at all? A sense of temptation. An awareness of our position on a slippery slope. Experience.
The test would be to see how you perform with novel depleting tasks (like writing without certain letters) where no such learned heuristics might apply.
That's what I mean by a meta-adaptation. It just makes more sense to me that you can learn to manage the demands on your self-control than to boost the total supply of it. It's more like time, to me.
It may of course be some mixture of both, but I always find it interesting when people's mental models of seemingly universal experience differ.
I find that I do gain ability in self-control. As you mention, the same task becomes effortless, but also, new ones become easier to confront. Practicing self-control has, for me, led to it becoming easier in general.