There's good evidence that socioeconomic status correlates positively with Self-Control. There is also good evidence that people with high socioeconomic status live in a more stable environment during childhood. The signals of a stable environment correlating with Self-Control is his speculation as far as I'm aware, but in light of the data it seems plausible.

I agree they would function better in a crisis, but a crisis is a situation where fast response matters more than self-control. In a crisis you will take actions that are probably wrong during stable periods. I would go on to say, as my own speculation, that hardship - as else being equal - make people worse.