Conscientiousness is a finite resource
So picture this: Students come into a lab. It smells amazing—someone has just baked chocolate-chip cookies. On a table in front of them, there are two bowls. One has the fresh-baked cookies. The other has a bunch of radishes. Some of the students are asked to eat some cookies but no radishes. Others are told to eat radishes but no cookies, and while they sit there, nibbling on rabbit food, the researchers leave the room – which is intended to tempt them and is frankly kind of sadistic. But in the study none of the radish-eaters slipped – they showed admirable self-control. And meanwhile, it probably goes without saying that the people gorging on cookies didn’t experience much temptation.
Then, the two groups are asked to do a second, seemingly unrelated task—basically a kind of logic puzzle where they have to trace out a complicated geometric pattern without raising their pencil. Unbeknownst to them, the puzzle can’t be solved. The scientists are curious how long they’ll persist at a difficult task. So the cookie-eaters try again and again, for an average of 19 minutes, before they give up. But the radish-eaters—they only last an average of 8 minutes. What gives?
If you add in to this that people naturally vary in conscientiousness, an act of self-control that is easy for one person can be nearly impossible for another.
h/t John D Cook