I am not certain it has to do with straight intelligence. My sibling is intelligent and he was very easy to motivate. My partner is also quite intelligent and loves apps that give her stars. They both were really into grades and such.
I think I am probably just gullible in other ways. I did read very early and growing up it was suggested about the adult thing from time to time. I have always hated feeling manipulated. And a grade or a star was not going to make me do something I was not already inclined to do.