It kinda makes sense. There's a personality trait that measures "working hard" called conscientiousness. It's stable and hard to impossible to change among ad

It kinda makes sense. There's a personality trait that measures "working hard" called conscientiousness. It's stable and hard to impossible to change among adults. But it's possible that's it's learned in childhood. So a smarter child will need to exert less/little work in getting good enough grades. So he may develop less of that personality trait, and become less hardworking. And that shows. BTW, choosing a more interesting job instead of a higher paying one - like many mentioned here - is directly related to that trait: it's about choosing play over work, basically.