Only math homework, apparently, lends itself to deliberate practice, with the kind of feedback that drives improvement. Writing an English paper and getting a B doesn’t seem to teach a student how to write or analyze literature better for next time.

Of course it doesn’t. Feedback systems are the issue. Homework functions when you’ve got a parent sitting with you telling you when you’re right or wrong, or else you’re checking your answers in the back of the book.

If (a) you’re not checking your answers, or (b) someone else isn’t checking your answers, or (c) you’re just copying answers from the back of the book or (d) you’re not in any danger of getting anything wrong, then homework is the equivalent of letting someone get better at baseball pitching by throwing the ball with no plate to check against. No feedback, no value.

I bet you could rerun the experiment, and find that the kids who got better were those who were (a) super-conscientious, or (b) whose parents helped, the kids who didn’t get better were those who (a) were not super-conscientious (b) parents didn’t help (couldn’t or didn’t care).

Would explain Harlem Children’s Zone awfully well.