I'm wondering just what this really means. There is research showing that expertise requires an intense amount of "effortful study" (see The Expert Mind in Scientific American), and according to John Anderson (of ACT-R Theory), effective time on task is the most important element in learning something. Without any research in hand, I'm pretty certain that my table tennis skills (long ago) were directly related to the amount of time I practiced. So, I'm having a hard time reconciling the solid research on learning and expertise with the notion that homework, which would seem to be a sort of more time-on-task activity, doesn't aid achievement. The only thing that comes to my mind is that homework is busy work, is perhaps not related to learning that counts, or is not "effective time on task." Another possibility is that the homework, being spread over a variety of subjects, means that the amount of extra time on a particular subject is not significant enough to aid achievement in that subject or overall. With just these few possibilities, it seems that we need to know more to understand the effect, or non-effect, of homework on school achievement.