Duke University Professor Harris Cooper, perhaps the best known homework researcher and the author of The Battle Over Homework, explains his thinking on homework in Homework Within Reason Makes Grade.
The homework question is best answered by comparing students who are assigned homework with students assigned no homework but who are similar in other ways. The results of such studies suggest that homework can improve students’ scores on the class tests that come at the end of a topic. Students assigned homework in second grade did better on math, third- and fourth-graders did better on English skills and vocabulary, fifth-graders on social studies, 9th- through 12th-graders on American history and 12th-graders on Shakespeare.
The salient point is that homework improves students’ scores only on the class tests created by the teachers. In other words, teachers create homework so that students will do better on unit tests, and students who do that homework do better on those tests. But, Cooper makes no claim that homework has any broader benefits such as a better understanding of the material in the long run, a better educated student body, or passionate or creative or self-directed learners.
Read his views in The Atlanta Journal-Constitution.