I commented the other day that a study was misleading because it didn't take into account motivation. This paper documents that effect. "Separate analyses of ability and motivation groups are conducted," write the authors. "We find that motivation and ability explain variation in both homework and exam scores." The literature explains the link: "motivation influences performance through its effect on selfregulatory behaviors and study strategies... Self-regulated students engage in increased effort by completing supplemental problems, managing time effectively, and seeking help in solving problems." 31 page PDF.