Since the release of results from the Third International Mathematics and Science Study (TIMSS) in 1996, scholars have recognized that the central importance of TIMSS lies in its contribution to a better understanding of factors that are responsible for cross-national differences in average student achievement. Among many such factors may be differences in student ability and motivation to perform the task of completing the TIMSS achievement tests in math and science. In fact, national differences in math and science achievement scores may be determined more by differences in student test-taking ability and motivation than by . differences in student knowledge of math and science content. This possibility is explored in the research reported here.