achievement data were analyzed, U.S. students scored below the international mean in mathematics and just above the mean in science. U.S. students slipped even farther at the twelfth grade, where, in science and mathematics general achievement, they outperformed only one other country's students.

There may be a number of reasons that the science assessment performance of the fourth-grade students in the United States was relatively high compared to those of students in other countries, including the fact many other countries do not include formal science instruction in the early grades; however, education analysts found it most startling that achievement appears to decline over time in the United States relative to other countries. Whatever the cause, it appears to be cumulative, contributing to the decrease in U.S. scores as schooling progresses through the grade levels.

One possible cause for this decrease is the nature of the curricula that many U.S. students experience over their 13 years in the schools (Schmidt et al., 1998). Most mathematics and science curricula in the United States lack coherence and focus, and that has caused some researchers associated with TIMSS to characterize the typical curriculum in the United States as a "mile wide and an inch deep" (Schmidt et al., 1997). When Schmidt compared the number of topics in U.S. textbooks and curriculum guides with those of other countries, he found that textbooks in the United States contained considerably more. As an example, Figure 1 displays data obtained in the analysis of science textbooks for three age levels. In addition, in the United States, fourth-grade mathematics and science textbooks contain an average of 530 and 397 pages, respectively, whereas in Japan, mathematics and science text