Gender gaps in the development of mathematical and scientific literacy have important implications for the general public's understanding of scientific issues and for the underrepresentation of women in science, technology, engineering, and math. We subjected data from the National Assessment of Educational Progress to a meta-analysis to examine whether there were sex differences in mathematics and science achievement for students in the United States across the period 1990-2011. Results show that there were small but stable mean sex differences favoring males in mathematics and science across the past 2 decades, with an effect size of d = 0.10 and 0.13, respectively, for students in 12th grade. Furthermore, there were large sex differences in high achievers, with males being overrepresented by a factor of over 2:1 at the upper right of the ability distribution for both mathematics and science. Further efforts are called for to reach equity in mathematics and science educational outcomes for all students.