Writing is an essential skill that is critical for learning, communication and employment. Female advantages in writing test scores have been reported in several studies. The aims of this study were to systematically quantify sex differences in writing achievement as measured by individually administered achievement tests via multilevel meta-analysis, break down differences based on components of writing, and investigate why effect sizes differ across studies. In all, 126 effect sizes were identified from 43 independent samples (N = 58,793) for the overall analysis. An overall effect of g = -0.31 (95% CI [-0.41, -0.22]) was found indicating small to moderate female advantages in writing. Area of writing was a significant moderator, with female advantages in written expression (g = -0.34, 95% CI [-0.52, -0.17]) exceeding those in spelling (g = -0.20, 95% CI [-0.32, -0.08]). Female advantages in writing fluency (g = -0.40, 95% CI [-0.52, -0.28]) were the largest sex differences that were statistically significant. Female advantages in writing have remained stable over the last fifty years and age or test did not alter those advantages. Experimenter derived tests produced larger female advantages than commercially produced tests, and convenience samples produced the largest female advantages among sample types. Exploratory moderator analyses of sex differences in written expression revealed similar results to those found in the overall sample. Small to moderate female advantages in writing have been consistent for decades. These advantages exist in all components of writing and across many different types of measures; future research should investigate the causal mechanisms that produce those advantages. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]