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Description: Sex differences in mathematical ability were examined in a nation-wide sample of 32,346 Jordanian 4th graders (age 9-10) on a 40-item mathematics test. Overall, girls were found to perform slightly better (d = 0.12), while boys had slightly more variation in scores (standard deviations of 1.02 and 0.98 for boys and girls, respectively). 2 parameter item response theory analysis showed that the item parameters were almost identical across sexes; correlations were .97 [95% confidence interval: .95 to .99] and .90 [.82 to .95] for difficulties and loadings, respectively. A bootstrapping analysis revealed that these could not be explained by sampling error alone, but reflected genuine departures from measurement invariance. However, sex differences in items showed no relationship to the relative size of the departure from measurement invariance. A correlation between item difficulty and male advantage was observed, r = .57 [.32 to .75] (.68 after exclusion of an outlier), such that the relative male advantage increased with increased item difficulty. Results were interpreted to be congruent with Richard Lynn’s developmental theory of sex differences in general intelligence.