Women's underrepresentation in mathematics-related careers continues to concern policymakers, economists, and educators. This study addressed the issue by examining data from two international databases, namely IEA's "Trends in International Mathematics and Science Study 2015," and the World Economic Forum's "Global Gender Gap Report 2017." Using country as the unit for our observations and meta-analysis techniques, the question of gender mathematics differences was investigated using standardized mean difference comparisons and variance ratios. Fourth- and eighth-grade girls and boys were also compared in terms of the number of students who reached the advanced international benchmark. The findings mostly supported previous findings in the related literature; no statistically significant large differences were observed comparing the performance of girls and boys in mathematics achievement and the number of high achievers. Moreover, boys were found to have more variability in mathematics achievement than girls. This finding further isolates the potential cause of women's underrepresentation in mathematics-related careers as policies driven by implicit and explicit cultural biases.