This open access book examines why reading and math test scores for boys and girls have differed since the origins of testing in the United States. It details the pattern of differences that have remained largely unchanged for more than 100 years in the United States and worldwide. The book explores why boys have modestly larger math test score means than girls, and why girls have far larger reading test score means than boys. Boys have larger test score variances for both tasks. The only data of focus—and thus the only data to be explained—are boys’ and girls’ test score sample means and variances.