In the last century many studies have revealed the advantages of girls in reading and superiority of boys in science. However, the international tests detected no difference in science test results in the 21st century. The aim of the study was to find the sex effect size and variances in reading achievement in recent international studies. The analysis of PISA 2000, 2003, and 2006 data and the PIRLS 2001 and 2006 data revealed that the advantages in reading achievement of ten-year old girls was 0.23d and that of 15-year old girls was 0.42d. One explanation of girls’ higher achievement in reading is in their deeper engagement in language related activities. Comparisons with other studies and possible implications are shown.