Investigations reported during the past century have generally shown girls to be better readers than boys in the United States. Some researchers have attributed these differences to biological-maturational factors; others have suggested cultural-societal causes. The present study sought to examine this dichotomy by testing the reading ability of boys and girls in four English-speaking nations. More than a thousand elementary children from Grades 2, 4, and 6 in Canada, England, Nigeria, and the United States participated. Each subject was tested for reading comprehension, vocabulary and various word analysis skills. Analysis of variance, in which sex was nested within grade and grade within country, was applied to the data for each of eight analyses. Dependent variables were raw test scores, grade equivalent scores, or composite scores. In two countries, England and Nigeria, boys generally scored higher than girls and the sex differences favoring boys increased by sixth grade. Conversely, in Canada and the United States girls generally scored higher than boys and the sex differences favoring girls diminished or disappeared by sixth grade. Results of the study indicate that sex differences in reading ability are culturally related. (Author)