Results of a previous meta-analysis of gender differences in reading achievement at the secondary school level showed significant differences between major assessment programs. Thus, the gender gap in favour of girls was more pronounced for the assessment programs conducted by the National Assessment of Educational Programs in the United States (NAEP), for the more recent assessment programs in Australia and the Programme for the International Student Assessment (PISA) conducted by the OECD. In contrast, no such effect was found for earlier studies conducted by the International Association for the Evaluation of Educational Achievement (IEA), namely the International Reading Comprehension Study 1970-71 and the International Reading Literacy Study 1990-91. Hence, this article seeks to investigate whether or not an effect exists that could be associated with the time period in which a study was conducted. In other words, the article examines whether or not the reasons for the greater gender differences in more recent assessment programs might be related to the scaling of reading scores before and after 1992.