mathematics was the only one of the four sets of models where the final gender difference was no longer statistically significant. This implies that other factors, not examined in the models, contribute to the substantial residual gender difference in grade 4 science and grade 8 mathematics and science. One possibility is that selection effects could be driving the observed differences — that is, if only highachieving girls attend school or sit for assessments, but most boys do so, there is a possibility of bias, such that girls’ average achievement would appear inflated. However, as enrollment in primary and intermediate education in Saudi Arabia is almost universal among both genders, selection effects are unlikely to be playing a role in this analysis.

Given evidence from other studies, it is likely that differences in reading proficiency play a role in explaining at least part of the remaining gender differences. This is particularly so in relation to science achievement, where test items are by necessity embedded in a context that often requires a greater degree of reading comprehension. Differences in reading proficiency between boys and girls might also contribute to explaining the remaining gender differences observed here in mathematics achievement in grade 8, given that grade 8 mathematics assessments require more reading skills compared to mathematics in early grades. For example, test items assessing applied reasoning or problem-solving skills in grade 8 are more likely to be embedded in a short scenario requiring some level of reading. International assessments in recent years have consistently shown that reading achievement is, on average, substantially lower in Saudi Arabia than in many other countries, both among grade 4 students (Mullis et al. 2012; 2017) and among 15-year-old students (OECD 2019).

Moreover, in Saudi Arabia, the reading achievement gap in which girls outperform boys is among the largest gender differences in the world. Differences between boys and girls in reading proficiency have been found to exceed half a standard deviation in both the PIRLS and PISA studies (Mullis et al. 2017; OECD 2019). Previous research on TIMSS mathematics and science items has shown that items with a higher reading load (those requiring a higher volume of reading or more complex reading skills) tend to be more difficult for students to answer correctly than items with a lower reading load, and also that weaker readers tend to be disproportionately disadvantaged by a higher reading load (Mullis, Martin, and Foy 2013). For this reason, the magnitude and consistency of Saudi boys’ relative disadvantage in reading, seen across various studies, seems likely to play a role in contributing to their poorer results found here in mathematics and science achievement even after accounting for a range of contextual variables. The NALO 2018 results provide further support for this view, with boys at both grade levels being more likely than girls to report not having read a book (although it should be noted that this was the case even for a substantial minority of girls).

The proposed importance of reading skills in underpinning mathematics and scientific achievement is consistent with the pattern of residual (unexplained) variance reported in the models, which indicates a role for other factors operating largely at the student level. After accounting for a range of other student- and class/school-level factors, the majority (approximately three-quarters) of class/school-level variance was explained in the models, whereas a majority of student-level variance remained unexplained. This suggests that the residual gender differences in achievement are likely to be associated more strongly with student-level factors, such as reading skills, social and behavioral skills, or aspects of the home background, than with additional school-level factors. The finding from NALO 2018 that more boys than girls have repeated a grade at school because of poor academic performance is worth noting in this regard. Similar data on the extent of grade repetition are available from PISA 2018 (OECD 2019), where 13.0 percent of 15-year-old boys in Saudi Arabia reported repeating at least one grade, compared to 9.8 percent of girls. These findings hint at the likelihood that early disadvantages and difficulties with learning in the early grades may compound over time, and that there is a need for stronger learning supports for students with special educational needs to enable progression through the education system. Although this issue affects both boys and girls in Saudi Arabia, the figures from NALO and PISA indicate that such compounding educational disadvantage is more clearly apparent among boys.

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