International assessments are important to benchmark the quality of education internationally. However, ignoring students' efforts in low-stakes tests can lead to biased interpretations of test performance. We use data from the PISA to analyse the potential role of student effort in explaining gender achievement gaps across countries. We build two effort measures based on students' response times to test questions (i.e. rapid-guessing rates) and the student effort on the post-test survey (i.e. item non-response rates). We find a strong association between student efforts and test scores. After accounting for differences in effort, the estimated gender achievement gap in mathematics and science could be up to 36% and 40% of an standard deviation wider, respectively, in favour of boys. Altogether, our effort measures on average explain between 51% and 56% of the cross-country variation in test scores.