International comparative assessments of student achievement, such as Trends in Mathematics and Science (TIMSS) and Programme for International Student Achievement (PISA) are becoming increasingly important in the development of evidence-based education policy. The potentially far-reaching influence of such studies underscores the need for these assessments to be valid and reliable. In education, increasing recognition is being given to motivational factors which impact on student learning. This research considers a possible threat to the validity of such studies by investigating the influence the amount of effort invested by test-takers has on their outcomes. Reassuringly, it is found that the reported expenditure of effort by students is fairly stable across countries. This finding counters the claim that systematic cultural differences in the effort expended by students invalidate international comparisons. Realistically reported effort expenditure is related to reading achievement with an effect size similar to variables such as single parent family structure, gender and socioeconomic background. Finally, when reporting trends, taking effort into account should be considered and may well facilitate the interpretation of national and gender trends in reading achievement.