Research on how socioeconomic status (SES) and gender interact to affect student achievement has produced contradictory findings. Some studies suggest boys are more vulnerable to socioeconomic disadvantage; others find the opposite. This paper argues that these conflicting results stem from a methodological artifact: because boys report their parents’ education less accurately than girls do, the source of parental education data in large-scale assessments fundamentally shapes estimates of the SES–gender interaction. PISA 2006 and 2009 provide data on 151,269 fifteen-year-old students from 20 countries for whom both parent-reported and student-reported parental education were available. This within-subject design allowed a direct comparison of the parental-education–gender interaction in mathematics achievement, isolating the effect of the data source. The direction of the interaction systematically reverses depending on the data source. When using reliable parent-reported data, the parental-education gradient is steeper for boys in the majority of country-waves (18 of 31), consistent with the “vulnerable boys” hypothesis. In 12 of those 18 cases, the results reverse to indicate “vulnerable girls” when using the more commonly available student-reported proxy data. A sign test confirmed that the interaction estimate was more negative with student data in 28 of 31 country-waves (p < 0.001). The choice of data source is not a minor technical detail but a factor that can reverse conclusions about educational equity.