Education researchers who study the effect of family social background on student achievement often use students’ survey reports of parental education to investigate these effects. However, past research has demonstrated that students misreport their parents’ education levels. We expand upon this research in two ways. First we use cognitive theories about the response process to develop and test hypotheses about reporting inconsistencies across these variables. Second we evaluate the impact of student misreporting on estimates of the relationship between parental education levels and student math achievement. Using data from the German administration of PISA 2000 (OECD Programme for International Student Assessment) in which both students and parents were asked to report parental variables, we show that reporting inconsistencies are a function of student achievement: students with higher math scores tend to provide reports that are more consistent with their parents’ reports. This interesting case of differential measurement error has consequences for comparisons of the effects of parental background on student achievement across different subgroups of the population and across countries (a common use of PISA data and other international studies similar to PISA).