Despite a consensus that children's socio-economic status (SES) is linked to their educational outcomes, research is inconsistent. Differing variables are used to quantify SES; this can artificially boost its explanatory power. Policy-makers are encouraged to infer overly optimistic potential for redistributive tools. The PISA survey dataset was interrogated. The association between academic performance and two variables, parental educational attainment, and household income – was analysed. Results indicated that their link to academic performance was modest. Parental educational attainment was more important. Collapsing these into one single concept, rather than clarifying patterns of academic performance, concealed important differences between wealthy and poorer countries. The discussion proposes eschewing the unitary concept of SES.