Lower levels of academic achievement amongst children of lower SES have long interested researchers. Influences of SES are often addressed by studying predictors of achievement in economically advantaged and disadvantaged samples. Levels of engagement in school, argued to contribute to academic achievement independently of intelligence, have been found to be relatively lower amongst children from low SES families. Low SES is thus argued to have a destructive influence on engagement and achievement compared to the more constructive influences of high SES. The relative strength of influences of SES on engagement and academic achievement have been largely ignored. The present study used a difference-in-score approach to assess the strength of SES influences on engagement and achievement at different levels of SES. Regression analyses predicting difference in engagement and difference in grades were carried out, controlling for gender and IQ. SES influenced engagement more strongly at higher levels of SES, unifying siblings in their relatively high engagement levels independently of gender or IQ. A similar relation between SES and academic achievement was partially mediated by engagement, indicating that SES influenced engagement levels, which then contributed to academic achievement. This trend was evident only in the biologically-related sibling samples, indicating that the influences of SES on engagement and academic achievement are primarily genetic. The implications of these findings are discussed.