Previous studies have associated several variables concerning motivation and other domains with underachievement, i. e. a student's academic achievement falling short of what their cognitive abilities, as the best predictor of academic performance, would indicate. The present study extends these findings using a more rigorous approach in defining underachievers and suitable control groups. Using discriminant analysis, underachievers identified in a German twin family study were compared not only to achievers with comparable IQ scores, but also with students of lower aptitude, but comparably low grades, as well as overachieving students. Results confirm previous findings that compared to successful students, underachievers report lower levels of motivation and parental support; beyond this comparison, underachievers also differed from other low achievers, mostly in terms of their personality. In total, 40% of the variance between the groups were explained. Additionally, the data shed doubt on the common assumption that underachievers are an unusually heterogenous group of students.