Scores on the ACT college entrance exam predict college grades to a statistically and practically significant degree, but what explains this predictive validity? The most obvious possibility is general intelligence—or psychometric “g”. However, inconsistent with this hypothesis, even when independent measures of g are statistically controlled, ACT scores still positively predict college grades. Here, in a study of 182 students enrolled in two Introductory Psychology courses, we tested whether pre-course knowledge, motivation, interest, and/or personality characteristics such as grit and self-control could explain the relationship between ACT and course performance after controlling for g. Surprisingly, none could. We speculate about what other factors might explain the robust relationship between ACT scores and academic performance.