The present study tests two predictions stemming from the hypothesis that a source of difficulty with rational numbers is interference from whole number knowledge. First, inhibitory control should be an independent predictor of fraction understanding, even after controlling for working memory. Second, if the source of interference is whole number knowledge, then it should hinder fraction understanding. US children (N=765, 337 female) in 3rd, 5th, and 7th grade completed a battery of computerized tests. Inhibitory control predicted fraction task performance over and above working memory. Whole number knowledge predicted performance only for some forms of fractions. These results highlight a role for inhibitory control in rational number understanding and suggest that its contribution goes beyond inhibiting whole number knowledge.

Relational thinking, the ability to represent abstract, generalizable relations, is a core component of reasoning and human cognition. Relational thinking contributes to fluid reasoning and academic achievement, particularly in the domain of math. However, due to the complex nature of many fluid reasoning tasks, it has been difficult to determine the degree to which relational thinking has a separable role from cognitive processes collectively known as executive functions (EFs). Here, we used a simplified reasoning task to better understand how relational thinking contributes to math achievement in a large, diverse sample of elementary and middle school students (N = 942). Students also performed a set of ten adaptive EF assessments, as well as tests of math fluency and fraction magnitude comparison. We found that relational thinking was significantly correlated with each of the three EF composite scores previously derived from this dataset, albeit no more strongly than they were with each other. Further, relational thinking predicted unique variance in students’ math fluency and fraction magnitude comparison scores over and above the three EF composites. Thus, we propose that relational thinking be considered an EF in its own right as one of the core mid-level cognitive abilities that supports cognition and goal-directed behavior.