Believing that one is either a "math person" or a "language person" can have important implications for students' engagement and performance in different activities and for their educational and career choices. One important source of information children use to form such self-relevant motivational beliefs is dimensional comparisons; that is, students engage in intraindividual comparisons of their subjective abilities across domains such as math and language arts when making self-evaluations. Despite their fundamental impact on students' educational outcomes, our understanding of when dimensional comparisons begin to influence children's self-perceptions, how this influence might change as children grow older, and whether the pattern varies across different types of motivational beliefs is still limited. A meta-analysis of 142 independent samples, 210,954 participants, and 426 effect sizes showed that the correlations between students' math- and language-related motivational beliefs decreased from r = .32, 95% CI .26, .37 for Grades 1-4 to r = −.01, 95% CI −.07, .04 for Grades 9-12/13. A meta-regression revealed a significant moderating effect of students' grade level on the association between students' math- and language-related motivational beliefs, b = −.06, 95% CI −.06, −.05, indicating an increasing differentiation in these beliefs. Findings generalized over samples across the world and studies using different research designs and held true for motivational constructs such as ability self-concepts and interests. Our results suggest that dimensional comparisons are involved in the formation of students' domain-specific motivational beliefs across the childhood and adolescent years and that their relative importance increases over the school years. Public Significance Statement Dimensional comparisons-that is, intraindividual comparisons of a person's relative and subjective strengths and weaknesses across different domains-are an important source of information people use to determine what they are good at and what they like. This meta-analysis shows that students' reliance on dimensional comparisons across the math and verbal domains increases over the school years. The role of dimensional comparisons is strongest in the high school years, a time when students form specialized identities (e.g., a "math person") and make achievement-related choices with long-term consequences for their educational and career trajectories.