How good am I at doing this? How much do I like doing that? These questions are related to what psychologists call self-relevant motivational beliefs. Decades of research show that these beliefs impact students’ school performance and their college major and career choices. When students try to determine what they are good at and what they like, they may use information from dimensional comparisons—i.e., comparisons of a person’s relative and subjective strengths and weaknesses across different domains such as math and language arts. These comparisons play a key role in students’ formation of specialized identities (e.g., “I’m a math person”), and they influence students’ achievement-related choice-making with long-term consequences for their educational and career trajectories. Despite their impact on students’ educational outcomes, our understanding of when dimensional comparisons begin to influence children’s self-perceptions and how this influence might change as children grow older is still limited due to the lack of longitudinal research on this topic. To address these research gaps, I conducted three studies (Chapters 2-4 in this dissertation). First, I meta-analyzed 103 published studies involving 142 independent samples and 210,954 participants across 16 countries. I found an increasing differentiation of students’ math and verbal self-concepts over the school years, suggesting that students increasingly use dimensional comparisons across the math and verbal domains over the K-12 school years. Second, using cross-sequential data spanning Grades 1-12 (n = 1,069, ages 6-18, 91% White, 51% female), I charted age-related changes in the role of dimensional comparisons in students’ ability self-concept formation. This study provided evidence that the increase in students’ use of dimensional comparisons substantially contributed to the increasing differentiation in students’ math and verbal self-concepts over time. Finally, I analyzed an existing longitudinal dataset following students from Grade 6 to Grade 7 (n = 1,763, 91% White, 53% female). This study investigated the effects of between-class math ability grouping on 7th-graders’ use of dimensional comparisons in the formation of their math self-concepts. I found that students who went to a school with a between-class grouping policy for 7th grade math class were more likely to use dimensional comparisons to form their math self-concept than those who attended 7th grade in a no-grouping school. Collectively, these studies help us understand when dimensional comparisons become an important mechanism shaping students’ motivational beliefs during K-12, and some of the reasons why this happens. In the final chapter, I discuss the implications of my studies and potential future research directions.