Since the end of World War I, a new way of thinking about individuals’ fit (and misfit) in social and educational institutions spread in the United States. Key to this evaluation of fitness was various conceptions of psychological “adjustment” and “maladjustment.” This dissertation explores the emergence and circulation of these conceptions of psychological adjustment in the human sciences and public schools. It also assesses the social implications of the scrutiny of emotional fitness among its citizenry in the U.S.Chapter 1 examines the intellectual and social origins of the new conception of personality adjustment during the 1910s and 1920s. It traces how ideas of adjustment emerged in theories of dynamic psychiatry, the mental hygiene movement, and psychological assessment of emotional fitness. Meanwhile, practical demands from school officials, industrial employers, and state bureaucracies deeply shaped the use of scientific knowledge about personality adjustment. Chapter 2 focuses on the child guidance clinic, one of the first innovations scientific experts promoted to intervene in students’ emotional and personality adjustment. While child guidance contributed new theories and methods to dealing with students’ emotional and behavioral problems, child guidance practices struggled to transform the operation and function of public schooling. Instead, a more economical way – personality testing – emerged as a popular tool to manage children’s behavior. Chapter 3 focuses on the construction and application of personality tests that aimed to detect students’ emotional and social maladjustment in the late 1920s and 1930s. It interrogates the claims of scientific objectivity in these tests and reveals embedded conceptions of human differences and normality within the standards of adjustment. Chapter 4 explores the “personality and culture” approach to studying the adolescent personality during the 1930s. Compared to personality testing, this relatively dynamic approach treated personality as a complex whole in cultural contexts. Yet the approach ended up reifying the line between the “normal” and “maladjusted” personality based on race. By revealing embedded perceptions of human differences and hierarchy in scientific constructions of the “well-adjusted citizen,” this study sheds new light on the roots of what became commonsensical understandings about children’s socio-emotional development and mental health.