This article charts the history of mental testing in the context of the rise and fall of Russian child science between the 1890s and the 1930s. Tracing the genealogy of testing in scientific experimentation, scholastic assessment, medical diagnostics and bureaucratic accounting, it follows the displacements of this technology along and across the boundaries of the child science movement. The article focuses on three domains of expertise – psychology, pedagogy and psychiatry, examining the key guises that mental testing assumed in them – namely, the experiment, the exam and the diagnosis. It then analyses the failed state-bureaucratic harnessing of mental testing in early Soviet attempts to manage mass education, discussing the peculiar dynamics of the (de)legitimation of testing, as it swung between black-boxing and instrumentalization, on the one hand, and scandal and controversy, on the other. The article argues that mental testing thrived in Russia as a strategically ambiguous and flexibly interpreted ‘boundary object’, which interconnected a highly heterogeneous field, enabling the coexistence and cooperation of diverse occupational agendas and normative regimes.