When a strange new test of perceptual style called the Rorschach reached the New World in the 1920s, it became almost immediately popular. Developed as a psychoana lytic "X ray" of the psyche, it succeeded because American psychologists wanted and needed it to do so, and to do so as that kind of test. Over a decade later, the MMPI was constructed as a more orthodox personality inventory geared to traditional psychiatric categories While this medical legacy was soon removed or (...) obscured, success was more gradual. After the war, clinical psychologists adopted a professional identity independent of psychiatry. Their personality assessment tools, and what counted as success, came to reflect a reclaimed disciplinary genealogy. Standardized mappings and rule-by-numbers tended to displace a trust in experience and expert Judgment. In this context, "proper" Rorschach use came to be seen as indulgent or sadly mistaken. Supporters of the MMPI were, in contrast, able to claim both science and efficiency on their side and colonized the field. The history of these tests clearly illustrates the process of co-production, of how the right tool can become very wrong as networks dissipate and professional time goes by. (shrink)