PROJECT SUMMARY Diagnostic errors are pervasive in medicine and often result from failures in clinical reasoning. Yet, little is known about how to improve clinical reasoning, and even less is known about how emotional experiences may contribute to diagnostic failure. This significant gap in knowledge is important for all populations, but is especially important for highly vulnerable and stigmatized populations who tend to evoke negative emotions. Individuals with mental illnesses comprise nearly 20% of the U.S. population and represent one such group. The primary objective of this research is to identify specific ways in which emotional factors may contribute to errors in diagnosis in the practice of emergency medicine (EM), and to develop and test cognitive interventions that may reduce these errors. Specific aims are to: 1) identify factors that serve as barriers to and facilitators of diagnostic reasoning in EM from key stakeholders (physicians, nurses, and patients) in the diagnostic process; 2) determine the impact of emotional factors (e.g., anger) on clinical reasoning processes among providers assessing patients with physical health complaints either with or without comorbid mental illnesses in emergency contexts; and 3) assess the influence of cognitive interventions that may mitigate affect-induced errors and improve the reliability of the diagnostic process. To achieve these objectives, qualitative interviews will first be conducted with EM physicians, nurses, and patients to inform the experimental studies that follow. Using rigorous, highly-reliable, and well-established methods from social psychology, EM physicians and nurses will participate in highly- controlled experiments designed to examine the causal influences of experimentally-manipulated (a) emotional experiences and (b) cognitive interventions. In these experiments, participants will assess and diagnose patient cases (presented in multimedia formats) to determine the causal effects of these manipulations on providers? clinical reasoning. This approach is theoretically and methodologically innovative in that it employs cutting-edge theory, research, and methods from social cognitive and affective science to investigate a critical knowledge gap that has not yet been systematically investigated. The proposed research is significant because it will expand our understanding of how emotions influence diagnostic reasoning and investigate possible ways to reduce adverse influences and promote diagnostic accuracy. Such knowledge has the potential to enhance patient safety not only for vulnerable, stigmatized populations such as those with mental illness, but for all patient populations.