Psychiatric disorders profoundly impair many aspects of decision making. Poor choices have negative consequences in the moment and make it very hard to navigate complex social environments. Computational neuroscience provides normative, neurobiologically informed descriptions of the components of decision making that serve as a platform for a principled exploration of dysfunctions. Here, we identify and discuss three classes of failure modes arising in these formalisms. They stem from abnormalities in the framing of problems or tasks, from the mechanisms of cognition used to solve the tasks, or from the historical data available from the environment.