The last 2 decades have produced a vast literature describing relationships between cognitive performance and neuropsychological data. This literature has provided the foundation for countless theories about the neural correlates of cognitive processing and specific theories regarding the role of different cortical areas in human cognition. In this paper, we examine a particular theory – the error likelihood model (Brown & Braver, 2005) – that attempts to account for the function of a particular brain area (the anterior cingulate cortex). A careful evaluation of behavioral data from humans raises questions about the error likelihood model and the implications of neuropsychological data for understanding cognitive performance.