An extensive body of research exists in both philosophy and social psychology on the subject of self-deception. Self-deception denotes the relative lack of insight that people have into what drives their thinking and behavior. Self-deception, then, would seem to be a critical concept for decision-making capacity that evaluates how well patients can apprehend and reason about a piece of information in order to make a decision that aligns with their desires and values. Issues surrounding capacity frequently arise in the context of patients with mental illness as these patients often lack insight into their condition. Yet self-deception remains unaddressed in the ethical frameworks of capacity assessments. This essay first gives an overview of various models for determining capacity and of the social psychological research on self-deception and then analyzes a short story that represents the consequences of not accounting for self-deception's role in human thought and behavior. This essay ultimately argues that self-deception is a concept that should be considered in the ethics research that underpins capacity and that taking self-deception into account allows for more rigorous capacity assessments.