Overdetection and underdetection of depression and anxiety in primary care are common and may partly reflect individuals' misperceptions of the severity of symptoms they experience. Here, we explore how people's judgments about the severity of their own symptoms are influenced by their beliefs about the distribution of symptoms experienced by the rest of the population. We apply the rank-based decision by sampling cognitive model of judgment to symptom severity. The model proposes that judgments depend on the relative rank of an item within a mental sample of comparable items. It is predicted that judgments of symptom severity will be context dependent and more specifically that an individual's judgments will be invalid to the extent that the individual has inaccurate beliefs about the relevant social context. Two studies found that participants' assessments of symptom severity were rank based. Study 1 elicited participants' beliefs about the social distribution of symptoms and found that participants' judgments of whether they were depressed or anxious were mainly predicted not by their symptoms' objective severity but rather by where participants ranked the severity of their symptoms in comparison with the believed symptoms of others. Study 2 varied symptom distributions experimentally and again found relative rank effects as predicted. It is concluded that the real-world application of contextual models of judgment requires investigation of individual differences in participants' background beliefs. Copyright © 2012 John Wiley & Sons, Ltd.