DO SYMPTOMS OF ILLNESS SERVE SIGNALING FUNCTIONS? (HINT: YES).

Symptoms of illness provide information about an organism's underlying state. This notion has inspired a burgeoning body of research on organisms' adaptations for detecting and changing behavior toward ill individuals. However, little attention has been paid to a likely outcome of these dynamics. Once an organism's fitness is affected by others' responses to symptoms of illness, natural selection can favor individuals who alter symptom expression to influence the behavior of others. That is, many symptoms may originate as cues, but will evolve into signals. In this paper, I develop the hypothesis that symptoms of illness serve signaling functions, and provide a comprehensive review of relevant evidence from diverse disciplines. I also develop novel empirical predictions generated by this hypothesis and discuss its implications for public health. Signaling provides an ultimate explanation for otherwise opaque aspects of symptom expression, such as why symptoms fluctuate in social contexts, and can exist without underlying pathology, and why individuals deliberately generate symptoms of illness. This analysis suggests that signaling theory is a major organizing framework for understanding symptom etiology.