Undergoing medical treatment legitimizes illness: An experimental study of medical treatment as a costly signal of need.

While the primary goals of medical treatment are typically to shorten illness or relieve symptoms, we explore the idea that an important additional goal for some patients is to communicate need. Drawing on costly signalling theory, we argue that treatments – and in particular aversive treatments – can help patients legitimize their illness and thereby enable access to crucial support during convalescence. Four pre-registered within-subjects experiments (n = 874) show that participants are more inclined to provide care to people who undergo treatment, especially when that treatment is painful. Results show this incentivizes the use of antibiotic treatments for viral infections as well as risky drug treatments for mental illness. A cross-sectional of 194 chronic pain patients shows that those who experience stigma and doubt over the legitimacy of their illness are more likely to accept aversive treatments. Furthermore, two experiments (n = 653) indicate that subtle manipulations to one’s sense of social support may increase willingness to accept treatment. These results indicate that people make decisions to provide care in part based on the presence/absence of treatment and furthermore that patients’ treatment decision making is informed by the social consequences of their choices. Signalling theory may help explain the surprising longevity of some ineffective and costly medical procedures.