'Concept creep' in perceptions of mental illness - an experimental examination of prevalence-induced concept change.

The perception of what constitutes mental illness is influenced by various social and medical developments. Prevalence-induced concept change is a phenomenon where decreasing the prevalence of a category leads people to expand their judgment of that concept. In this study, we tested whether changing the prevalence of statements describing mental illness results in a change in the concept of mental illness. Based on a population survey (n = 1031), we created a validated set of 273 brief statements depicting either clear symptoms of mental illness, clear examples of healthy behaviour, or ambiguous situations. We presented a subset of statements to 138 students, asking them to judge whether each statement represented mental illness, or not. After 96 statements, we reduced the prevalence of clearly mentally ill statements in one group, while the proportion of statements denoting clear mental illness remained the same in the other group. In the group where the proportion of clearly mentally ill statements was reduced during the experiment, a concept change of mental illness evolved: participants were more likely to identify a statement as denoting a mental illness. The results indicate that the perceived prevalence of symptoms of mental illness is important for conceptualizing mental illness and that decreasing prevalence broadens the concept of mental illness. These findings add a novel perspective to current debates around diagnostic thresholds, the treatment-prevalence paradox, the medicalization of emotions, and the focus of anti-stigma campaigns.