Identification of acutely sick people: individual differences and social information use.

Abstract

Can humans discriminate between healthy and sick individuals, and if so by what cues? In order to study these important questions, in a recent study, Axelsson et al . let 62 untrained raters decide for 32 facial photos whether the person in the photo was healthy or sick [1]. Photos were taken of 16 healthy volunteers, injected once with a lipopolysaccharide injection ( Escherichia coli endotoxin) and once with a placebo injection (with a three to four week interval in between). Photos were taken 2 h after injection. While the authors used this ingenious study design to estimate the average discrimination ability of raters, their analysis did not address two keys aspects pertaining to their research questions. First, they did not report whether and how individuals differ in discrimination ability. Individuals may differ in their ability to discriminate between healthy and sick individuals, and in how they balance the trade-off between sensitivity (i.e. the frequency of sick individuals classified as sick) and specificity (i.e. the frequency of healthy individuals classified as healthy), a.k.a. response bias [2]. Importantly, while the authors did not use their data to investigate the presence and structure of such differences, the implications of their work for disease dynamics—and consequently prevention strategies—will differ substantially between populations with and without individual differences [3–5]. For example, groups and populations with a large number of expert and/or ‘cautious’ individuals (i.e. individuals with a relatively low response bias) are predicted to have a lower prevalence and weaker infectious outbreaks than groups with fewer expert and/or cautious individuals. Second, while the authors investigated the discrimination ability of socially isolated raters, in many real-world contexts, individuals will be able to observe the responses of others to potentially sick individuals. In such situations, the social information present …