Identification of acutely sick people and facial cues of sickness

Abstract

Detection and avoidance of sick individuals have been proposed as essential components in a behavioural defence against disease, limiting the risk of contamination. However, almost no knowledge exists on whether humans can detect sick individuals, and if so by what cues. Here, we demonstrate that untrained people can identify sick individuals above chance level by looking at facial photos taken 2 h after injection with a bacterial stimulus inducing an immune response (2.0 ng kg^-1 lipopolysaccharide) or placebo, the global sensitivity index being d' = 0.405. Signal detection analysis (receiver operating characteristic curve area) showed an area of 0.62 (95% confidence intervals 0.60-0.63). Acutely sick people were rated by naive observers as having paler lips and skin, a more swollen face, droopier corners of the mouth, more hanging eyelids, redder eyes, and less glossy and patchy skin, as well as appearing more tired. Our findings suggest that facial cues associated with the skin, mouth and eyes can aid in the detection of acutely sick and potentially contagious people.

Trial registration: ClinicalTrials.gov NCT02529592.

Keywords: disease avoidance; facial cues; health; social perception.

Figure 1.

Illustrations of the timing of when the photos were taken (2 h and 10 min post injection) and (a) mean circulating concentrations of IL-6 and (b) development of sickness (subjective sickness rated on the sickness questionnaire, SQ), after injections with placebo and LPS. All 16 subjects participated in both conditions, and the dotted grey lines show their raw data in the LPS condition.

Figure 2.

Effects of LPS-induced acute sickness on (a) apparent sickness and tiredness, and cues relating to (b) the skin, (c) the mouth and (d) the eyes, when compared with placebo. The regression lines are estimated after the removal of variation between the observers using empirical Bayes' estimates. Thus, the regression lines represent the average change in the average observer. All effects are significant at p < 0.001. The scales for cues range from 1 ‘no symptoms’ to 7 ‘very high symptoms’. The health–sickness scale ranges from 1 ‘very poor’ to 7 ‘very good’ and reversed for the figure, and tiredness 1 ‘very alert’ to 7 ‘very tired’.

Figure 3.

Averaged images of 16 individuals (eight women) photographed twice in a cross-over design, during experimentally induced (a) acute sickness and (b) placebo. Images made by Audrey Henderson, MSc, St Andrews University, using Psychomorph. Here, 184 facial landmarks were placed on each image before composites displaying the average shape, colour and texture were created [20].

Figure 4.

Relationships between apparent sickness and facial characteristics. All significant regressions illustrated by dashed lines in black, and non-significant regression lines by solid light grey lines (see text for detailed statistics). The regression lines and the data points (individual data points in grey being jittered to better illustrate the distributions) are estimated after the removal of variation between the observers using empirical Bayes estimates. Thus, all observers have been adjusted (in level) to represent an average observer. The plots consist of 2856–2873 ratings each (60 observers rated the 32 photos, some photos were rated twice by each observer) on 7-point Likert scales (1 = ‘no symptoms’, 7 = ‘very high symptoms’).

Figure 5.

The effect of LPS on apparent sickness, directly and via the mediators patchy skin (Pat_S), droopy mouth (Dro_M), pale lips (Pale_Lips), glossy skin (G_S), swollen face (S_F), red eyes (R_Eye), hanging eyelids (H_Eye) and pale skin (Pale_Skin). The effects of cues on apparent sickness are β-weights (to the right, under the heading ‘effect on apparent sickness’) and the effects of LPS correspond to Cohen's d (to the left, under the heading ‘effect on cues’). The placement of the mediator along the x-axis corresponds to the degree of mediation (percentage mediation, vertical line inside box), with 95% CI (width of the box). The scales for cues range from 1 ‘no symptoms’ to 7 ‘very high symptoms’. *p < 0.001, ^†p < 0.05.