The ability to detect and avoid contagious individuals would be advantageous to reduce the toll of infection. Some animals can indeed detect infection in conspecifics, and there are some data showing that sick humans smell worse, walk more slowly, and appear less socially desirable in photos. However, little is known regarding how accurately humans detect sick people and how this process works. Authors of a recently published Proceedings B article, Identification of acutely sick people and facial cues of sickness, explored this subject and made some interesting findings.

Since the human face is the primary source for information in social communication, the authors of this study wanted to determine whether people could detect sick and healthy people from facial photos. Since they were interested in the first signs of infection (when people are acutely sick) and in reducing the influence of other factors disease factors that may develop across time, the authors used a highly controlled sample of healthy individuals that were made sick experimentally. Two photos were taken of each individual, one 2 hours after injection with an endotoxin (lipopolysaccharide) and the other 2 hours after injection with a placebo.
These photos were subsequently rated by people to determine whether the person on the photo was sick or healthy (the detection rate being 62%, i.e., not absolute but well beyond the random level of 50%). Another group of people were then asked to rate the features commonly expected to indicate sickness. This included features related to the eyes, the skin and the mouth (e.g. pale skin, red eyes and pale lips), and apparent sickness from the same photos. It was found that pale lips, pale skin, red eyes, hanging eyelids and droopy corners of the mouth were the most apparent indicators during acute sickness.

The researchers involved in this study are based at Stockholm University and Karolinska Institutet in Sweden and are experts within psychoneuroimmunology, social psychology, perception and cognitive neuroscience. They have long been involved in delineating some of the behavioural and neural changes resulting from acute sickness, changes which are today widely accepted as supporting recovery in the short term, but may nourish chronic problems in the long-term. It is well known that humans and other animals change their behaviour substantially when sick: they become feverish, fatigued, pain sensitive, and develop depression-like symptoms. Recently, the researchers have been focussing on the social consequences of being sick, as well. A sick person can be considered a threat to other people, as they may be contagious and potentially risky to be around. A central model in this research field is ‘disease avoidance’, which proposes that humans avoid people who are potentially contagious, while favouring healthy-looking, attractive individuals. The authors have therefore carried out a series of studies describing cues by which other people detect potentially sick and contagious individuals, including changes in body odour, skin colour, and gait. A sick person is also often in need of care; they have recently studied how sick people communicate their sickness symptoms and suffering to a caretaker.

The authors stated that

‘We are very satisfied with the easy submission and swift and direct review process provided by the editors and reviewers at Proceedings B. We want to stress that the ability to detect these early sickness signs has probably helped our forefathers avoid infections, and we are happy to share our results with the readership of this historically important journal.’

Proceedings B is looking to publish more high-quality research articles and reviews in the fields of behaviour and psychology. If you have an idea for a review, we strongly encourage you to submit a proposal by completing our proposal template and sending it to the journal.


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