© Alan Thornton/Getty ImagesObvious signs of illness such as sneezing and coughing are easy to spot, but more subtle cues such as pale lips or droopy eyelids may help humans to tell when another person is sick.
Coughing, sneezing and clutching the stomach might be obvious signs of sickness, but humans can also spot if someone is healthy simply from a glance at their face, new research suggests.
Scientists have found that signs of a person being acutely unwell - such as pale lips, a downward turn of the mouth and droopy eyelids - are visible just hours after an infection begins.
"We use a number of facial cues from other people and we probably judge the health in other people all the time," said John Axelsson, a co-author of the research and a professor at the stress research institute at Stockholm University.
While previous work has shown that besides overt symptoms - such as sniffing - changes in skin colour can serve as a guide to health, experts say the latest study highlights the ways in which humans might use a host of early signals to avoid contracting infection from others.
Writing in the journal Proceedings of the Royal Society B, Axelsson and colleagues described how they injected 16 healthy adults with a placebo and, at a separate point in time, molecules from E coli which are known to rapidly trigger flu-like symptoms. The participants were unaware which injection they had received, and were photographed about two hours after each injection.
The team then showed the portraits to 62 participants who were asked to judge whether the pictured person was sick or healthy, with each picture shown for a maximum of five seconds.
The results reveal that the participants were able to spot a sick person slightly better than if they were guessing, correctly identifying someone as being unwell 52% of the time. However, more impressively, they correctly labelled individuals as being healthy 70% of the time.
Axelsson said the judgement of whether someone was sick or healthy might vary depending on the people analysing the images, noting that people looking for a partner might be better at spotting signs of health, while those afraid of catching an infection might be better at noticing cues for sickness. "I think it depends a bit on the context you are in, on what you are sensitive for," he said.
To delve deeper a new group of 60 participants were shown the photographs, without being told which injection had been given.
Individuals photographed after being injected with the E coli molecules were, on average, rated as looking more sick and more tired than in photos taken after they were given the placebo. They were also rated as having a more swollen face, redder eyes, less glossy and less patchy skin as well as a more drooping mouth, hanging eyelids and - in particular - paler lips.
However further analysis showed that just how sick a person was judged was most reliably linked to paler skin and droopier eyelids.
Professor Ben Jones of the Face Research Lab at the University of Glasgow welcomed the research. "This study adds to growing evidence for the existence of facial cues associated with acute sickness and help us understand how, unfortunately, social stigmas about people suffering illnesses might emerge," he said.
But he noted that the study did not replicate real life, where faces can show many types of variation, even in the same person. Also, other studies have used the same set of photographs, meaning caution was needed against conclusions based on only a small group of individuals.
Dr Carmen Lefevre of the centre for behaviour change at University College London also raised concerns about the small number of people who had been photographed. Nonetheless, she said the research supports the idea that humans have developed a range of behavioural mechanisms to help avoid catching diseases. "This is the first study [to show] that illness [is detectable] shortly after onset," she added.
Dr Rachel McMullan of the Open University said that it would be helpful to look at whether the results held for a wide range of ethnic groups and for different diseases, but added the results suggested ill individuals could be spotted soon after infection.
"Being able to quickly identify and avoid potentially sick, contagious individuals will certainly be an evolutionary advantage and this study is a good starting point for further research into the how we detect early signs of infection," she said.
Comment: Very interesting research. It goes to show that we humans have a very sophisticated system of reading facial cues for gathering all sorts of information we're usually completely unaware of. See also: