How can humans tell when another person is sick? There are obvious signs, surely. If they're coughing, wheezing, or doing something out of the ordinary that suggests they're in pain, they might be sick.

As it turns out, humans are also actually adept at detecting sickness just by looking at people's faces.

"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, coauthor of a new research published Tuesday, Jan. 2, in the Proceedings of the Royal Society B.

You Look Sick

Unlike humans, other species have other ways of detecting sickness. For instance, some species rely on their sense of smell to determine sickness in another animal. For humans, facial cues are key. It's the primary source for "social information for communication," according to Axelsson.

Axelsson's team, which includes psychologists and neuroscientists from Germany and Sweden, gathered 16 subjects, half of which were white men and the other half being white women. They injected them with lipopolysaccharide, a molecule found in bacterial membranes. It didn't actually make them sick. It simply triggered their immune systems, which then brought feelings of sickness — a natural response of the body when it thinks it's under attack.

The researchers took photos of the participants hours after injecting them with the bacteria, when they felt most unwell. They also took pictures of them at some other time after the subjects were injected with a placebo.

The team then took in 60 students to look at the photos. They asked them to determine which ones looks sick and which ones don't, with each person given a mere five seconds to observe the photo.

The students correctly identified someone as being unwell 52 percent of the time. Conversely, they were able to identify which ones were healthy 70 percent of time.

They brought another set of observers and asked them to rate different facial cues, such as pale lips, redder eyes, drooping mouths, hanging eyelids, and other features potentially indicative of a health problem.

"The change of skin color seemed to be the most robust," according to Axelsson. They also considered heavy eyelids as a strong determining factor of illness based on looking at a person's face alone.

Ben Jones, a professor from the University of Glasgow's Face Research Lab, praised the study but noted that it might not accurately replicate real-life scenarios. That's because faces can show many types of variation, even if it's the same person.

Is This Useful?

If a person is indeed able to recognize if a person is sick just by looking at their face, it might help them avoid risks of infection, especially if the other person has a contagious disease.

The researchers are currently trying to look at other ways for sickness detection in faces to be more accurate. Axelsson said the next step is to find out whether doctors and medical personnel are better at detecting sickness in people than regular folks.

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