These subtle cues are what make sick people look sick

The next time a friend tells you that you look sick, hear the person out. We are better than chance at detecting illness in others simply by looking at their faces, according to new research led by a Swedish psychologist.

“We can detect subtle cues related to the skin, eyes and mouth,” said John Axelsson of the Karolinska Institute, who co-wrote the study published Tuesday in the journal Proceedings of the Royal Society B. “And we judge people as sick by those cues.”

Other species have more finely tuned disease radars, relying primarily on the sense of smell. And previous research, Axelsson noted, has shown that animals can sniff sickness in other animals. (A Canadian hospital enlisted the help of an English springer spaniel trained to smell bacterial spores that infect patients.)

Yet while there is some evidence that an unhealthy person gives off odors that another individual can identify as sickness, the face is our primary source of “social information for communication,” Axelsson said.

He and his colleagues, a team that included neuroscientists and psychologists in Germany and Sweden, injected eight men and eight women with a molecule found in bacterial membranes. Like animals — from insects to mammals — people react very strongly to this substance, lipopolysaccharide. “People did not really become sick from the bacteria,” Axelsson said, but their bodies did not know the bacteria weren't actually attacking. Their immune systems kicked into action, complete with feelings of sickness. The subjects, all white, received about $430 for their trouble.

The scientists photographed the subjects two hours and 10 minutes after the injection, around the time participants said they felt the most unwell. They also photographed the subjects on a different date after they received a placebo injection of saline solution.