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.”

Scientists combined 16 photo portraits into one composite image. On the left, the composite "sick" face, and on the right, the composite healthy one. Audrey Henderson/St. Andrews University

He and his colleagues 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 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.

Axelsson and the team asked 60 students recruited from universities in Stockholm to assess the photos. The students looked at the portraits one at a time. A sick face and healthy face from the same individual never appeared consecutively. Observers were given five seconds to look at each photo and identify the person in it as sick or healthy.

The students’ detection of immune responses was somewhat more accurate than sheer chance.

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