Participants were infected with a bacterium that caused an inflammatory response. Two hours later, photos were taken of them. Photos were also taken of healthy volunteers who had a placebo injection.
The next part of the experiment involved another set of volunteers. The sixty-two volunteers were shown thirty-two composite pictures of the healthy and sick people randomly. The volunteers had 5 seconds to decide if the person in the photo was sick or healthy. The guesses were correct 64% of the time. The probability of volunteers knowing whether someone was sick was greater than just by chance.
In another trial, sixty people looked at the same thirty-two photos and noted if they looked sick or tired and rated the presence of certain sickness symptoms. People that had gotten the E. coli injection were rated as looking sicker and tireder and with paler skin, more swollen faces, and redder eyes than those who had gotten a fake injection. It appears that these facial cues influence how viewers perceive whether someone is sick.
The reason that humans could be able to recognize sick people so quickly could have had an evolutionary origin. It may have been a matter of survival. For most of human history, vaccinations and antibiotics did not exist so someone had to be able to tell if someone else was sick to know to stay away and survive. In real life, this ability to detect if someone is sick is likely even more sensitive because there are many other telling cues in real life, such as posture and voice.
However, this detection ability is not fail-proof, someone who is sad or tired could be incorrectly thought of as “sick”. These people were often indiscriminately avoided because it’s a lot more dangerous if you stay near a sick person than avoid someone who is healthy. Perhaps these evolutionary reactions still affect our behaviour.