It is January, my dudes, and classrooms and offices have become a mucous minefield for those of us trying to maintain our health. Most of us can get a good sense of which people are sick just by looking at them, but this instinct has not been explored much by scientists until recently. Now, neuroscientists say this ability is rooted in a few specific facial characteristics that could tip you off to a friend’s — or a stranger’s — illness.

In a paper published Wednesday in the journal Proceedings of the Royal Society B, researchers at Karolinska Institute in Stockholm, Sweden, report that participants in their study were more likely to rate sick people as having some specific facial characteristics: “paler lips and skin, a more swollen face, droopier corners of the mouth, more hanging eyelids, redder eyes, and less glossy and patchy skin, as well as appearing more tired.”

The team’s conclusions matched up well with what we already intuit about sick people, but the researchers are careful to point out that some of these characteristics are easily confused with symptoms of people who are simply tired, not sick.

This composite of 16 different people's photographs shows how slight changes in facial features can make the difference between someone looking sick (a) and looking healthy (b).

To conduct this study, they injected 16 volunteers with lipopolysaccharide produced by E. coli bacteria, a substance that doesn’t actually make people sick but triggers an immune response that makes people look sick. They also injected them, on a separate occasion, with a placebo solution of salt water. Under each of the two conditions, they took photographs of the test subjects, as in the photo above.

In one trial, the researchers had 62 people who don’t have any particular expertise in recognizing disease symptoms examine the pictures and identify whether the people in them looked sick or healthy. In a second trial, a different group of 60 equally naive observers “rated the photos with respect to health (scale from 1 ‘very poor’ to 7 ‘very good’), tiredness (scale from 1 ‘very alert’ to 7 ‘very tired’) and eight facial cues.” These eight facial cues were identified by experts as those most closely associated with sickness.

The observers correctly identified 13 out of 16 individuals as sick, indicating that even people with no knowledge of the facial cues that indicate someone’s sick can usually tell, after just a few seconds, whether someone is ill. This study is different from past studies on the topic that showed pictures of more obviously sick people in an effort to elicit and measure disgust reactions: In this case, the photos were taken a mere two hours after exposure to bacterial toxins, so the changes in people’s appearances were subtle.

“This supports the notion that humans have the ability to detect signs of illness in an early phase after exposure to infectious stimuli,” the study’s authors write. Rapid detection at early phases of infection would be beneficial since that is when people could be most contagious.

Human brains have developed to quickly judge faces for traits that convey emotional states such as aggression, so it’s not that surprising that we can make rapid judgments about sickness. However, a few of the characteristics they examined, the researchers point out, tend to send mixed signals.

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Pale Lips

The researchers note that pale lips are one of the most prominent changes that occur as a result of sickness. Surprisingly, observers did not rate pale lips as being strongly related to sickness.

Hanging Eyelids

As a part of this study, the researchers measured the overlap between facial features that make someone look sick and features that make them look tired. And while hanging eyelids are a sign that someone is sick, they’re also a sign that someone is tired … which can be a sign that they’re sick, but isn’t Since looking tired can be confused for being sick, the researchers point out that looking tired can cause people to avoid you, even if you’re not sick.

“This is backed up by the finding that subjects are less inclined to socialize with individuals who have got insufficient sleep,” they write.

Appearing More Tired

As an overarching characteristic, observers associate the appearance of tiredness with sickness. The researchers note that this conflation of people’s judgments of tiredness and illness could actually contribute to disability stigma since sadness could trigger this disease avoidance response. In other words, our built-in tendency to steer clear of sick people could also subconsciously discourage us from interacting with someone who looks sad.

This overlap, the researchers say, “suggests that perceived deviations from a healthy or functional state, based on cues that overlap between sickness and other conditions, perpetuate prejudices.”

Further study will help researchers understand why we give greater weight to some facial clues and less weight to others when evaluating whether someone looks sick, and perhaps we’ll be able to understand why we are so inconsistent. In the meantime, though, it’s best practice to just cover your darn mouth when you sneeze.

Abstract: Detection and avoidance of sick individuals have been proposed as essential components in a behavioural defence against disease, limiting the risk of contamination. However, almost no knowledge exists on whether humans can detect sick individuals, and if so by what cues. Here, we demonstrate that untrained people can identify sick individuals above chance level by looking at facial photos taken 2 h after injection with a bacterial stimulus inducing an immune response (2.0 ng kg−1 lipopolysaccharide) or placebo, the global sensitivity index being d′ = 0.405. Signal detection analysis (receiver operating characteristic curve area) showed an area of 0.62 (95% confidence intervals 0.60–0.63). Acutely sick people were rated by naive observers as having paler lips and skin, a more swollen face, droopier corners of the mouth, more hanging eyelids, redder eyes, and less glossy and patchy skin, as well as appearing more tired. Our findings suggest that facial cues associated with the skin, mouth and eyes can aid in the detection of acutely sick and potentially contagious people.

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