1. A Smile Is a Team Effort Between the Cheeks and the Eyes
When you smile, two muscles do most of the work. The zygomatic major in the cheek pulls the corners of the mouth upward. The orbicularis oculi, a ring of muscle around the eye socket, squeezes the outer corners of the eyes into the little creases sometimes called crow's feet. [1]
Psychologists call a smile that uses both muscles a "Duchenne smile," after Guillaume Duchenne, a French anatomist who studied expressions by stimulating facial muscles with electric current. In an 1862 book, he wrote that people can switch on the cheek muscle at will, but that the eye muscle moves only when a person feels what he called the "sweet emotions of the soul." [1]
That idea has held up fairly well, with a few warnings. A review of smile research describes the Duchenne smile as a better sign of enjoyment than other kinds of smiles, not a perfect one. Smiles can also show embarrassment, deceit, or grief, and how a smile is read depends a lot on the situation. [1] So "the eyes are smiling" is a good clue about a smile, but it is not a lie detector.
2. Athletes Who Were Born Blind Make the Same Faces as Everyone Else
Here is the puzzle. If you have never seen a smile, how would you know how to make one? To get at this, psychologist David Matsumoto of San Francisco State University studied photographs from the 2004 Olympic and Paralympic judo competitions. The collection had more than 4,800 photos of athletes from 23 countries, including athletes who were born blind and athletes who could see. [2]
The expressions of blind and sighted athletes after winning or losing were almost the same. Matsumoto said the statistical match between sighted and blind athletes' expressions was "almost perfect," and he took it as a sign that something built into us is a source of facial expressions of emotion. [2] It is hard to explain these faces as copied by watching others, so at least part of the link between feeling and face seems to be inborn.
"At least part" matters. Blind children can still learn from voices, touch, and the way family members react, so this study does not prove that every detail of a smile is inherited. It supports an inborn starting point, not the whole story.
The photos also showed something else. At the medal ceremony, 85 percent of the silver medalists, who had just lost their final match, produced "social smiles" that used only the mouth muscles and not the eyes. This was true for blind and sighted athletes alike. [2] A face is not only a window onto a feeling. People also steer it, even when they cannot see anyone else do the same.
3. Can a Smile Change Your Mood? A Famous Pen Experiment Was Repeated in 17 Labs
So far the arrow pointed from feeling to face. The next question asks about the reverse: can the way you move your face affect how you feel? This idea is called the facial feedback hypothesis. Its best-known test is a 1988 experiment by Fritz Strack, Leonard Martin, and Sabine Stepper. People held a pen in their teeth, which forces the mouth into a smile-like shape without the person knowing it, and then rated how funny cartoons were. In the original study, the pen-in-teeth group rated the cartoons about 0.82 points funnier than the comparison group did, on a 10-point scale. [3]
Scientists rarely trust one result on its own, so in 2016, 17 labs ran the same experiment using the same plan, which they had agreed on and published before collecting any data. Added together, the difference came out at 0.03 points, with a 95% confidence range from −0.11 to 0.16. [3] The range includes zero, so the repeat did not find a clear effect. It also does not prove that no effect exists.
This does not mean the original researchers did something dishonest. It means that when the experiment was repeated, the result did not show up again. Sorting out why is part of how science corrects itself.
4. Big Studies Say the Effect Is Small and Depends on the Method
Because a single repeat is not the last word either, researchers looked at all the evidence together. In 2019, a team led by Nicholas Coles combined 138 studies with more than 11,000 participants. They found that facial expressions have a small effect on feelings: smiling makes people feel a little happier, and scowling a little angrier. Coles was careful to add, "We don't think that people can smile their way to happiness." [4]
In 2022, the Many Smiles Collaboration tested the idea again. According to the paper's abstract, the study included 3,878 participants in 19 countries and was planned in advance. Tasks that asked people to copy smiling faces or to move their facial muscles on purpose both increased happiness. When the smile was created indirectly, as with a pen in the mouth, the evidence was less conclusive. [5]
Taken together, a careful summary is this: smiling can nudge your mood a little, sometimes, depending on how the smile is produced. The nudge is not large and it is not guaranteed. Headlines such as "Fake smiling makes you happy" claim more than these studies show. A mood does not simply switch on because the face does.
5. How to Observe Your Own Smile
Stand in front of a mirror and make a smile on purpose. Then think of something that really made you laugh, and let a real smile come. Compare the skin around the corners of your eyes in the two cases. You may see a difference, or you may not. Even researchers find it hard to judge a smile from the eyes alone, so a "no difference" result is still a fair observation.
To read more, search for "facial feedback hypothesis" or "Duchenne smile." The 2016 replication report [3] and the Many Smiles paper [5] are good places to see how scientists check each other's work.