US scientists have investigated the makeup of the perfect smile, saying the findings could be useful for clinicians working to restore facial movement

If you want your smile to appear pleasant, you might want to avoid a dazzling beam, research suggests. A study by scientists in the US has found that wide smiles with a high angle and showing a lot of teeth are not the best at creating a positive impression.

“A lot of people don’t understand how important their smiles are and how important this aspect of communication we do with each other every day is,” said Stephen Guy, a co-author of the research from the University of Minnesota. The authors say the findings could prove valuable for clinicians working to restore facial movement and expression to those who have experienced facial paralysis.

“When you have different surgical options, how do you choose which one is better?” Guy said, pointing out that some options might offer more extent of smile – referring to breadth – but others might improve the angle. “In order to do that, you need to say, ‘Oh, this smile is better or worse than that smile.’”

Facebook Twitter Pinterest A heat-map plotting the three-way interaction between the smile parameters. The three vertical bars behind each face denote the predicted score for the three response variables: effective, genuine, and pleasant. Greener colours correspond to better (ie higher-rated) smiles, redder colours to worse. Photograph: University of Minnesota

To find the perfect smile, the team showed a 3D, computer-animated virtual face smiling in a range of different ways to 802 members of the public, ranging in age from 18 to 82. All had consumed fewer than six alcoholic drinks – the study was carried out at the Minnesota state fair.

Each animation ran for 250 milliseconds and the faces showed differences in the angle of the smile, how broad it was, and the amount that teeth on show. In addition, the team took one smile – featuring a high angle, low extent and medium amount of dental show – and tinkered with the symmetry of the smile, changing the length of time it took the left side of the face to smile compared with the right.

Participants were shown a number of the animations, selected at random, and asked to rate the smiles on a series of scales, from “fake” to “genuine”, and “creepy” to “pleasant”. Participants also rated emotional intent and effectiveness as a smile for each face.

The results, published in the journal Plos One, reveal that all of the different factors appeared to influence the ratings given by the participants. From analysing data from 27 symmetric smiles, the authors say the best smiles on average have mouth angles between 13 degrees and 17 degrees, and a width between 55% and 62% of the distance between the pupils of the eyes.

“While this is the sweet spot that we found, it is important to be clear that there is not just one really, really good smile,” said Guy. “Just because your smile is different … doesn’t mean it is in any way bad.”



The effect of having teeth on show was found to depend on how broad the smile was, and the angle of the mouth. However, unsuccessful smiles included those that had simultaneously high angles, were very broad, and had plenty of pearly whites on show, while smiles of small extent and small angles were the most likely to be thought to show “contempt”.

The team say that, together, the explored effects account for 40% of the variation in ratings. “About half [the variation] is really coming from factors that we are not modelling, maybe cultural differences in different people who are taking our studies, or a different number of drinks, or different opinions of what is good or bad,” said Guy.

While a slight asymmetry was found to yield higher ratings than a symmetrical smile, the team found ratings were lower if the delay extended beyond 125ms.



Guy said the research could also prove useful in helping to make characters in video games seem more realistic and natural. But the team admit that the study has limitations, not least that it only models the impact of the mouth on the perception of a smile, while previous work has shown that the eyes are also crucial.



“The context of your whole face can be important and, in general, a larger context of what the conversation is, and your body’s disposition can be important too,” Guy said. “Those are important limitations that we should look at in the future, and other researchers as well.”



Rachel Jack, a psychologist at the University of Glasgow, welcomed the research. “Most research has been based on static images, which omits potentially important information about facial dynamics,” she said. “This study is among a new generation of work that aims to precisely understand how the dynamic features of the same face movement, such as a smile, can change our perception of it.”