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The Influence of Eye-Lip Interaction on Facial Attractiveness and Orthodontic Treatment Willingness: A Cross-Sectional Study

This cross-sectional study demonstrates that larger eyes significantly increase observers' tolerance for lip protrusion and reduce the perceived necessity for orthodontic treatment, with laypersons showing greater sensitivity to this eye-lip interaction than dental professionals.

Original authors: Baodong Hao, Mingjin Zhang, Liang Lyu, Tingting Yu, Jiajing Li, Runzhi Guo, Dawei Liu

Published 2026-06-28

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CC BY 4.0

Original authors: Baodong Hao, Mingjin Zhang, Liang Lyu, Tingting Yu, Jiajing Li, Runzhi Guo, Dawei Liu

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine your face is a painting. For a long time, orthodontists (the artists who fix teeth and jaws) have been obsessed with one specific corner of that painting: the bottom third, specifically the lips. They've asked, "Are the lips sticking out too much? Do they need to be pushed back?"

But this study suggests that looking at the lips in isolation is like judging a painting by only looking at the bottom edge. You're missing the most important part of the picture: the eyes.

Here is what the researchers found, broken down simply:

The Experiment: A Digital "Face Factory"

The researchers didn't just take photos of real people. Instead, they built a perfect, average Chinese face using 3D computer software (think of it like a high-tech video game character creator).

They took this "average" face and started tweaking two things:

The Eyes: They made them small, medium, or large (like changing the size of a window).
The Lips: They moved the lips forward and backward, from very tucked-in to very sticking out.

This created 27 different versions of the same face. Then, they showed these faces to three groups of people:

The Pros: Experienced orthodontists.
The Trainees: Dental residents (students).
The Regulars: Everyday people (laypersons) with no dental training.

Everyone was asked two questions: "How attractive is this face?" and "Does this person need braces to fix their lips?"

The Big Discovery: The "Eye-Lip" Trade-Off

The study found a fascinating relationship, like a seesaw between the eyes and the lips.

The Analogy: Think of the eyes as a spotlight.

If you have small eyes, the spotlight is dim. If your lips are sticking out even a little, they look like a huge, distracting problem. The face looks unbalanced.
If you have large, bright eyes, the spotlight is huge and bright. Suddenly, the lips can stick out a bit more, and they don't look like a mistake anymore. They look like they belong there.

What the Data Said:

Larger eyes = More tolerance for big lips. When the eyes were bigger, people thought the lips could stick out further and still look attractive.
Smaller eyes = Less tolerance. When the eyes were smaller, even slightly protruding lips were seen as unattractive.

The "Professional Gap"

Here is where it gets interesting regarding who is looking at the face:

The Regular People (Laypersons): They were the most sensitive to this eye-lip connection. If a face had big eyes, regular people were very forgiving of protruding lips. They saw the "whole picture" and felt the balance.
The Orthodontists: They were much stricter. They focused heavily on the exact position of the lips, almost ignoring the eyes. They were less likely to say, "Oh, the eyes are big, so the lips are fine." They wanted the lips to be in a specific, flatter position regardless of the eyes.
The Trainees: They fell right in the middle, acting like a bridge between the pros and the regular people.

The Treatment Decision

Because of this "spotlight effect," the decision to recommend braces changed based on the eyes:

When the eyes were larger, people were less likely to say, "This person needs braces to fix their lips."
When the eyes were smaller, people were more likely to say, "Yes, fix those lips."

Specifically, the study found that for every small increase in eye size, the chance of someone recommending treatment for protruding lips dropped by about 8%.

The Bottom Line

The paper concludes that beauty isn't just about one feature; it's about how features talk to each other.

Big eyes make big lips look okay. They increase our tolerance for lips that stick out.
Regular people notice this connection more than doctors do. Doctors are trained to look at the lips like a ruler, while regular people look at the face like a whole painting.
Context matters. If you have big eyes, you might not need your lips pulled back as much to look attractive, because your eyes are doing the heavy lifting in the "attractiveness" department.

The study suggests that when doctors and patients talk about treatment, they need to remember that the eyes change the rules for the lips. What looks like a "problem" to a doctor might look perfectly balanced to a patient with big eyes.

Technical Summary: The Influence of Eye-Lip Interaction on Facial Attractiveness and Orthodontic Treatment Willingness

Problem Statement
Facial attractiveness is a primary motivator for seeking orthodontic treatment, with lip protrusion traditionally serving as a central diagnostic and planning consideration. While the relationship between lip position and the lower third of the face (e.g., nose-lip-chin relationships) is well-documented, the holistic perception of facial aesthetics often overlooks the "middle third," specifically the eyes. Although larger eyes are independently associated with higher perceived attractiveness, the manner in which eye size interacts with lip protrusion to influence overall facial harmony and the willingness to undergo corrective treatment remains poorly understood. This study addresses the gap in knowledge regarding whether eye-lip interaction modulates aesthetic judgments and treatment recommendations for lip protrusion.

Methodology
The study employed a cross-sectional design utilizing a three-dimensional (3D) standard morphable facial model constructed in Maya software based on normative data of the Chinese Han adult population.

Model Construction: A baseline model with perfect bilateral symmetry was created to match mean craniofacial dimensions. Two key parameters were quantitatively morphed:
- Eye Size: Defined by palpebral fissure height, modified to three levels (10.0 mm, 12.0 mm, and 14.0 mm).
- Lip Protrusion: Quantified by the distance from the lower lip (Li) to the E-line (Li-PrnPog'), modified to nine levels ranging from −8 mm (retrusion) to +8 mm (protrusion) in 2.0 mm increments.
- This generated 27 unique facial variations, each presented in frontal, 45°, and lateral views, plus a GIF.
Participants: A total of 108 observers were recruited and categorized into three groups: 35 orthodontists (>5 years experience), 37 dental residents (<5 years experience), and 36 laypersons.
Procedure: Observers rated the facial attractiveness of each model on a 100-point Visual Analog Scale (VAS) and indicated whether orthodontic treatment to reduce lip protrusion was necessary. The presentation order was counterbalanced using a Latin-square design to mitigate sequence effects.
Statistical Analysis: Reliability was assessed via Cronbach's alpha and test-retest correlations. Multivariate linear regression was used to analyze aesthetic ratings, while multivariate logistic regression assessed treatment recommendations. Stepwise regression controlled for confounding variables, and ANOVA with Bonferroni post-hoc tests examined group differences.

Key Results

Interaction Effects: A significant interaction effect was found between eye size and lip protrusion on facial attractiveness ( $\beta = 0.62, P < 0.01$ ). As eye size increased, the peak of the parabolic curve representing aesthetic scores shifted rightward, indicating a preference for more protrusive lips.
Treatment Recommendations: Larger eye size was associated with a reduced likelihood of recommending orthodontic treatment. Specifically, a 2 mm increase in palpebral fissure height corresponded to an 8% reduction in the odds of selecting treatment ($OR = 0.92, P < 0.01$).
Observer Group Differences:
- Sensitivity: Laypersons demonstrated significantly higher sensitivity to the eye-lip interaction compared to both orthodontists and residents ( $P < 0.01$ ).
- Professional Bias: Orthodontists showed greater intra-group consistency and were generally stricter in their evaluation of lip protrusion, requiring less protrusion to deem it "acceptable" compared to laypersons. Residents exhibited an intermediate pattern.
- Demographics: Female observers, older observers, and those with self-reported willingness to undergo treatment showed greater tolerance for lip protrusion.

Key Contributions

Holistic Aesthetic Modeling: This is the first study to utilize a standardized 3D morphable model based on Chinese population norms to systematically quantify the interaction between eye size and lip protrusion.
Quantification of Interaction: The study provides empirical evidence that eye size acts as a modulator for the optimal lip position, shifting the aesthetic "sweet spot" toward more protrusive lips as eyes become larger.
Perceptual Gap Identification: The research highlights a significant discrepancy between professional (orthodontist) and layperson perceptions, particularly regarding the influence of non-lip features (eyes) on the necessity of treatment.

Significance and Claims
The authors claim that eye-lip interaction plays a notable role in the perception of lip protrusion and should be integrated into patient-clinician communication and treatment planning. The study suggests that clinicians must recognize that larger eyes may increase a patient's tolerance for lip protrusion, potentially reducing their willingness to seek treatment. Consequently, treatment goals should be discussed collaboratively, acknowledging that professional assessments of lip position may not align with patient preferences driven by holistic facial harmony. The authors emphasize that while their findings are specific to the Chinese Han population and a female average model, they underscore the necessity of considering facial component interactions rather than isolated features in orthodontic aesthetics.

Limitations
The study acknowledges several limitations: the use of a single female average model (excluding male or other ethnic variations), a restricted observer age range (18–52 years), and the assessment of only one eye parameter (palpebral fissure height) rather than a comprehensive set of ocular morphological features. Subgroup analyses were noted as exploratory due to sample size calculations based on the primary regression analysis.