What Makes a Smile Attractive? Vietnamese Study Maps the Facial Blueprint

A smile is one of the most instantly recognizable expressions in human communication, yet the precise anatomy behind a beautiful smile has remained surprisingly difficult to quantify. A new study published in PLOS One has taken a systematic look at the problem, recording and analyzing the dynamic smiles of 490 young Vietnamese adults to determine which facial and oral parameters actually shape how a smile looks and how attractive it is perceived to be. The research, led by Thuy Anh Vu Pham, Phong Phu Le, and Vu Pham Phi Nguyen, offers one of the most detailed portraits to date of smile morphology in a Southeast Asian population, a group that has been underrepresented in the orthodontic and aesthetic dentistry literature.

The study focused on young adults with a mean age of just over 21 years, an age range in which facial growth is complete and the dentition is typically stable. Rather than relying on static photographs, which can flatten or distort the nuances of expression, the researchers recorded dynamic smiles, capturing the full motion of the lips, teeth, and surrounding soft tissues as participants smiled naturally. This dynamic approach matters because a smile is not a fixed structure; it is a movement, and the way tissues travel during that movement can change which teeth are visible, how the lips curve, and how much of the gingiva and buccal corridor is exposed.

To characterize each smile, the team assessed a battery of categorical features that orthodontists have long used to describe smile aesthetics. These included the anterior smile line, which describes the vertical relationship between the curvature of the upper lip and the incisal edges of the upper front teeth; the smile arc, which concerns whether the curvature of the upper teeth follows or diverges from the curve of the lower lip; upper lip curvature, which can be straight, upward, or downward; the most posterior tooth displayed during the smile; and the relationship between the upper teeth and the lower lip. Each of these categories captures a different dimension of the smile, and together they provide a structured vocabulary for what is often described only subjectively.

Beyond these categorical descriptors, the researchers measured quantitative parameters that lend themselves to statistical analysis. The buccal corridor space, the dark triangular region between the corners of the mouth and the outer surfaces of the upper back teeth, has long been debated in aesthetic dentistry, with some clinicians arguing that wide corridors detract from a smile and others finding little effect. The smile index, calculated as the ratio of smile width to smile height, condenses the overall geometry of the expression into a single number. The team also quantified dynamic smile symmetry, tracking whether the two halves of the face move comparably during the expression, a feature that is difficult to judge from a single photograph.

The study went further by pairing these smile-specific measures with a broad set of facial and dentofacial measurements taken from the same participants. These included upper lip length, lip mobility, philtrum height, proportional indices of the face, the cant of the lip line and occlusal plane relative to the horizontal, dental midline deviation, mouth size ratio, and incisal edge visibility, the amount of upper front tooth structure that shows when the lips are at rest or during speaking. By relating the smile variables to this wider anatomical context, the researchers could ask which underlying structures actually drive the visible characteristics of the smile, rather than merely cataloging the smiles themselves.

The results revealed that three of the categorical smile features, the anterior smile line, the smile arc, and upper lip curvature, showed broad associations with several dentofacial parameters. In other words, the way the upper lip curves and the way the smile line is positioned relative to the teeth are not isolated quirks of soft tissue; they are systematically linked to the proportions of the underlying facial skeleton and dentition. This finding supports the clinical intuition that altering tooth position or lip support through orthodontics or restorative dentistry can meaningfully change the overall character of a smile, because these elements are mechanically and anatomically intertwined.

Other relationships were more specific. The most posterior tooth displayed during the smile, essentially how far back into the mouth the visible teeth extend, was significantly related to the ratio of philtrum height to upper lip length and to incisal edge visibility. This suggests that a patient whose philtrum is short relative to the upper lip, or who shows more of the upper incisors at rest, is likely to display more posterior teeth when smiling, a detail that matters when clinicians plan how far distally to extend aesthetic restorations or how to manage vertical tooth position. By contrast, the buccal corridor space and the smile index showed comparatively limited correlations with the measured facial and dental variables, hinting that these features may be governed more by individual muscular patterns than by skeletal or dental anatomy alone.

Dynamic smile symmetry, a feature that many patients and clinicians intuitively value, was significantly associated with upper lip length during smiling and with the mouth size ratio at rest. This is a notable result because it ties a movement-based property of the smile to measurable static anatomy. Individuals with particular lip lengths and mouth proportions appear predisposed to more or less symmetric smiles, which raises the possibility that some aspects of smile symmetry could be anticipated, and potentially addressed, before treatment even begins. For orthodontists and surgeons planning interventions around the mouth, such predictive relationships are valuable because they shift the conversation from correcting visible asymmetry after the fact to understanding its anatomical origins in advance.

Perhaps the most clinically consequential part of the study was its analysis of smile attractiveness. Participants were classified according to how attractive their smiles were judged to be, and the researchers then identified the soft tissue and dentofacial variables that contributed to those classifications. Several such determinants emerged, confirming that attractiveness is not reducible to a single ideal measurement but instead reflects a coordinated interaction between soft tissue dynamics, such as lip length, mobility, and curvature, and dentofacial proportions, including tooth display and facial ratios. This multivariate picture aligns with a growing consensus in aesthetic dentistry that beautiful smiles arise from harmony among many components rather than from any single parameter hitting a textbook value.

The population-specific dimension of the findings deserves emphasis. Much of the existing guidance on smile aesthetics derives from studies of Western populations, and norms derived from one ancestry or facial morphology may not transfer cleanly to another. By documenting the smile characteristics of Vietnamese young adults in detail, the study provides reference data that clinicians working with similar populations can use for aesthetic evaluation and treatment planning in orthodontic and restorative practice. At the same time, the study’s methodological approach, combining dynamic smile recording with a comprehensive set of facial measurements, offers a template that researchers elsewhere can adapt, moving the field toward a more quantitative, evidence-based understanding of what makes a smile not just healthy, but genuinely appealing.

Subject of Research: Smile morphology and its facial and oral determinants in Vietnamese young adults

Article Title: Facial and oral factors affecting smiles in young people

Article References: Pham, T. A. V., Le, P. P., & Nguyen, V. P. P. (2026). Facial and oral factors affecting smiles in young people. PLOS One, 21(10), e0360529. https://doi.org/10.1371/journal.pone.0360529

Image Credits: AI Generated

DOI: 10.1371/journal.pone.0360529

Keywords: smile aesthetics, smile morphology, orthodontics, dentofacial parameters, buccal corridor, smile arc, lip curvature, dynamic smile symmetry, Vietnamese adults, PLOS One, aesthetic dentistry, incisal edge visibility