In addition to improving various oral functions such as occlusion, pronunciation, and respiration, an important goal [1,2] of orthodontic treatment is to improve the esthetics of the mouth, particularly the smile, which is important for communication [3]. In the field of prosthodontics, factors known to influence the attractiveness [4] of the smile include the shape of the maxillary teeth, as well as the width/length ratio [5], and color of each tooth. In the field of orthodontics, the smile arc [1,6
-10], buccal corridor [9,11
-14], tooth and gingival exposure [1,6,13,15,16], and position of the incisors [3,15, 17
-19] are all factors that can increase the attractiveness of the smile.

However, these reports mainly involve subjects of Euro-pean and American nationalities, and there is little research on comparisons of perceived smile attractiveness and consciousness among different nationalities. In 2010, Funaki [20] compared mouth shapes between 96 Japanese and 44 Caucasian individuals and found a significant difference in the types of mouth shape that were considered attractive. These results suggest the need for further comparisons between countries. The present study compared smile attractiveness and smile consciousness among Japanese, Americans and Indonesian women from diverse cultural and anthropological back-grounds.

1. Subjects

The respondents were female patients or their female guardians (aged 15-50 years) from Japan, the U.S.A., and Indonesia who visited orthodontic clinics. In total, 262 participants (mean age, 25.9±7.7 years) responded to the survey, including 109 Japanese participants, 70 Indonesian participants, and 83 American participants (Table 1). All participants provided prior informed consent, and the study was approved by the Research Ethics Committee of the Faculty of Dentistry, Tokyo Medical and Dental University (No. D2016-065), according to the Declaration of Helsinki.

Table 1 . Ages and occupations of the subjects by country

	Japan (n=109)	U.S.A. (n=83)	Indonesia (n=70)	p-value
Age (yr)	25.3±8.4	27.9±8.1	24.5±5.4	0.015*
Occupations				0.007*
No/Student	69 (63.3)	51 (61.4)	58 (82.9)
Yes (full-time/part-time)	40 (36.7)	32 (38.6)	12 (17.1)

Values are presented as mean±standard deviation or number (%). *p< 0.05.

2. Questionnaire

There were two possible answers to the question regarding occupation: “No/Student” and “Yes (full-time/part-time).”

The questionnaire used frontal posed smile photographs [21]. Smile patterns [22] (Figure 1) were classified according to mouth shape: type A (V shape), in which the mouth corners are higher than the center of the upper lip (M point); type B (cup shape), in which the mouth corners are aligned with the M point; type C (oval shape), in which the mouth corners are lower than the M point and higher than the center of the lower lip (N point); and type D (cap shape), in which the mouth corners are aligned with the N point. In addition, the smiles were categorized in terms of tooth and gingival exposure: type 1 was a smile in which the front teeth and gingiva of the maxilla were visible; type 2 was a smile in which the maxillary and mandibular front teeth were visible; and type 3 was a smile in which the front teeth and the gingiva of the mandible were visible.

Figure 1. Twelve types of smile patterns.

Thus, a total of 12 smile patterns (A-1 to D-3) were presented to the respondents. The attractiveness of these patterns was rated as “most attractive” (3 points), “attractive” (2 points), “neutral,” (1 point), and “unattractive” (0 points). A comparative analysis of perceived attractiveness was conducted using the mean values for each mouth corner position (A-D) and for each tooth and gingival exposure type (1-3).

Smile consciousness was assessed by the question “Do you have confidence in your smile?” The percentages of “Yes” and “No” responses to the question were examined. Participants could selected multiple reasons for the “No” response including (1) tooth alignment, (2) tooth color, (3) tooth shape, (4) mouth corner position, (5) gingival exposure, and (6) others.

3. Statistical analysis

One-way analysis of variance (ANOVA) was used to evaluate the differences in age among the survey respondents of different nationalities, and a Pearson chi-squared test was used to evaluate the differences in employment status. ANOVA was used to compare the perceived attractiveness of the 12 types of smiles among the countries. In addition, the mouth- corner positions and the exposure of the front teeth and gingiva were compared among the countries using analysis of covariance (ANCOVA) to adjust for age and occupation. Pearson’s chi-squared test was used to compare smile consciousness among the three countries. IBM SPSS Statistics ver. 20.0 (IBM Japan, Tokyo, Japan) was used for statistical analyses, with the significance level set at p<0.05.

1. Basic subject information

The ages and occupations of all respondents are shown in Table 1. Participant age significantly differed among the three countries, with Americans being the oldest (mean age, 27.9±8.1 years) and Indonesians the youngest (mean age, 24.5±5.4 years). The occupation response “No/Student” was significantly more common among Indonesian respondents than among Japanese and American respondents.

2. Three-country comparison of mouth shape when smiling

The attractiveness scores for the following smile patterns exhibited significant differences among the three countries: A-1, A-2, A-3, B-1, B-3, D-1, and D-2. The scores for patterns A-1, A-2, A-3, B-1, and B-3 were the highest in Japan and lowest in Indonesia, while a significantly higher number of American participants classified the patterns D-1 and D-2 as unattractive (Table 2).

Table 2 . Attractiveness (smile) scores of 12 types of smile patterns by country

	Japan (n=109)	U.S.A. (n=83)	Indonesia (n=70)	p-value
A-1	1.84±0.93	1.07±0.87	0.67±0.70	<0.001*
A-2	1.82±0.92	1.16±0.94	0.76±0.79	<0.001*
A-3	0.50±0.65	0.30±0.60	0.03±0.24	<0.001*
B-1	1.61±0.87	1.39±0.88	1.20±0.71	0.006*
B-2	1.42±0.81	1.30±0.91	1.20±0.73	0.204
B-3	0.28±0.55	0.19±0.43	0.03±0.17	0.001*
C-1	0.97±0.92	0.81±0.88	0.66±0.68	0.050
C-2	0.70±0.81	0.75±0.75	0.51±0.63	0.133
C-3	0.16±0.45	0.18±0.42	0.10±0.35	0.478
D-1	0.34±0.71	0.42±0.61	0.13±0.38	0.010*
D-2	0.28±0.64	0.54±0.70	0.31±0.67	0.022*
D-3	0.14±0.52	0.11±0.31	0.03±0.17	0.181

Values are presented as mean±standard deviation. *p<0.05.

3. Mouth corner position and tooth and gingival exposure

In Japan, the V-shaped smile (A type: A-1, A-2, A-3) and cup-shaped smile (B type: B-1, B-2, B-3) showed significantly higher scores than in the U.S.A. and Indonesia, in that order.

The cap-shaped smile (D type: D-1, D-2, D-3) had a significantly higher score in the U.S.A., followed by Japan and then Indonesia. The order of attractiveness by mouth corner position was A, B, C, and D in Japan, and B, A, C, and D in the U.S.A. and Indonesia.

With respect to the tooth and gingival exposure of the front teeth, type 1, type 2, and type 3 smiles all scored significantly higher in Japanese subjects, followed by Americans and then Indonesians. The scores for the front teeth and gingival exposure were, in decreasing order, type 1, type 2, and type 3 in Japan, and type 2, type 1, and type 3 in both the U.S.A. and Indonesia (Table 3).

Table 3 . Attractiveness (smile) scores of mouth corner position and tooth and gums visible classification by country

	Japan (n=109)	U.S.A. (n=83)	Indonesia (n=70)	p-value
Mouth corners position
A	4.17±1.70	2.53±1.92	1.46±1.34	<0.001*
B	3.31±1.43	2.88±1.60	2.43±1.23	<0.001*
C	1.83±1.63	1.73±1.60	1.27±1.31	0.054
D	0.76±1.62	1.07±1.31	0.47±0.96	0.027*
Tooth and gums
1	4.76±2.35	3.69±2.19	2.66±1.68	<0.001*
2	4.22±1.89	3.75±2.24	2.79±2.18	<0.001*
3	1.08±1.55	0.78±1.35	0.19±0.64	<0.001*

Values are presented as mean±standard deviation.*p<0.05.

4. ANCOVA for comparison of mouth corner position and tooth and gingival exposure

Figure 2 shows the ANCOVA results for mouth corner position and tooth and gingival exposure, adjusted for age and occupation. The score for type A in Japan was significantly higher than that in the other two countries, and those for types B and C were significantly higher than those in Indonesia. The score for type D in the U.S.A. was significantly higher than that in Indonesia. Furthermore, the score for type 1 in Japan was significantly higher than that in the other two countries. Types 2 and 3 showed significantly lower scores in Indonesia than in Japan and in the U.S.A.

Figure 2. Attractiveness smile scores compared among Japan, the U.S.A., and Indonesia for each mouth shape and tooth and gingival visibility classification (6=very attractive, 0= unattractive). Statistical analysis was performed using ANCOVA. *p<0.05, **p<0.01.

5. Smile consciousness

Table 4 shows the responses for smile consciousness in each country. Significantly more respondents in Japan covered their mouth when smiling and lacked confidence in their smile. In the assessment of the reasons for the lack of confidence in smiles, the three countries showed significant differences in the number of responses mentioning “tooth color,” “tooth alignment,” and “mouth corner position.” The frequency of each of these responses was significantly higher in Japan than in the other two countries. The most common reasons were “tooth alignment” in Japan, “tooth color” in the U.S.A., and “tooth alignment” and “gingival exposure” in Indonesia.

Table 4 . Percentage of smile consciousness by country

	Japan (n=109)	U.S.A. (n=83)	Indonesia (n=70)	p-value
Gesture of covering the mouth with the hand	55 (50.5)	13 (15.7)	13 (18.6)	<0.001*
Reasons of lacking confidence	79 (72.5)	14 (16.9)	11 (15.7)	<0.001*
(1) Tooth alignment	50 (45.9)	3 (3.6)	7 (10.0)	<0.001*
(2) Tooth color	27 (24.8)	7 (8.4)	1 (1.4)	<0.001*
(3) Tooth shape	12 (11.0)	2 (2.4)	3 (4.3)	0.039*
(4) Mouth corners position	23 (21.1)	1 (1.2)	1 (1.4)	<0.001*
(5) Visibility of gums	13 (11.9)	4 (4.8)	7 (10.0)	0.230
(6) Color of gums	1 (0.9)	0 (0.0)	0 (0.0)	0.494
(7) Halitosis	5 (4.6)	1 (1.2)	0 (0.0)	0.098
(8) Others	4 (3.7)	6 (7.2)	0 (0.0)	0.067
Goodness of smile for mind and body	108 (99.1)	80 (96.4)	68 (97.1)	0.434

Values are presented as number (%). *p<0.05.

The results of this study showed clear differences in smile pattern preferences and smile consciousness among the three countries. A V-shaped smile with lifted mouth corners (type A) was preferred in Japan, suggesting that this preference is a characteristic of Japanese individuals. This may have been influenced by the fact that Japanese magazines and books often cite lifted mouth corners as an essential factor in the ideal attractive smile [23]. With regard to gingival and tooth exposure, smiles in which the maxillary teeth could be seen (type 1) had significantly higher attractiveness in Japan than in the other two countries. We surmise that type 1 (i.e., visible maxillary front teeth) and 2 smiles (i.e., visible maxillary and mandibular front teeth) are preferred in Japan because they are difficult to achieve without lifting the corners of the mouth.

Meanwhile, the cup-shaped smile (type B), in which the right and left corners of the mouth are lifted in a flat manner, was preferred in the U.S.A. and Indonesia. Previous studies in the U.S.A. have found that the requirements for a beautiful smile include (1) clearly visible front teeth and a smile arc [6
-9,11], or a smile line with continuity, (2) small buccal corridor [9,11
-14], and (3) maxillary gingival exposure [1,6,13,15, 16]. However, our study suggests that tooth and gingival exposure do not influence smile attractiveness in the U.S.A. as much as in Japan and Indonesia.

In all three countries, the type D smile (cap shape), in which the corners of the mouth are in a lower position and aligned with the center of the lower lip, was perceived as the least attractive. In Japan, the mouth turned down at the corners has traditionally been considered to show dissatisfaction. In the U.S.A., a low lip line smile is defined by coverage of the maxillary teeth with the upper lip, while the corners of the mouth do not drop and are considered unattractive. Indeed, Ameri-cans may not consider this mouth shape to be a smile at all. This type of smile is considered markedly unattractive in Indonesia as well, suggesting that Indonesians, like Ameri-cans, may not consider such a shape to be a smile.

There were major differences in smile consciousness among the three countries. The percentage of respondents who smiled with a hand covering their mouth in Japan was significantly higher than in the other two countries, and the percentage of respondents with confidence in their smile was significantly lower. Moreover, as seen in the woodblock prints of Japanese people from the Edo period (1603-1868), the gesture of using a hand to cover the mouth was used by married women to hide the black pigment that they painted on their teeth. Therefore, this was seen to reflect a kind of shy elegance [24]. In addition, in comparison with the participants from the other two countries, a significantly higher number of people in Japan lacked confidence in their tooth alignment, tooth color, and tooth shape.

Conversely, the percentage of Americans with confidence in their smiles was almost the same as that of Indonesians. Among Americans, the primary reasons for a lack of confidence in their smiles were tooth color (8.4%) and tooth alignment (3.6%), a trend that contrasted with the findings from the other two countries. This may be because many people in the U.S.A. have good tooth alignment, since orthodontic treatment is widely available. Americans frequently cited tooth color as a reason for lack of confidence, probably because beautiful, healthy-looking white teeth are more highly valued in the U.S.A.

Smiling with a hand covering the mouth was significantly less common in Indonesia than in Japan, and the percentage of Indonesian respondents with confidence in their smile was higher than that in the U.S.A. Tooth alignment (10.0%) and gingival exposure (10.0%) were the most frequently cited reasons for lack of confidence among Indonesian respondents, while “tooth color” was cited by a lower percentage of re-spondents. Indonesians’ confidence in their smiles may be attributable to the influence of the country’s warm weather on their personality [8] and friendly nature [25,26].

This study suggests that there are significant differences among the three countries in terms of both the perceived attractiveness of various smile patterns and smile con-sciousness. Cultural and anthropological factors, such as race, religion, history, and national character, are likely to influence these findings. The limitations of this study include the small sample size and lack of information about oral conditions and facial patterns from the questionnaire.

In conclusion, there were major differences in the opinions on attractive smile patterns and smile consciousness among Japanese, American, and Indonesian respondents. Ortho-dontists should ensure that they have a deeper understanding of attractive smile patterns and consciousness.

No potential conflict of interest relevant to this article was reported.