Skin coloration is a culturally-specific cue for attractiveness, healthiness, and youthfulness in observers of Chinese and western European descent.

Facial skin coloration signals information about an individual and plays an important role in social interactions and mate choice, due its putative association with health, attractiveness, and age. Whether skin coloration as an evolutionary significant cue is universal or specific to a particular culture is unclear and current evidence on the universality of skin color as a cue to health and attractiveness are equivocal. The current study used 80 calibrated, high-resolution, non-manipulated images of real human faces, either of Chinese or western European descent, which were rated in terms of attractiveness, healthiness, and perceived age by 44 observers, 22 western European (13 male; mean age ± SD = 24.27 ± 5.30) and 22 Chinese (7 male; mean age ± SD = 26.05 ± 3.96) observers. To elucidate the associations between skin coloration and these perceptual ratings and whether these associations are modulated by observer or image ethnicity, a linear mixed-effect model was setup with skin lightness (L*; CIELAB), redness (a*) and yellowness (b*), observer and image ethnicity as independent variables and perceived attractiveness, healthiness, and estimated age as dependent variables. We found robust positive associations between facial skin lightness (L*) and attractiveness, healthiness, and youthfulness, but only when Chinese observers judge facial images of their own ethnicity. Observers of European descent, on the other hand, associated an increase in yellowness(b*) with greater attractiveness and healthiness in Chinese facial images. We find no evidence that facial redness is positively associated with these attributes; instead, an increase in redness (a*) is associated with an increase in the estimated age of European facial images. We conclude that observers of both ethnicities make use of skin color and lightness to rate attractiveness, healthiness, and perceived age, but to a lesser degree than previously thought. Furthermore, these coloration cues are not universal and are utilized differently within the Chinese and western European ethnic groups. Our study adds to the growing body of work demonstrating the importance of skin color manipulations within an evolutionary meaningful parameter space, ideally using realistic skin models based on physical parameters.

Introduction

As one of the most significant features of a human face, skin coloration has been implied as a factor in sexual selection [1] and increased facial skin lightness, redness and yellowness, have been positively associated with healthy appearance and facial attractiveness [2, 3]. Facial redness in particular, has been shown to enhance perceived healthiness and attractiveness equally [4], possibly reflecting cardiovascular fitness of humans. Skin color homogeneity, driven by the distribution of skin chromophores melanin and hemoglobin, is positively associated with a younger perceived age and greater health and attractiveness [5-7].

In virtually all experiments, with the exception of several more recent studies [8-11], observers were either presented with color-manipulated, often morphed, facial images [4, 12, 13] or were asked to manipulate the facial image along fixed dimensions in color space, such that perceived healthiness or attractiveness was optimized [2, 3]. Computer-generated or morphed facial images, however, may lose skin texture and appear to be unrealistic after image processing. Furthermore, a uniform color shift applied to each pixel of the facial image, is not necessarily consistent with naturally occurring coloration changes since the variation in the color pattern depends on the distribution of blood vessels across the face which is not uniform. Human observers have been shown to be sensitive to these spatial color variations and perceived health is affected differentially by the color changes in particular facial regions [14].

With some notable exceptions, the impact of facial color on subjective attributes (perceived healthiness, attractiveness, and youthfulness) has been studied using samples of European descent, both as participants and as stimulus material. Ethnicity-specific effects have primarily focused on structural facial features; average features, neotenous features, and feminine features have been shown to affect perceived attractiveness [15-17]. When viewing black and white facial images, observers agreed more strongly in their attractiveness ratings for own-ethnicity faces [16, 17]. In relation to skin coloration as a cue to perceived health and attractiveness, similar preferences for skin coloration have been demonstrated in Caucasian and African samples and observers [18, 19], not only for their own but also across ethnicities. Mainland Chinese observers have been shown to prefer lighter skin and decreased yellowness in contrast to Caucasian observers, but only when judging faces of their own ethnicity [20]. In contrast, when asked to optimize perceived healthiness, Malaysian-Chinese observers increased facial yellowness and redness and decreased lightness, irrespective of the ethnicity of the faces which were Chinese-Malaysian, Caucasian or African [21]. One of the aims of the present study was to shed light on these mixed results and to dissociate the effect of observer and facial image ethnicity on these subjective judgements.

Some of the associations between skin color and perceptual ratings are clearly grounded in physical skin changes, e.g. the negative association between Lightness (L*) and both physical and perceived age is linked to sun exposure over time [22]. Other associations between skin coloration and preference ratings might reflect the aesthetic differences between western and eastern culture, which are likely to result from the development of multiple social and cultural factors over a long period of time [23-26]. We therefore hypothesize that associations between skin coloration and preferences will depend on both, the ethnicity of the observer and the ethnicity of the viewed facial image.

The aim of our study was to establish (1) the strength of the association between skin coloration (L*,a*,b*) and the perceptual attributes (attractiveness, healthiness, youthfulness), and (2) whether the utilization of these skin coloration cues (L*, a*, b*) is modulated by the ethnicity of the observer, the viewed image, or both. We used a large set (n = 80) of calibrated, high-resolution images of real human faces (Chinese and western Europeans) comprising a range of ethnicity-specific skin colors (lightness L*, redness a*, yellowness b*), without digitally manipulating skin color or texture. Two groups of observers (Chinese and western Europeans) rated both sets of facial images along three dimensions: perceived attractiveness, healthiness, and youthfulness.

Materials and methods

Photography and image processing

All facial images used in this study were selected from the Liverpool-Leeds Skin-color Database (LLSD), which included data for 188 subjects from four ethnic groups (western European, Oriental, South Asian and African, including both genders) and was established by the Universities of Liverpool and Leeds [27, 28]. The facial image of each subject was obtained by photography in a VeriVide DigiEye® light booth, which provided a uniform matte mid-grey background and even, diffuse illumination that simulated CIE illuminant D65 (daylight). There was no other lighting in the room where the photography took place. During data collection, the participant sat in the viewing cabinet and their target facial area was adjusted to fit within the camera image. A digital SLR camera (Nikon D7000), controlled by the DigiEye system software, was used to capture images of training color charts for camera characterization and of each subject’s face. The distance from the participant to the camera was approximately 57.5 cm and the participant looked straight into the camera.

Eighty real facial images, 40 Chinese images and 40 of western European descent, all with a neutral facial expression, were selected from the LLSD database for this study. All the faces are from people of the same age range between 20–40 and both genders were included in each ethnic group. The RGB data of each pixel of each image was first transformed to spectral reflectance [29] and later into CIELAB color coordinates [30]. To truly represent the color appearance of those facial images, a BenQ professional color display was used, with the white point set to D65 (the same as the illumination for facial image capturing). The method of piecewise linear interpolation assuming constant chromaticity (PLCC) [31, 32] was used for the color characterization of the display and the CIELAB values for each pixel were transformed to display RGB values for each facial image. Subsequently, each facial image was edited to remove the hair, ears, and any visible clothing manually and the image was then scaled to be in the center of the screen. Finally, a mid-grey background (L*, a*, b* = 50, 0, 0) was set to display all the images. All the images were processed in MATLAB. Fig 1 shows an example of a Chinese real facial image used in this study.

Fig 1

An example of a Chinese facial image.

Stimulus description

The mean color specification, in terms of CIELAB coordinates, of 80 test facial images (40 Chinese and 40 western Europeans) were calculated as the overall mean of each pixel in the facial area, excluding the mouth, nose, eyes, and eyebrows. CIELAB color space is a device-independent standard color appearance space where skin color is described by three dimensions: Lightness L*, Redness a* and Yellowness b* [33]. Additionally, chroma is used to roughly represent the saturation of colors. As shown in Fig 2, skin colors of both western Europeans and Chinese images were plotted in the a*-b* chromaticity diagram as well as the Chroma-Lightness diagram. There are systematic mean differences in lightness and chromaticity between the two ethnic groups. The western European images (average L* = 59.0, a* = 8.3, b* = 14.1) have, on average, higher lightness and lower yellowness (b*) compared to the Chinese images (average L* = 55.0, a* = 8.9, b* = 16.9).

Fig 2

The distribution of the mean facial colors of the test facial images in CIELAB a*b* space (left) and L*C* space (right). ◆ Western European (WE), ▲ Chinese (CH).

Observers

A psychophysical experiment was conducted using 44 observers, including 22 western Europeans (13 male; mean age ± SD = 24.27 ± 5.30) and 22 Chinese (7 male; mean age ± SD = 26.05 ± 3.96), who each evaluated the appearance of the 80 facial images (40 Chinese and 40 western Europeans) using three subjective attributes: perceived attractiveness, perceived healthiness, and perceived age. The sample size needed was estimated using G-power 3.1 software package [34] and a total sample of at least 34 was needed to ensure 80% power to detect a medium effect size of 0.5 at p<0.05. All observers were given instructions in English and confirmed their understanding before the experiments. The Chinese observers were from mainland China, and at the time of the study, they were at Leeds University, UK as students or visiting scholars. On average, they spent about 1–3 years in UK. This study was approved by the Ethics Committee at the University of Leeds (LTDESN-090) and all participants gave written informed consent prior to taking part in the study. The individual in this manuscript has given written informed consent (as outlined in PLOS consent form) to publish these case details.

Experimental procedure

The experiment was divided into three separate sessions. In each session, the observer viewed 80 facial images presented in random order and rated the skin color of each image with respect to one of the three attributes. The observer viewed each image for eight seconds and then made a judgement of the facial skin color without a time limit. The following question was asked after the observation of each image, “based on the skin color, what attractiveness score (or healthiness score or the estimated age, depend on different sessions) you would give for the last image?” Using a categorical judgment method, the perceived facial attractiveness and healthiness were rated on a 7-point Likert-type scale where 1 represented ‘least attractiveness’ / ‘healthiness’ and 7 represented ‘best attractiveness’ / ‘healthiness’. The visual age was rated on a single-year step scale from 1 to 99 years. The sex effect was not tested in this study and both observer and image sex were balanced. The observers were not either told the image sex or asked to judge the image sex when making judgements. Since each facial image was edited to remove the hair, ears, and any visible clothing, the sex of the face was not that obvious or highlighted in this study.

Statistical analysis

Inter-observer variability was examined by calculating Cronbach Alpha Coefficients [35]. For each of the three dependent variables (DV—attractiveness, healthiness, perceived age) a linear mixed-effect model was set up with the following fixed effects: lightness (L*), redness (a*) and yellowness (b*) as continuous predictors and image ethnicity and observer ethnicity as categorical predictors, including random intercepts for both images and observers.

All linear mixed-effect models were implemented in lme4 R package [36]. Deviation coding was used to convert both image ethnicity and observer ethnicity into deviation-coded factors (code ‘-0.5’ for western European images/observers and code ‘0.5’ for Chinese images/observers) for testing the main effects of each model. For each DV, a simple and a full model was considered; the full model allowing for all interactions between color (L*, a*, b*) and ethnicity (observer ethnicity, image ethnicity). In all cases the two model predictions differed significantly from each other and both AIC/BIC favored the model including the interactions [37]. P values for the fixed effects in each linear mixed-effect model were calculated using F tests with type III sums of squares and Satterthwaite’s degrees-of-freedom approximation in the lmerTest R package [38]. Significant interactions revealed in the tests were followed up with a further analysis of the simple effects for each subgroup. In addition, Pearson’s correlation coefficients (two-tailed) were also used to test the associations of the perceptions of all three attributes for both sets of observers.

Results

Consistency within and between ethnicities

The internal consistency in the ratings of attractiveness, healthiness, and age for both the western European and Chinese groups of observers is very high, with values of Cronbach’s alpha coefficient all greater than 0.87 (Table 1). On average, both set of observers show high levels of agreement on the subjective ratings in their own-ethnicity faces and other- ethnicity faces.

Table 1

The Cronbach Alpha Coefficient for assessing the inter-observer variability of the western European (WE) and Chinese (CH) observers (sample size).

	WE	CH	WE & CH
WE images
Attractiveness	0.96 (22)	0.93 (22)	0.96 (44)
Healthiness	0.96 (22)	0.93 (22)	0.97 (44)
Age	0.90 (22)	0.91 (22)	0.95 (44)
CH images
Attractiveness	0.95 (22)	0.96 (22)	0.97 (44)
Healthiness	0.96 (22)	0.96 (22)	0.98 (44)
Age	0.87 (22)	0.92 (22)	0.94 (44)

The effect of skin coloration, observer and image ethnicity on perceived healthiness, attractiveness, and age

For all perceptual attributes, we first evaluate a linear mixed-effect model with and without interactions. In all cases, a model allowing for interactions outperforms the model without interactions, as shown in Table 2. While the BIC for age weakly favors the model with no interaction, all the AIC strongly prefers the model with interactions, as does the significant likelihood ration test. We will therefore report the analysis of the model with interactions.

Table 2

Model comparisons: Mixed models with and without interactions for all three attributes.

model	npar	AIC	BIC	logLik	deviance	χ2	χ2df	P
DV = attractiveness
no interaction	9	9725	9781	-4854	9707
+ interactions	19	9534	9651	-4748	9496	211.03	10	<0.001***
DV = healthiness
no interaction	9	9471	9526	-4726	9453
+ interactions	19	9344	9460	-4653	9306	147.1	10	<0.001***
DV = age
no interaction	9	20013	20068	-9997	19995
+ interactions	19	20002	20119	-9982	19964	31.09	10	<0.001***

*P≤0.05

** P≤0.01

***P≤0.001.

Table 3 shows all the main effects of the linear mixed-effect model for facial attractiveness. For attractiveness, neither skin coloration (L*,a*,b*) nor observer ethnicity are significant, but image ethnicity, interactions between image ethnicity and lightness (p = 0.017) and interactions between observer ethnicity and lightness/redness/yellowness are significant (p = 0.001/<0.001/<0.001). The interactions indicate that the effect of lightness on attractiveness is different when faces of different ethnicity are viewed and the effects of lightness/redness/yellowness on attractiveness are different for western European observers and for Chinese observers. The regression lines in S1 Fig in S1 File show all these interactions between colour and ethnicity (observer/image) in the linear mixed-effect models.

Table 3

Linear mixed effects model estimates of fixed effects, their SE, t-value, lower (2.5%) and upper (97.5%) confidence intervals and P-values for attractiveness.

Fixed effects	Estimate	SE	t-value	2.5% CI	97.5% CI	P-value
(Intercept)	2.346	4.465	0.525	-6.513	11.206	0.601
L*	0.021	0.058	0.359	-0.094	0.136	0.720
a*	-0.113	0.085	-1.328	-0.282	0.056	0.188
b*	0.106	0.061	1.739	-0.015	0.228	0.086
Im	-20.703	8.929	-2.319	-38.419	-2.987	0.023 *
Ob	-1.531	1.780	-0.860	-5.020	1.958	0.390
Im:Ob	-6.266	3.546	-1.767	-13.218	0.686	0.077
L*:Im	0.284	0.116	2.448	0.054	0.515	0.017 *
L*:Ob	0.077	0.023	3.353	0.032	0.123	0.001 ***
a*:Im	0.187	0.170	1.102	-0.150	0.525	0.274
a*:Ob	-0.166	0.034	-4.901	-0.232	-0.099	<0.001 ***
b*:Im	0.182	0.122	1.487	-0.061	0.425	0.141
b*:Ob	-0.089	0.024	-3.646	-0.136	-0.041	<0.001 ***
L*:Im:Ob	0.079	0.046	1.707	-0.012	0.169	0.088
a*:Im:Ob	0.137	0.068	2.030	0.005	0.270	0.042 *
b*:Im:Ob	0.050	0.049	1.024	-0.046	0.145	0.306

*P≤0.05

** P≤0.01

***P≤0.001. Im = Image ethnicity, Ob = Observer ethnicity.

Table 4 shows the results of the main effects in the full perceived healthiness model. For perceived healthiness, there was no significant main effect of skin coloration, but a significant effect of image ethnicity. We also found significant interactions between image ethnicity and lightness (p = 0.024) and between observer ethnicity and redness and yellowness, respectively (p<0.001; p<0.001). Similar to facial attractiveness, the effect of lightness on perceived healthiness is different to faces of different origins and the effect of redness/yellowness on healthiness is different for the two groups of observers.

Table 4

Linear mixed effects model estimates of fixed effects, their SE, t-value, lower (2.5%) and upper (97.5%) confidence intervals and P-values for healthiness.

Fixed effects	Estimate	SE	t-value	2.5% CI	97.5% CI	P-value
(Intercept)	4.829	4.998	0.966	-5.088	14.745	0.337
L*	-0.015	0.065	-0.231	-0.144	0.114	0.818
a*	-0.100	0.095	-1.055	-0.289	0.088	0.295
b*	0.093	0.068	1.357	-0.043	0.229	0.179
Im	-20.374	9.995	-2.039	-40.205	-0.544	0.045 *
Ob	2.141	1.725	1.241	-1.242	5.523	0.215
Im:Ob	-4.515	3.434	-1.315	-11.248	2.218	0.189
L*:Im	0.300	0.130	2.306	0.042	0.558	0.024 *
L*:Ob	0.027	0.022	1.208	-0.017	0.071	0.227
a*:Im	0.133	0.190	0.697	-0.245	0.510	0.488
a*:Ob	-0.141	0.033	-4.309	-0.205	-0.077	<0.001 ***
b*:Im	0.137	0.137	1.002	-0.134	0.409	0.320
b*:Ob	-0.147	0.024	-6.237	-0.193	-0.101	<0.001 ***
L*:Im:Ob	0.060	0.045	1.339	-0.028	0.147	0.181
a*:Im:Ob	0.102	0.065	1.563	-0.026	0.231	0.118
b*:Im:Ob	0.034	0.047	0.726	-0.058	0.126	0.468

*P≤0.05

** P≤0.01

***P≤0.001. Im = Image ethnicity, Ob = Observer ethnicity.

For estimated age, as shown in Table 5, there are significant main effects of redness (a*), image ethnicity, and the interaction between the ethnicity of the image and observer. Significant main effects of interactions between colorations and ethnicity include the interaction between lightness and image ethnicity (p = 0.015), the interaction between redness and image ethnicity (p = 0.036) and the three-way interaction of L*:Im:Ob (p = 0.035). Facial lightness/redness have different effects on the perceived age of European faces and Chinese faces, and the influence of lightness on age perception also depends on the ethnicity of the observer.

Table 5

Linear mixed effects model estimates of fixed effects, their SE, t-value, lower (2.5%) and upper (97.5%) confidence intervals and P-values for estimated age.

Fixed effects	Estimate	SE	t-value	2.5% CI	97.5% CI	P-value
(Intercept)	23.676	14.317	1.654	-4.731	52.082	0.102
L*	-0.112	0.186	-0.603	-0.482	0.257	0.548
a*	0.550	0.273	2.017	0.009	1.091	0.047 *
b*	0.237	0.196	1.210	-0.152	0.626	0.230
Im	70.588	28.617	2.467	13.805	127.37	0.016 *
Ob	4.551	8.080	0.563	-11.290	20.392	0.573
Im:Ob	31.712	16.046	1.976	0.254	63.170	0.048 *
L*:Im	-0.926	0.372	-2.487	-1.665	-0.187	0.015 *
L*:Ob	-0.072	0.104	-0.693	-0.277	0.132	0.488
a*:Im	-1.165	0.545	-2.136	-2.247	-0.083	0.036 *
a*:Ob	0.294	0.153	1.922	-0.006	0.593	0.055
b*:Im	-0.602	0.392	-1.536	-1.380	0.176	0.129
b*:Ob	-0.142	0.110	-1.294	-0.358	0.073	0.196
L*:Im:Ob	-0.440	0.209	-2.106	-0.849	-0.030	0.035 *
a*:Im:Ob	-0.133	0.306	-0.434	-0.732	0.467	0.665
b*:Im:Ob	-0.313	0.220	-1.425	-0.744	0.118	0.154

*P≤0.05

** P≤0.01

***P≤0.001. Im = Image ethnicity, Ob = Observer ethnicity.

To further understand the interactions above and reveal the simple effects of L*, a* and b* within each set of image and observer, parameter estimates for the fixed effects within each subgroup are computed from the three linear mixed-effect models, as shown in Table 6. When Chinese observers rated Chinese faces, an increase in lightness is strongly associated with greater attractiveness (p = 0.002); for western European faces, a decrease in redness predicts greater attractiveness (p = 0.018). Observers of European descent associate an increase in yellowness with higher attractiveness, but only when viewing European facial images (p = 0.010). Lighter skin is associated with greater healthiness but only when Chinese observers rate Chinese images (p = 0.037). Western European observers associate an increase in yellowness with healthiness (p = 0.021) when viewing facial images of Chinese. This association is driven by the western European images: an increase in redness is associated with an older perceived age for western European images when viewed by western European (p = 0.033) or Chinese Observers (p = 0.004). Lightness is a strong predictor for perceived youthfulness when Chinese observers rate Chinese faces (p = 0.002). S2 Figs 2–4 in S1 File show the associations between perceptual ratings and skin color for western European observers and Chinese observers. The regression lines are drawn for the significant fixed effects in the linear mixed-effect model.

Table 6

Parameter estimates of the simple effects in the linear mixed-effect models.

Fixed effects	WE observers	CH observers
DV = attractiveness
WE images
Model
L*	β = -0.140, P = 0.149	β = -0.102, P = 0.291
a*	β = -0.090, P = 0.507	β = -0.324, P = 0.018*
b*	β = 0.072, P = 0.425	β = -0.041, P = 0.647
CH images
Model
L*	β = 0.105, P = 0.133	β = 0.221, P = 0.002**
a*	β = 0.029, P = 0.790	β = -0.068, P = 0.538
b*	β = 0.229, P = 0.010**	β = 0.166, P = 0.059
DV = healthiness
WE images
Model
L*	β = -0.164, P = 0.131	β = -0.166, P = 0.124
a*	β = -0.071, P = 0.638	β = -0.263, P = 0.083
b*	β = 0.106, P = 0.292	β = -0.058, P = 0.567
CH images
Model
L*	β = 0.106, P = 0.170	β = 0.163, P = 0.037*
a*	β = 0.011, P = 0.930	β = -0.079, P = 0.519
b*	β = 0.226, P = 0.021*	β = 0.097, P = 0.318
DV = age
WE images
Model
L*	β = 0.277, P = 0.381	β = 0.424, P = 0.180
a*	β = 0.952, P = 0.03*	β = 1.312, P = 0.004**
b*	β = 0.531, P = 0.074	β = 0.545, P = 0.066
CH images
Model
L*	β = -0.429, P = 0.060	β = -0.721, P = 0.002**
a*	β = -0.146, P = 0.684	β = 0.081, P = 0.821
b*	β = 0.085, P = 0.763	β = -0.213, P = 0.451

*P≤0.05

** P≤0.01

***P≤0.001. DV = dependent variable.

Correlations between the perceptual attributes

Ratings of attractiveness and healthiness are highly correlated across both image and observer ethnicities (Table 7, also see S3 Fig 5 in S1 File) but are negatively correlated with estimated age. The latter negative correlations are highly significant for Chinese observers. The strongest negative correlations are observed when Chinese observers rate Chinese image, consistent with interactions between ethnicity and skin coloration cues.

Table 7

The Pearson Correlation Coefficients of age, healthiness, and attractiveness scores for the western European (WE) and Chinese (CH) observers.

	WE images	CH images	Overall images
WE observers
Attractiveness-Healthiness	0.912***	0.946***	0.929***
Attractiveness-Age	-0.343	-0.354	-0.351*
Healthiness-Age	-0.293	-0.295	-0.298
CH observers
Attractiveness-Healthiness	0.881***	0.927***	0.893***
Attractiveness-Age	-0.632***	-0.828***	-0.730***
Healthiness-Age	-0.651***	-0.818***	-0.726***

*P≤0.05/18

** P≤0.01/18

***P≤0.001/18. N = 40, 40, 80 for WE, CH and overall images, respectively. All p-values were Bonferroni-corrected.

Discussion

80 color-calibrated images of real western European and Chinese human faces were used to study how facial coloration was utilized by Chinese and western European observers when rating the healthiness, attractiveness, and youthfulness of facial images of their own or the other ethnicity. The lightness and color variations in these images were representative of the color variations in the respective populations [28]. Our main finding is that skin coloration is not a universal predictor for perceived attractiveness, health, or age, but is utilized differently by Chinese and western European observers when rating own- or other-ethnicity facial images.

Perceived attractiveness and healthiness

Ratings of attractiveness and healthiness are driven by the same facial cues (Tables 3,4,6), which is also supported by the high correlations between attractiveness and healthiness ratings (Table 7), consistent with Han et al. [39]. However, these cues are utilized differently by Chinese and western European viewers. Chinese observers use skin lightness as a robust cue for judging both attractiveness and healthiness, but only when judging Chinese faces (Table 6). In contrast to our study, Han et al. [39] found an effect of lightness on attractiveness/perceived health for both, Chinese and Western faces, whereas in our study we find significant associations only when Chinese faces were rated. A possible explanation for this discrepancy is that Chinese skin is characterized by a lower average L* value, and crucially, by a smaller variation in lightness compared to the skin of western Europeans [40]. We speculate that it is the smaller variability in L* for Chinese faces, which leads to a more informative and more reliable lightness cue.

For Chinese viewers, redness (a*) is negatively associated with both attractiveness and healthiness but reaches significance (p<0.05) only for attractiveness when viewing European faces (Table 6). In contrast, western European viewers associate an increase in yellowness (b*) with increased attractiveness and healthiness, but only when viewing images of Chinese faces (Table 6). The association between an increase in skin yellowness (b*) and perceived attractiveness and healthiness is likely to reflect the western European preference for ‘tanned’ skin which is characterized by a concurrent increase in b* and decrease in L* [41]. Skin yellowness as a significant predictor for perceived health is consistent with previous studies (e.g. [42]). Whether skin yellowness is associated with physical health is controversial; some studies found correlations between physical fitness and skin yellowness [43], whereas others found associations between skin yellowness and perceived health, but not with physical health [10]. Experiments linking high-carotenoid diet to objective skin color changes have reported consistent associations between carotenoids intake and skin yellowness (b*), however, the results for the carotenoids-redness (a*) linkage are inconclusive. Fruit and vegetable (FV) intake has been associated with a significant increase in skin yellowness (about 2 b* units, much smaller in a* values) and perceived attractiveness, consistent with the putative carotenoid-linked health-signaling system for mate choice [12, 44]. Pezdirc et al [45] reported an increase of 0.6 b* but no measurable increase in redness (a*), which was confirmed by Appleton et al. [8] in a randomized controlled trial where they documented a small but significant effect of FV intake on skin yellowness (about 1–2 b* units), but no change in skin redness and no effect on perceived health. On the contrary, Tan et al. reported significant increments in both skin yellowness and redness (p<0.001) after 4-week FV intervention [46]. Highly elevated skin yellowness levels are likely to reflect underlying health issues such as jaundice [47]. Therefore, one would predict that only those small changes in yellowness that reflect objectively measurable health benefits, to be positively associated with perceived health. This non-linear relationship between physical skin coloration changes and perceptual preferences supports the argument that associations between physical skin changes and perceptual attributes such as healthiness and attractiveness need to be investigated within a physically plausible, evolutionary relevant, parameter range.

Perceived age

The strongest negative associations are found between skin lightness and estimated age, but only when Chinese observer rate own-ethnicity faces (Table 6). This robust relationship between skin lightness and perceived youthfulness in the Chinese culture may reflect the physical lightness changes as function of age: a decrease of one L* unit is equivalent to a 10-year increase in age in female Chinese skin [48]. Western European observers, on the other hand, show no robust associations between skin lightness and any of the measured attributes. Skin lightness as a cue to attractiveness and youthfulness is therefore deployed differently in these two cultures. For western European images, facial redness (a*) is positively associated with estimated age, irrespective of the ethnicity of the viewer (Table 6). The role of facial redness is discussed in more detail below.

The role of other facial cues

Two facial coloration cues are used robustly, but differentially between ethnicities: skin lightness (L*) within the Chinese sample as an indicator for attractiveness, health and youthfulness, whereas skin redness (a*) is negatively associated with youthfulness in western European facial images (Table 6). Since we used non-manipulated images of real faces, skin coloration co-varied necessarily with other facial shape features. The high observer consistency both within and across ethnicities (Table 1) suggest that observers may rely on additional facial cues in their judgments, consistent with Fink et al.’s study [49] in which ratings for perceived age, health, and attractiveness were obtained for full facial images and for isolated cropped cheek patches. Associations between these two sets of ratings were modest: R2 were 0.31 (perceived age), 0.14 (health) and 0.06 (attractiveness), which demonstrates that participants must make use of other facial features (not contained in the isolating skin patch) when rating the skin patches/faces along these attributes. In the Fink study, the most robust association were reported for perceived age, where the isolated skin image explains about 31% of the variation in the full-face image ratings, consistent with the melanin distribution being a strong indicator of youthfulness [50]. Our current study was designed to estimate the strength of association between mean skin coloration (L*, a*, b*) and the perceptual ratings of perceived health, attractiveness, and perceived age; it does not allow us to estimate the contribution of skin coloration relative to the contribution of other cues, such as skin texture, localized coloration changes or other anatomical facial features. A useful follow-up study would be to include in the linear fixed-effect model spatial location and skin texture as independent variables, which would allow us to evaluate the relative contributions of mean skin coloration vs 2nd-order skin characteristics.

No evidence for a positive association between skin redness and perceived attractiveness, health, and youthfulness

We find no evidence that facial skin redness is positively associated with perceived attractiveness, healthiness or youthfulness (see Table 6), in contrast to previous reports with western European (Stephen et al., 2009a; Stephen et al., 2009b; Pazda et al., 2016; Stephen et al., 2012) [2, 3, 13, 18] and Chinese observers (Han et al., 2018; Tan et al., 2019) [20, 21]. Our results are, however, consistent with recent studies using a large image data base of real female faces that did not find any association between facial redness and objective health measures, neither any positive association between redness and attractiveness [9]. Other studies employing non-manipulated real facial images, found a weak positive association between skin yellowness and facial attractiveness, but skin redness as a mediator showed a small but negative association with facial attractiveness [8]. Facial redness, caused by increased blood oxygenation, has been postulated to be an important signal for mating choice, by serving as an indicator for fertility [51-53], but recent evidence suggests a more complex relationship between female facial redness and its role in signaling fertility. While perceived attractiveness and healthiness has been shown to be higher in the follicular phase, the changes in redness (a*) were either not measurable [54], below perceptual threshold [55] or not cycle-specific [56], which suggests that other facial cues, such as facial shape and skin homogeneity, may play a more prominent role.

Studies that have reported a strong positive association between facial redness and attractiveness or healthiness, have involved color-manipulated facial images [2, 3, 12, 13]. More recent studies, including the current one, using non-manipulated facial images, have failed to show these strong associations [8-11]. This discrepancy could be partly due to methodological differences; including the magnitude of redness changes (in excess of 10 a* units) and color shifts being applied uniformly across the face. Skin color manipulations were often restricted to the CIELAB dimensions, a*, b*, L*, whereas in the natural skin color universe, skin color dimensions are highly correlated [28, 43]. Crucially, changes in skin color reflecting evolutionary relevant or life-style changes (fertility, exercise, diet, physical health) are not restricted to one of these dimensions, but are characterized by co-variations along all three dimensions (e.g. [8, 55]). It is conceivable that our sample of facial images (n = 80) did not contain a sufficiently large color range, in particular redness variations, to reveal associations with the subjective ratings. We believe this is unlikely to explain our results, since the range of redness values covered by our images is about 6 a* units (Fig 2.), whereas, for color-normal observers, two a* units are easily discriminable for facial skin patches [4, 57]. A potential limitation of our study is that we used average skin color as a predictor instead of using the color values at specific facial locations, which has been shown to be an important factor [14].

All these methodological differences taken together may explain some of the discrepancies in the suggested role of skin coloration for subjective ratings of health and attractiveness. Broadly speaking, rating experiments using manipulated facial images showed strong associations [2, 3, 12, 19, 20, 58] between skin coloration and subjective ratings of health and attractiveness. While color manipulation of the facial images has the advantage of dissociating the role of skin color cues from facial shape cues, they may not reflect evolutionary significant color changes (as discussed above), and recent studies employing natural, non-manipulated images [8–11, 43] or plausible physics-based skin image manipulations [59, 60] reported much weaker associations between skin color and healthiness and attractiveness, suggesting that the former studies may have led to an overestimation of the role of skin color for health and attractiveness, particularly for associations with facial redness. Jones [11] concluded that perceived health and attractiveness relies almost exclusively on facial shape features with mean facial color playing a minor role. To fully characterize the differential contributions of facial shape, average skin color and skin homogeneity requires all features to be manipulated independently, but within an evolutionary meaningful parameter space, ideally using a realistic skin model based on physical parameters including chromophore distribution and magnitude [59], blood oxygenation, skin moisture, translucency, sub-surface skin scatter, all of which affect skin appearance and therefore potentially provide potential cues to healthiness, attractiveness, and youthfulness.

In summary, the most robust positive associations were found between facial skin lightness (L*) and attractiveness, healthiness, and youthfulness, but only when Chinese observers judge facial images of their own ethnicity. These associations between ratings and skin lightness are grounded in known physical changes: a decrease of one L* unit is equivalent to a 10-year increase in age in female Chinese skin [48]. In contrast to previous studies, we find no evidence that facial redness is positively associated with perceived attractiveness or health. A possible explanation for this discrepancy with previous results is that we used naturally occurring skin coloration variations, instead of manipulating the facial images. We speculate that previous experiments may have overestimated these associations by using skin color manipulations well beyond the gamut found in non-manipulated images as well as changing the coloration of the entire face instead of specific areas.

The effect of ethnicity has been investigated in our experiments using observers of both genders, but we appreciate that observer gender is a possible additional variable. While our sample is too small to conduct a meaningful gender-based analysis, previous studies found a strong effect of facial redness that impacts on perceived health and attractiveness for both male and female skin by skin color manipulations [61, 62]. Whether there is a perceptual difference for facial colour appearance between gender and, if there is, how large the effect is in the realistic skin model compared to the cultural difference, requires further work.

We conclude that observers of both ethnicities make use of skin color and lightness to rate attractiveness, healthiness, and perceived age, but the utilization of these cues is more subtle than previously thought. Crucially, skin coloration cues are not universal and are utilized differently within the Chinese and western European ethnic groups, reflecting different aesthetic preferences in eastern and western cultures. Such ethnic differences in objective aesthetic criteria should be considered in many applications of preferred skin colour reproduction including aesthetic surgery [63].

(RAR)

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24 Jun 2021

PONE-D-21-13133

Skin coloration is a culturally-specific cue for attractiveness, healthiness and youthfulness in observers of Chinese and Western European descent.

PLOS ONE

Dear Professor Wuerger,

Dear Professor Wuerger,

Thank you for submitting your work to PLOS ONE. I have now had chance to secure the opinion of two reviewers with significant expertise in the area of both mixed models, and the role of facial appearance and colouration in social perception.

As you will see, the reviews are quite favourable, and centre mostly around including more detail in the introduction and method, as well as more clearly illustrating the results from the mixed models. Reviewer 1 highlighted that some of the results were quite confusing (as can often be the case with mixed models) and looking at the raw data may clear things up, but was unable to access it. I would strongly encourage the authors to deposit their raw data somewhere that can be accessed, even if only for review purposes.

I hope the authors find these comments useful. I am in the unusual position of acting as a handling editor after reviewing an earlier version of this manuscript, and I find that while I agree with both reviewers comments, I find it significantly improved and commend the authors on the hard work they have put in already.

I look forward to receiving a revised manuscript in due course.

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We look forward to receiving your revised manuscript.

Kind regards,

Alex Jones

Academic Editor

PLOS ONE

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Partly

Reviewer #2: Yes

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2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: I Don't Know

Reviewer #2: I Don't Know

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3. Have the authors made all data underlying the findings in their manuscript fully available?

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Reviewer #1: No

Reviewer #2: Yes

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Reviewer #1: Yes

Reviewer #2: Yes

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5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: The current manuscript investigates cultural differences in the preference for and social perception of facial coloration in a sample of Chinese and Western European participants. I think there is great value in complementing existing findings on experimentally manipulated images with work on unmanipulated, natural images. However, there are several points I would like to see the authors address before I would recommend this manuscript for publication.

General comments

- I would ask the authors to make their data and analysis script(s) available for the reviewing process; I could not find these at the provided link. I have several questions regarding the analyses (see below), some of which I might have been able to answer myself with access to the data

- Han et al. (2018) found effects of both stimulus and observer sex. I assume the authors did not test for such effects due to sample constraints, but this should be acknowledged somewhere

Materials and Methods

Stimuli:

- How were images selected from the LLSD? Demographics of stimulus set should be reported (could age of stimulus faces have had an effect on results?)

- What software was used to mask non-face cues, how were images scaled/centered (i.e. algorithmically or manually), and how were CIELAB coordinates calculated?

Observers:

- How were observers recruited?

- It is stated Chinese observers had spent about 1-3 years in the UK – mean and SD would be more informative here

Procedure:

- Was the order of sessions randomized?

Statistical analysis:

- It is stated that “all interactions between the predictors were included”—this could be a bit clearer. Was a full factorial design specified? Only two-way interactions are reported in Table 2, while Table 3 and the text suggests three-way interactions were analysed (see also comments below)

Results

- It is not clear exactly which models were tested and how they were specified. Table 2 reports main effects and two-way interactions, while Table 3 is split by observer ethnicity. This only make sense if the respective three-way interactions were included in the main model and found to be significant, in which case the corresponding stats need to be reported. Moreover, Table 2 only presents F- and p-values, while Table 3 reports significant estimates. All of this makes it hard to evaluate results and the subsequent discussion of effects. Findings would also be easier to parse if they were presented separately for the three DVs. This extends to Tables 2 and 3: estimates (including 95% CIs) and corresponding stats should be presented together, and tables instead split by DV

- It would be helpful to include graphs of significant interactions of interest (either in the main manuscript or the supplemental material)

- When describing results in-text, effect sizes should be reported in addition to p-values

- Results should maybe start with info on inter-rater agreement (since low inter-rater agreement would render any further analyses meaningless). In that section it is also stated observers showed higher agreement within their own ethnicity— as the numbers are only marginally different, I am not sure this warrants extra mention?

- Presenting correlations between perceptual ratings separately for Chinese and Western observers and stimuli suggests that associations between these sub-groups differed—was this tested? It looks like age was more strongly associated with attractiveness and health in Chinese compared to Western participants, but I am not sure differences between Chinese and Western images would actually reach statistical significance?

Discussion

- Like the results, I feel the discussion would benefit from some restructuring. Again, I found it a bit hard to understand what exactly was found and how these results sit within the existing literature

- Attractiveness and b*. The positive effect of b* on attractiveness that was found for Chinese faces as judged by Western observers is interpreted in the context of a Western preference for tanned skin. This begs the question why no such preference was found for Western faces? In addition, there is some evidence that tan/melanin pigmentation is less preferred compared to carotenoid pigmentation. While there likely is some perceptual overlap between the two, the tan explanation does not appear entirely satisfactory. The authors also appear to doubt that high-carotenoid intake is linked to changes in skin pigmentation (line 252), but cite several studies that show such a link? Lastly, b* does not necessarily have to be linked to actual health in order to affect perceived health (or attractiveness); other mechanisms have been suggested (e.g., Han et al., 2018). I would be interested to hear the authors’ thoughts on this.

- Effects of L*. In line with the current study, Han et al. (2018) found a stronger association of L* and attractiveness/perceived health for Chinese compared to Western observers. This also appears consistent with the current finding that L* was linked to perceived age in Chinese observers (especially since ratings of age seem to be quite strongly correlated with ratings of attractiveness and perceived health in Chinese observers). However, Han et al. reported an effect for both Chinese and Western faces, whereas the current study only appears to have found such an effect for Chinese stimulus images; I wonder whether this might have to do with different base levels of L* in unmanipulated images of Chinese and Western faces? Again, I would be interested to hear the authors’ thoughts on this.

- While I agree with the authors that colour cues are perceptually integrated with other facial cues (such as shape), I felt in its current form the section on “the role of other facial cues” was a bit unclear: what are the implications for the current study/findings?

- Effects of a*. The discussion in this section switches in terminology to referring to “youthfulness” instead of age, which I thought was slightly confusing. It is reported that a* negatively affected attractiveness (lines 302 and 358); it should be clarified that this effect was only found for Chinese observers rating Western images. The authors provide some ideas as to why a* was not positively linked to attractiveness and health as in (some) previous studies; I was wondering whether these null- and negative findings on Western images in particular might also be linked to the association of a* and perceived age in the Western image set?

Minor comments:

- Table 2 is labeled main effects but also presents interaction effects

- Table 3 should specify whether the reported estimates are standardized or not

- Tense sometimes switches between past and present in results

- While most references are indexed by numbers, a few are referred to in the FirstAuthor et al. format

- Supplemental info is labeled a bit confusingly; Figs. 1-3 are labeled as S1, and then there is an unnumbered Fig. that is labeled as S2. In addition, it would be good to add a bit more info to figure captions

Reviewer #2: This is an interesting cross-cultures study set to examine the associations between skin colouration and subjective rating of attractiveness, apparent health and perceived age across two independent samples. The authors highlighted the methodological uniqueness (difference) of the current study, as compared to those examined the similar variables. The description of the method, as a whole, is sound, appropriate and with a decent amount of details.

A few minor suggestions and recommendations listed below, for the authors’ consideration:

1. The literature can be further expanded, both in breadth and also depth, to better established the scene – and justify the proposed hypothesis. The authors should consider discussing the potential mechanism involved, in brief. Also, worth discussing the reasons for potential cultural difference – apart from reporting the mixed results.

2. It would be better if the authors can introduce the term LAB-C, in brief, before discussing the individual component using only the label/acronym.

3. The presentation (display) of the stimulus should be better described for clarity. What software did the researcher use? And how was the actual protocol like? This info seems to be immersed with the image preparation part.

4. Please refer to line 132 – Were the participants being asked to attend only to the skin colour part of the image and not the overall face? If so, justification may be needed for this unconventional approach considering that there is a substantial amount of evidence showing that face is being processed holistically. If that is not the case, the writer may need to rephrase the specific statement.

5. It would be great if the writer can provide some simple justification about the sample size used – to ensure that it has enough power.

6. Refer to line 112 - It may be better if the author can explicitly report the mean values of LAB for the Asian and Caucasian faces used – instead of a generic description about the difference between the two sets of stimuli. Even the actual mean difference will be better.

7. Please refer to line 206-210 – The Result can be better phrased for clarity. Phrases such as “highly significant” or “highest negative correlation” may not be the conventional way of reporting the result – or at least not specific enough and the point of comparison is not clear. Please consider rephrasing this section for clarity.

8. Please refer to line 213 – I am not certain what the number 18 after the p values stand for.

9. Please refer to line 218-220 – A stat may be needed to support the inference made about the higher agreement of own ethnic faces rating than of the other ethnic’s.

10. Line 221-223, please explain the number (22)(44) in the bracket?

11. Line 228, maybe reconsider if the term “deployed” is best to be used here. This is merely a suggestion.

12. Table 3 is incomplete and hard to be seen clearly. I would assume the authors will also need to report those stats with a not-significant result, at least in the table.

13. Line 240-242, not sure if the conclusion stated here is accurate – I would refrain from implying that one (aspect) is higher than another if the result is indicating otherwise.

14. Line 252-253 – the author reported that - “Experiments linking high-carotenoid diet to objective skin color changes have produced inconclusive results.” However, (all) the evidence listed subsequently in the adjacent sentences seems to be indicating that the supplementation consistently induces an increment in skin yellowness. The inconclusive part, I assume should be more on redness (and the subjective rating of health)?

15. Line 260-263 – please consider rephrase this section, for better clarity.

16. I would avoid using the comparative terms (highest, higher etc) especially when there is no obvious reason for doing so (hypothesis wise, theoretically wise). Instead of saying “highest correlation”, a better way to go may be to merely indicate the valence (positive) and the strength (weak, medium, strong) – see, for example, line 265

17. Discussion can be expanded and better organized – This section seems to be mostly descriptive (the who and what – e.g., who found what). I would expect some explanation about the potential mechanism here (the why and how) – at least in brief. For example, why is luminance being associated with higher perceived health (and/or physical health)? Why redness is being related to the increment in perceived age?

18. Please refer to line 300 – the authors wrote ‘also failed to find any association between facial redness and objective health measures”. This may be implying that the authors were also measuring the objective health measures – which I assume is not the case.

19. Line 301, would it be better to explicitly cite the last name here – for clarity and ease of reading. This is merely a suggestion.

20. Line 301-302 may not be entirely accurate, as redness is not, as a whole, correlated with attractiveness.

21. Please check the in-text citation on Line 331.

22. Line 355-356, this argument needs to be rephrased – as it is unclear what do you mean by “manipulated facial images showed strong association”. You have mentioned the stimuli used here (facial images) but not the IVs and DVs.

23. Please check your reference list – a few items do not seem to be with complete info (e.g, item 2, 8, 16, 21, 22, 23, 28, 32, 44).

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Reviewer #1: No

Reviewer #2: Yes: Tan Kok Wei

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5 Aug 2021

We appreciate that both reviewers and editor Alex Jones have given us a lot insightful comments and valuable feedback. We have revised our manuscript according to the comments and suggestions (please find the manuscripts with and without track changes), and we have also responded to each point raised by reviewers (please see 'Response to Reviewers'). Again, thanks both reviewers and editor who really made us think more and helped us improve our paper. We hope the revised paper is improved and any new comments and suggestions are always welcome.

Submitted filename: Response to Reviewers.docx

Click here for additional data file.

1 Oct 2021

PONE-D-21-13133R1

Skin coloration is a culturally-specific cue for attractiveness, healthiness and youthfulness in observers of Chinese and Western European descent.

PLOS ONE

Dear Dr. Wuerger,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

Please submit your revised manuscript by Nov 14 2021 11:59PM. If you will need more time than this to complete your revisions, please reply to this message or contact the journal office at plosone@plos.org. When you're ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

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If you would like to make changes to your financial disclosure, please include your updated statement in your cover letter. Guidelines for resubmitting your figure files are available below the reviewer comments at the end of this letter.

If applicable, we recommend that you deposit your laboratory protocols in protocols.io to enhance the reproducibility of your results. Protocols.io assigns your protocol its own identifier (DOI) so that it can be cited independently in the future. For instructions see: https://journals.plos.org/plosone/s/submission-guidelines#loc-laboratory-protocols. Additionally, PLOS ONE offers an option for publishing peer-reviewed Lab Protocol articles, which describe protocols hosted on protocols.io. Read more information on sharing protocols at https://plos.org/protocols?utm_medium=editorial-email&utm_source=authorletters&utm_campaign=protocols.

We look forward to receiving your revised manuscript.

Kind regards,

Alex Jones

Academic Editor

PLOS ONE

Journal Requirements:

Please review your reference list to ensure that it is complete and correct. If you have cited papers that have been retracted, please include the rationale for doing so in the manuscript text, or remove these references and replace them with relevant current references. Any changes to the reference list should be mentioned in the rebuttal letter that accompanies your revised manuscript. If you need to cite a retracted article, indicate the article’s retracted status in the References list and also include a citation and full reference for the retraction notice.

Additional Editor Comments (if provided):

Dear Professor Wuerger,

Apologies for the delay in returning reviews to you; it is a busy time of year.

Both the original reviewers have now returned their comments to me on the revised version of the manuscript. As you can see, both are generally pleased and find the work much improved, with only a few outstanding comments. I am recommending a minor revision here to address these final points, and will not be sending the manuscript back out for further review once these have been completed.

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #1: (No Response)

Reviewer #2: (No Response)

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2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Partly

Reviewer #2: Yes

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3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: No

Reviewer #2: Yes

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4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: Yes

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5. Is the manuscript presented in an intelligible fashion and written in standard English?

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Reviewer #1: Yes

Reviewer #2: Yes

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6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: I appreciate the changes the authors’ have made in response to both reviewers. There are a few more points I’d like to see them address:

• The authors have now shared their data file, which is great! I wonder whether they also plan to share their R analysis script? This would make it much easier to check analyses and suggest any potential improvements. I also note that I still could not find any data specific to the manuscript under the link they provide

• The authors have now added some notes on sex of the images in the experimental procedure. I would still ask them to explicitly note this as a limitation of the current study; I’m not convinced that masking non-face cues actually also effectively masks any cues to sex/makes sex irrelevant in perception

• The authors have explained in their response that all faces were between the ages of 20 and 40. Just to clarify, did they use all faces that fell within this age bracket, or were there any other criteria for inclusion/exclusion (so if someone wanted to replicate this study using the same stimulus set they’d have all the info they needed)? Would it maybe be possible to report mean age of the used faces?

• It still wasn’t entirely clear to me whether masking and scaling were done manually or using some sort of algorithm. Either is fine, but in the interest of reproducibility, this could be explained in more detail

• Analyses/reporting of results. The paragraph on Statistical Analysis (lines 167 ff) should include info on effect-coding (how were image- and observer-ethnicity coded). Table 3 should also include estimates and 95% CIs (I can provide code for that if the authors want). Minor point, but p-values <.001 should be reported as such (instead of p=.000), and leading 0s should be omitted.

• I recommend breaking up reporting of the three DVs into separate subsections to make them easier to digest. Similar to my comment in the previous round, Table 2 should include corresponding estimates, and analyses should not be split by image and observer ethnicity (as for Table 4, and interpretation of effects). For example, re the significant two-way interactions of L*/b* x observer ethnicity on attractiveness: There is a positive effect of l* for Chinese but not Western observers; there is a positive effect of b* that’s even stronger in Western faces; but the model does not indicate these effects differ significantly for Chinese and Western observers. The three-way interactions are easily enough explained by visualizing them—I’ve attached an example of the sort of graph I have in mind (left panel Western observers, right panel Chinese observers)

• The authors have now included F-values in Table 3, but not really addressed my comment—I meant that, e.g., on line 209, “interaction between image ethnicity and lightness was significant (p=0.017)” should instead be “(estimate=0.28, 95%CI [0.06, 0.51], t78.74=2.45, p=.017)”

Reviewer #2: The manuscript improved significantly after the revision. Specifically, for the Introduction, the rationale has been strengthened. A substantial amount of details have also been added into the Method section – which enhances both the clarity and replicability. The result section is also clear and is supported with appropriate figures. Discussion is also more comprehensive now.

A few suggestions for minor change were as followed,

1. Argument on Lines 73-76 should be supported with evidence.

2. For attractiveness, I reckon there is a significant effect of image ethnicity? Please check. See Lines 207-210 and Table 3.

3. For estimated age, is there is a significant interaction between the ethnicity of the image and observer? If so, is there any reason why it was not being included in the writing?

4. Western European observers associate an increase in yellowness with healthiness – only when viewing facial images of Chinese. Can you please check again your writing in the Result: Lines 232-233 (or maybe Lines 232-235, as a whole).

5. In Discussion, Line 276-277, the claim made here should only be relevant for the viewing of the European face. Please check.

6. For the Discussion, the authors have not properly fixed the carotenoids-yellowness-redness linkage. The authors start their claim by saying that the observed change in skin redness with the consumption of carotenoids was not consistent (in the lit). They then reported only those studies that observed NO significant change in skin redness (with the consumption of carotenoids). Hence the evidence presented does not seems to be matching the core argument. Perhaps this paper can be used to illustrate the “inconsistent” part - https://pubmed.ncbi.nlm.nih.gov/26186449/

7. Also, since carotenoids colouration is related to both skin yellowness and skin redness – I would suggest the author start the sub-section by saying that – “even though the association between carotenoids consumption and skin yellowness has been consistently reported in previous studies, but such consistency cannot be applied to the linkage between carotenoids intake and skin redness (with their phrasing of course). It seems awkward to include a claim merely highlighting skin redness in between two chunks of discussion about skin yellowness. See Lines 277-301 but specifically also the claim made on Lines 287-288.

Thank you.

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Reviewer #1: No

Reviewer #2: Yes: Tan Kok Wei

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Submitted filename: axobsxim.png

Click here for additional data file.

14 Oct 2021

We really appreaciate the editor, Alex Jones, for allowing the revision of our manuscript, with an opportunity to address both reviewers' comments. The comments from both reviewers are very detailed and really valuable for helping us to improve our manuscript. We have read all the comments and have revised our manuscript accordingly. We are uploading our point-by-point response to the comments, an updated manuscript with all the changes marked in red, and a clean updated manuscript without marks. Please review the changes and feel free to get back to us with any further questions/suggestions.

Submitted filename: Response to Reviewers.docx

Click here for additional data file.

18 Oct 2021

Skin coloration is a culturally-specific cue for attractiveness, healthiness, and youthfulness in observers of Chinese and Western European descent.

PONE-D-21-13133R2

Dear Dr. Wuerger,

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

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Alex Jones

Academic Editor

PLOS ONE

Additional Editor Comments (optional):

Reviewers' comments:

20 Oct 2021

PONE-D-21-13133R2

Skin coloration is a culturally-specific cue for attractiveness, healthiness, and youthfulness in observers of Chinese and western European descent

Dear Dr. Wuerger:

I'm pleased to inform you that your manuscript has been deemed suitable for publication in PLOS ONE. Congratulations! Your manuscript is now with our production department.

If your institution or institutions have a press office, please let them know about your upcoming paper now to help maximize its impact. If they'll be preparing press materials, please inform our press team within the next 48 hours. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information please contact onepress@plos.org.

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on behalf of

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Academic Editor

PLOS ONE