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When artists create portraits, they rarely depict a full frontal view of the face of a given sitter. Instead, they tend to portray people in some degree of profile, emphasizing one cheek and dimensionality of a face (Murphy, 1994). Interestingly, vast majority of studies on facial perception uses frontal portraits (un/altered photographs, morphs, or line drawings) as stimuli (e.g., Thornhill and Gangestad, 1999; Rhodes, 2006; Kościnski, 2009; Calder et al., 2011; Little et al., 2011; Valentová et al., 2013; Little, 2014). Given, however, that in our daily lives we experience faces from multiple angles, it is far from certain that a frontal view is the optimal depiction and several studies even suggested that an individual's appearance can significantly vary depending on the viewing angle (Rule et al., 2009; Jenkins et al., 2011; Tigue et al., 2012; Kościnski and Zalewska, 2017; Sutherland et al., 2017). Faces are, after all, complex and highly variable morphological structures (Enlow et al., 1996) and some facial features are apparent only from some viewing angles (Danel et al., 2018). For example, Danel et al. (2018) reported only a moderate correlation in sexually dimorphic features between lateral and frontal facial configuration in both men and women. When frontal and lateral facial configurations were compared as to their averageness, a significant association was found only in women. It is therefore plausible to assume that complementary information may be provided by different viewing angles. A single frontal view could potentially obscure relevant visual cues used in assessing certain dimensions (e.g., determinants of facial masculinity, such as protrusion of the brow ridge and angularity of the jaw), thus reducing judgment accuracy.

In research on body perception, the use of other than just frontal view is becoming increasingly common (Tovée and Cornelissen, 2001; Perilloux et al., 2012; Sell et al., 2017; Cornelissen et al., 2018). The use of multiple body angles views allows for assessments of multivariate trait interactions (Brooks et al., 2015). Varying viewing angles of bodies allow raters to assess the shapes and sizes of various morphological characteristics, such as body fat, lean mass distribution, or breast morphology (Dixson et al., 2011, 2015) which all contribute to the resulting attractiveness rating.

Research on facial perception that employs other than frontal facial views remains, however, at best unsystematic (Kościnski, 2009) and mutual relations between the frontal and lateral dimensions of facial features have so far received very little attention (Danel et al., 2018). Profile views have been used primarily in orthodontics and aesthetic medicine because it is known that they have an impact on facial attractiveness judgments (Spyropoulos and Halazonetis, 2001; Johnston et al., 2005; Maple et al., 2005; Soh et al., 2007; Shafiee et al., 2008; Nomura et al., 2009). Results from several studies that investigated the averageness of facial profiles show patterns analogous to frontal images (Spyropoulos and Halazonetis, 2001; Minear and Park, 2004; Valentine et al., 2004; Valenzano et al., 2006). Some researchers, meanwhile, tried to overcome the limitations of a single view stimulus by presenting raters with both frontal and lateral views of targets on a single screen (e.g., Dixson and Rantala, 2015; Dixson et al., 2016; Valentova et al., 2017), while other studies found a medium to high correlation between the rating of attractiveness of frontal and lateral depictions (ranging from r = 0.52 to 0.83) (Diener et al., 1995; Valenzano et al., 2006; Davidenko, 2007; Shafiee et al., 2008; Kościnski and Zalewska, 2017).

Until recently, most studies used as stimuli static, two-dimensional images (photographs). Thanks to technological progress, including a considerable increase in computers' computing powers, 3D scanning and 3D reconstruction technology is now becoming more accessible to facial perception research (Toole et al., 1999; Caharel et al., 2009; Chelnokova and Laeng, 2011; Meyer-Marcotty et al., 2011; Jones et al., 2012; Lefevre et al., 2012; Tigue et al., 2012; Berssenbrügge et al., 2014; Mydlová et al., 2015; Holzleitner and Perrett, 2016; Hu et al., 2017; Kordsmeyer et al., 2018). Potential bias associated with a single 2D image (e.g., profile) might be minimized by the use of 3D images, which represent various viewing angles. To our best knowledge, however, only one study directly compared ratings based on 2D and 3D facial images (Tigue et al., 2012). Authors found a high correlation between 2D and 3D stimuli (r = 0.71), with mean ratings significantly higher for 3D images. On the other hand, it should be noted that in this study, only opposite-sex ratings were performed (female faces were rated by male participants), on a single scale (attractiveness), and the only 2D depictions used were frontal portraits.

Current evidence suggests a rather high level of congruence in judged characteristics (especially attractiveness) between frontal and lateral or frontal and 3D views of faces. It should, however, be taken into account that the development of morphological features between the frontal and lateral view does not always correlate (Danel et al., 2018) and one could thus expect that some socially relevant traits may be easier to assess from other than frontal view (Tigue et al., 2012).

In the two studies, we estimated the congruence in perception of three different views of male heads (frontal portrait, left profile, and 360° rotation photographs). We employed two characteristics relevant in the context of intra- and inter-sexual selection, namely the rating of formidability and attractiveness. We also explored whether the type of device used (mobile phones, laptop, and desktop computers) influences the ratings.

All procedures employed in this study conform to the ethical standards of the relevant committee on human experimentation and with the Helsinki Declaration. The study was approved by the Institutional Review Board of National Institute of Mental Health, Czech Republic (Ref. num. 28/15). All participants were informed about the goals of the study and gave their informed consent. The present study is part of a larger project which investigates multimodal perception of traits associated with sexual selection and characteristics related to competition outcome.

The main aim of this study was to examine whether perception of formidability and attractiveness varies depending on the angle under which a face is viewed. To this purpose, we used standardized sets of frontal portraits, left profiles, and 360° head rotations of male facial images. We found strong correlations between the three types of stimuli and no significant differences in the mean ratings of formidability and attractiveness. Our results thus showed that ratings based on the three different face views were highly congruent and both perceived formidability and attractiveness ratings appear to be view-invariant. As a subsidiary aim, we have also tested the effect of the device used on attractiveness rating. While there were no differences between ratings performed on desktop and laptop computers, ratings performed using the mobile phones were higher (targets were perceived as more attractive).

Majority of facial perception research uses as stimuli frontal images (Kościnski, 2009), which is in striking contrast with our daily life experience. Moreover, there is a long-standing debate on how human visual system recognizes objects viewed from different angles (Hayward, 2003) and whether object recognition is view-specific, i.e., linked to a specific viewing orientation (Tarr and Bülthoff, 1995), or view-invariant (Biederman and Gerhardstein, 1993). Some evidence suggests that human visual system may be view-specific and process objects differently depending on the viewing angle (Jeffery et al., 2007; but see Jiang et al., 2006). If this were the case, results from perceptual studies that rely solely on frontal portraits could not be generalized to the other viewpoints. Our results, however, at least when it comes to social perception, do not support this hypothesis.

Our data shows patterns analogous in direction and magnitude to those reported in previous studies that compared assessments based on different stimuli views of both faces and bodies (e.g., frontal × profile, frontal × 3D or oblique poses), which likewise showed strong correlations in ratings (Diener et al., 1995; Tovée and Cornelissen, 2001; Valenzano et al., 2006; Davidenko, 2007; Shafiee et al., 2008; Perilloux et al., 2012; Tigue et al., 2012; Dixson et al., 2015; Kościnski and Zalewska, 2017). A related study by Tigue et al. (2012) reported that attractiveness of frontal and 3D depictions of women's faces as rated by men were highly correlated (r = 0.71) but 3D stimuli received significantly higher mean ratings. Authors suggest that their findings may be an effect of novelty of the 3D visualization. Our study, on the other hand, found no differences between the mean ratings of 2D (frontal and profile) and 3D images. It should be noted, however, that we opted for an alternative to standard 3D visualization. By combining several individual photographs presented in sequence as a spin (360° rotation), we avoided possible bias based simply on differences in capture technology (such as noticeable differences in lighting and colors between 2D and 3D stimuli).

The 360° rotation photographs allowed us to present raters with an all-around view of stimuli heads without running the cost of acquiring 3D capture technologies. Although the resulting visualizations are indeed photorealistic, there is a notable drawback related to implementing this procedure. The capturing and subsequent processing of the images is rather time-consuming and physically demanding, especially for the photographed targets, because one spin takes approx. 5 min and during this time, targets have to sit completely still with fixed gaze, so that controlling for head tilts, yawns, and rolls thus becomes even more critical (Penton-Voak et al., 2001; Hehman et al., 2013; Sulikowski et al., 2015). The use of 3D stimuli captured with actual 3D scanning and 3D reconstructions technology would allow for a variety of target applications including stimuli capture and presentation. For instance, resulting models could be rotated to arbitrary angles relative to their position during capture, rather than simply displayed in an identical head position in all photographs. Such 3D stimuli would produce more realistic face reconstructions: the main obstacle is the relatively high initial investment into a 3D scanner. Moreover, although 3D facial models are remarkably human-like, they are certainly distinguishable from, and less familiar than, photographs and that could potentially reduce their validity in terms of being a realistic visualization of humans (Crookes et al., 2015). Future studies should investigate whether perception of 3D models differs from 360° rotation photographs.

Interestingly, we found that the device used for viewing the stimuli has a significant influence on the rating. Raters using mobile phones gave on average higher attractiveness ratings than users of laptop or desktop computers. To our best knowledge, no previous study investigated the influence of the device used for viewing on ratings. Although the screen size and resolution of mobile phones are increasing, screen size of handheld devices does limit the size of images that can be viewed on it. That negatively influences the amount of detailed visual information available to the observer, hence potentially limiting the visibility of cues that may affect some aspects of social perception (such as attractiveness). All this may result in ratings higher than those based on viewing images on larger screens which do show more detail. For instance, several studies have reported that more homogenous skin is perceived as more attractive (Jones et al., 2004; Fink et al., 2006; Tsankova and Kappas, 2016; Jaeger et al., 2018; Tan et al., 2018). It is thus possible that lower visibility of such types of imperfections on mobile phones may lead to higher scores. This is a potentially important issue since ever more researchers opt for online data collection. One ought to take into consideration the kind of devices raters decide to use for their viewing and rating, because if a specific subgroup of raters systematically chooses to use a particular kind of device, it could bias the results. In our case, mobile phones were used by nearly one quarter of raters. The results we report are correlations and we just assume that differences in ratings were influenced by the kind of device raters used. In theory, it is possible that a particular group of raters simultaneously tended to give higher ratings and used mobile phones for viewing and rating. It is likely, however, that the two phenomena are independent of one another, because we found no differences between rater groups in other characteristics.

A potential limitation of our study is the fact that we used a rather specific sample of targets, namely MMA athletes. This fact might limit the generalization of our results. One could expect that MMA fighters would be perceived as highly formidable opponents, as rather specific in appearance (“cauliflower” ears, broken noses, eyebrow scares, etc.), which is why their ratings of formidability and attractiveness might be less variable and/or skewed. In our study, however, raters were not explicitly told that the targets presented to them are MMA fighters. What we found was that mean formidability rating of all three stimuli types on a 7-point scale ≈ 4 (ranging from 2 to 6.2) and skewness of all three stimuli types were between 0.097 and 0.189 (Table 2) and data followed normal distribution. For attractiveness, mean ratings for all three stimuli types were between 2.8 and 2.93 (ranging from 1.7 to 4.8) and skewness between 0.111 and 0.578 (Table 2), hence comparable to average ratings of male facial attractiveness in other studies (e.g., Saribay et al., 2018). It thus seems that the specific nature of our sample does not impede generalization of our finding. Nevertheless, future studies based on less specific samples should further investigate this issue.

To conclude, the findings presented here, along with other recent studies, provide converging evidence that single and multiple view facial images convey highly overlapping information and a single angle view contains enough information about the spatial structural elements of a face to congruently assess formidability and attractiveness, at least in the case of male faces. These results also suggest that studies which use different types of stimulus depiction are, generally speaking, comparable: this ought to simplify the interpretation of individual studies.

VT, JF, and JH developed the study concept. KK contributed to the study design. Data collection was performed by VT, JF, DS, and ZŠ. VT performed data analysis and interpretation, VT and JF drafted the manuscript, and DS, ZŠ, KK, and JH provided critical revisions. All authors approved the final version of the manuscript for submission.