Red, Yellow, and Super-White Sclera

Beauty is in the iris: Constricted pupils (enlarged irises) enhance attractiveness

Physical attractiveness profoundly affects a broad array of life experiences and outcomes, and the eyes are an important determinant of physical attractiveness. We investigated whether a particular feature of the eyes - pupil size - affects perceived attractiveness. We present competing theoretical predictions of whether dilated (larger) or constricted (smaller) pupils should appear more physically attractiveness. Youthful features tend to be attractive (i.e., neoteny), and pupil size decreases across the lifespan, so dilated (enlarged) pupils may be more attractive as a signal of youth. Alternatively, constricted (small) pupils may be more attractive because, by revealing more of the iris, they increase both color and brightness of the eyes. The present experiments demonstrate that people appear more attractive when their pupils are constricted (Experiments 1-3). This effect is equally large with black-and-white images, indicating that color per se is not necessary for the effect (Experiment 4). Rather, constricted pupils make eyes appear brighter, which in turn renders the face more attractive (Experiment 5), even when controlling for how colorful the eyes appear (Experiment 6). These results identify constricted pupils as a novel facial feature that enhances attractiveness.

To Enhance, or not to Enhance: The Situational Context Shapes Women's Intentions on Amount and Diligence of Makeup Application

Makeup is a tool that women use to shape their image and gain benefits in both inter- and intrasexual selection. As makeup enables the hiding or enhancing of facial features, it allows women to strategically shape impressions in a given context. It affects interpersonal perceptions, workplace impressions, and can attract romantic partners. However, research has primarily focused on examining everyday makeup use, although the amount and type of makeup can vary depending on the situation and the motivation to make an impression. In two studies, we investigated how the intended amount and application of makeup differ depending on various situational contexts. Specifically, in Study 1 (N = 533), we explored the everyday and party contexts, while in Study 2 (N = 400), we additionally introduced the contexts of mating and threat. The results show that: 1) women intend to put on more makeup in a more diligent way in party contexts compared to everyday contexts, 2) the intended diligence of makeup application is the highest in contexts were women expect an attractive man to be present, and the lowest when a threatening context is introduced, 3) these effects are partially or fully mediated by women's motivation to make an impression, and 4) neither sociosexual orientation nor appearance orientation significantly moderated the obtained effects. Our studies extend previous research on makeup application as an appearance-enhancing or worsening strategy by further investigating the signaling function of women's makeup and its strategic use in various situational contexts. We discuss the results from a functional evolutionary perspective.

Eyelash length attractiveness across ethnicities

Eyelashes evolved to protect eyes. An optimum eyelash length functions to protect eyes from external hazards such as contaminations, excessive evaporation or shear stress from airflow. They can also be an indicator of a person's health as various congenital and noncongenital diseases can lead to short or long eyelashes. The current study aimed to extend a recent investigation on the preference for eyelash length in humans from an evolutionary adaptive perspective. Specifically, the current study tested whether the inverted-U function for eyelash length preference recently reported for White faces, generalises to other ethnicities, and whether ethnic background modulates preference for eyelash lengths. To investigate this question, men and women of Asian, Black, and White ethnicities from the U.S. rated the attractiveness of female Indian, Asian, Black, and White faces with varying eyelash lengths. The eyelashes ranged in length from no eyelashes to half the width of an eye. Results showed that Asian, Black, and White men and women preference for eyelash length followed an inverted-U function across all four ethnicities, supporting a general preference for human eyelash length that is approximately one-third the width of an eye. In addition, the results showed that the most attractive eyelashes for Black women were skewed toward a greater eyelash-length to eye-width ratio when compared to the other images. The source of this skew is presently unknown, as it could reflect a change in perceptual sensitivity to eyelash length with skin colour or changes in preference related to perceptions of participants' ethnicity.

Was Marilyn Monroe myopic and an early adopter of colored contact lenses? A review of the evidence and the early history of colored contact lenses

Wide availability and uptake of contact lenses came with the development of the first polymethyl methacrylate corneal lenses during the late 1940s. It is less well known that colored contact lenses were developed simultaneously. These innovations allowed both a degree of spectacle independence for ametropes and an ability to vary eye color. The impact on facial and ocular cosmesis was substantial, particularly for public figures such as actors. We have obtained contact lenses and matching casts manufactured by their inventor, Kevin M. Tuohy. Measurements of these suggest they were made for a myope, and we provide indirect evidence that the lenses were made for Marilyn Monroe. We also provide evidence that Monroe is likely to have been myopic, used colored contact lenses to change her eye color and may have been an early sufferer of contact lens overwear syndrome. The importance of ocular cosmesis can be overlooked in ophthalmic practice, yet it is of great interest to and importance for patients. It appears that discomfort, and even risk, will be tolerated to achieve a particular appearance. This is reflected in current techniques aimed at achieving improved cosmesis.

Five-year-old children show cooperative preferences for faces with white sclera

The cooperative eye hypothesis posits that human eye morphology evolved to facilitate cooperation. Although it is known that young children prefer stimuli with eyes that contain white sclera, it is unknown whether white sclera influences children's perception of a partner's cooperativeness specifically. In the current studies, we used an online methodology to present 5-year-old children with moving three-dimensional face models in which facial morphology was manipulated. Children found "alien" faces with human eyes more cooperative than faces with dark sclera (Study 2) but not faces with enlarged irises (Study 1). For more human-like faces (Study 3), children found human eyes more cooperative than either enlarged irises or dark sclera and found faces with enlarged irises cuter (but not more cooperative) than eyes with dark sclera. Together, these results provide strong support for the cooperative eye hypothesis.

The adaptive significance of human scleral brightness: an experimental study

Homogeneously depigmented sclerae have long been proposed to be uniquely human-an adaptation to enable cooperative behaviour by facilitating interpersonal coordination through gaze following. However, recent evidence has shown that deeply pigmented sclerae also afford gaze following if surrounding a bright iris. Furthermore, while current scleral depigmentation is clearly adaptive in modern humans, it is less clear how the evolutionarily intermediate stages of scleral pigmentation may have been adaptive. In sum, it is unclear why scleral depigmentation became the norm in humans, while not so in sister species like chimpanzees, or why some extant species display intermediate degrees of pigmentation (as our ancestors presumably did at some point). We created realistic facial images of 20 individually distinct hominins with diverse facial morphologies, each face in the (i) humanlike bright sclera and (ii) generalised apelike dark sclera version. Participants in two online studies rated the bright-sclera hominins as younger, healthier, more attractive and trustworthy, but less aggressive than the dark-sclera hominins. Our results support the idea that the appearance of more depigmented sclerae promoted perceived traits that fostered trust, increasing fitness for those individuals and resulting in depigmentation as a fixed trait in extant humans.

A songbird can detect the eyes of conspecifics under daylight and artificial nighttime lighting

Eyes convey important information about the external and internal worlds of animals. Individuals can follow the gaze of others to learn about the location of salient objects as well as assess eye qualities to evaluate the health, age or other internal states of conspecifics. Because of the increasing prevalence of artificial lighting at night (ALAN), urbanized individuals can potentially garner information from conspecific eyes under both daylight and ALAN. We tested this possibility using a visual modeling approach in which we estimated the maximum distance at which individuals could detect conspecific eyes under daylight and high levels of ALAN. We also estimated the minimum light level at which individuals could detect conspecific eyes. Great-tailed grackles (Quiscalus mexicanus) were used as our study species because they are highly social and are unusual among birds in that they regularly gather at nocturnal roosts in areas with high levels of ALAN. This visual modelling approach revealed that grackles can detect conspecific eyes under both daylight and ALAN, regardless of iris coloration. The grackles could detect conspecific eyes at farther distances in daylight compared to ALAN. Our results highlight the potential importance of lighting conditions in shaping social interactions.

A Novel Human Sex Difference: Male Sclera Are Redder and Yellower than Female Sclera

In a seminal study, Kobayashi and Kohshima (1997) found that the human sclera-the white of the eye-is unique among primates for its whitish color, and subsequent work has supported the notion that this coloration underlies the human ability to gaze follow. Kobayashi and Kohshima also claimed that there is no significant sex difference in sclera color, though no data were presented to support the claim. We investigated sclera color in a standardized sample of faces varying in age and sex, presenting the first data comparing male and female sclera color. Our data support the claim that indeed there is a sex difference in sclera color, with male sclera being yellower and redder than female sclera. We also replicated earlier findings that female sclera vary in color across the adult lifespan, with older sclera appearing yellower, redder, and slightly darker than younger sclera, and we extended these findings to male sclera. Finally, in two experiments we found evidence that people use sclera color as a cue for making judgements of facial femininity or masculinity. When sclera were manipulated to appear redder and yellower, faces were perceived as more masculine, but were perceived as more feminine when sclera were manipulated to appear less red and yellow. Though people are typically unaware of the sex difference in sclera color, these findings suggest that people nevertheless use the difference as a visual cue when perceiving sex-related traits from the face.

Experimental evidence that uniformly white sclera enhances the visibility of eye-gaze direction in humans and chimpanzees

Hallmark social activities of humans, such as cooperation and cultural learning, involve eye-gaze signaling through joint attentional interaction and ostensive communication. The gaze-signaling and related cooperative-eye hypotheses posit that humans evolved unique external eye morphologies, including uniformly white sclera (the whites of the eye), to enhance the visibility of eye-gaze for conspecifics. However, experimental evidence is still lacking. This study tested the ability of human and chimpanzee participants to discriminate the eye-gaze directions of human and chimpanzee images in computerized tasks. We varied the level of brightness and size in the stimulus images to examine the robustness of the eye-gaze directional signal against simulated shading and distancing. We found that both humans and chimpanzees discriminated eye-gaze directions of humans better than those of chimpanzees, particularly in visually challenging conditions. Also, participants of both species discriminated the eye-gaze directions of chimpanzees better when the contrast polarity of the chimpanzee eye was reversed compared to when it was normal; namely, when the chimpanzee eye has human-like white sclera and a darker iris. Uniform whiteness in the sclera thus facilitates the visibility of eye-gaze direction even across species. Our findings thus support but also critically update the central premises of the gaze-signaling hypothesis.

From an early age, we are able to detect the direction others are looking in (known as eye-gaze) and make eye contact with each other to communicate. The front of the human eye has a large white area known as the sclera that surrounds the darker colored iris and pupil in the center.

Compared to us, chimpanzees and other nonhuman great apes have sclerae that are much darker in color or at least not as uniformly white as human eyes. Some researchers believe that the white sclera of the human eye may have evolved to make it easier for other individuals to detect the direction of our gaze. However, there is a lack of experimental evidence as to whether white sclerae actually helps humans to distinguish the direction of eye-gaze.

Here, Kano, Kawaguchi and Yeow presented human and chimpanzee participants with images of other humans and chimpanzees on a computer screen and asked them to indicate the direction of eye-gaze in each image. The experiments found that both humans and chimpanzees were better able to discriminate the directions of eye-gaze from the images of humans than those of chimpanzees, particularly when the images were smaller or more shaded. Moreover, artificially altering the eyes in the chimpanzee images so that they were more human-like – that is, had a light-colored sclera and a darker iris – enabled both humans and chimpanzees to better discriminate the eye-gaze directions of the chimpanzees.

Kano, Kawaguchi and Yeow’s findings indicate that white sclerae do indeed help both humans and chimpanzees to discriminate the direction of eye-gaze, even though only humans have white sclerae. This is likely because humans use eye-gaze in key social activities (including learning languages, coordinating to complete complex tasks and transmitting cultural information), indicating that white sclerae may have evolved to enhance human-specific communication. To learn more about this type of communication, future research could focus on finding out when white sclerae first evolved.

An Evolutionary Perspective on Appearance Enhancement Behavior

Researchers have highlighted numerous sociocultural factors that have been shown to underpin human appearance enhancement practices, including the influence of peers, family, the media, and sexual objectification. Fewer scholars have approached appearance enhancement from an evolutionary perspective or considered how sociocultural factors interact with evolved psychology to produce appearance enhancement behavior. Following others, we argue that evidence from the field of evolutionary psychology can complement existing sociocultural models by yielding unique insight into the historical and cross-cultural ubiquity of competition over aspects of physical appearance to embody what is desired by potential mates. An evolutionary lens can help to make sense of reliable sex and individual differences that impact appearance enhancement, as well as the context-dependent nature of putative adaptations that function to increase physical attractiveness. In the current review, appearance enhancement is described as a self-promotion strategy used to enhance reproductive success by rendering oneself more attractive than rivals to mates, thereby increasing one's mate value. The varied ways in which humans enhance their appearance are described, as well as the divergent tactics used by women and men to augment their appearance, which correspond to the preferences of opposite-sex mates in a heterosexual context. Evolutionarily relevant individual differences and contextual factors that vary predictably with appearance enhancement behavior are also discussed. The complementarity of sociocultural and evolutionary perspectives is emphasized and recommended avenues for future interdisciplinary research are provided for scholars interested in studying appearance enhancement behavior.

Diversity in Primate External Eye Morphology: Previously Undescribed Traits and Their Potential Adaptive Value

Comparative examinations of external eye morphology in primates initially focused on communicative functions of the eye. Subsequent work has failed to find consistent associations between specific eye morphologies and communicative functions. In this article, we review the field of primate external eye morphology and inspect publicly available and unpublished photographs. We identify and describe five commonly occurring traits that have not received attention so far. We cross-examined the clinical and psychological literature to propose potential adaptive functions. These potential adaptive functions include communicative functions, but also photoregulatory functions and photoprotective functions.

Sclera and Iris Color Interact to Influence Gaze Perception

The white sclera is important in facilitating gaze perception in humans. Iris color may likewise influence gaze perception but no previous studies have directly assessed its effect. We therefore examined how the interaction between sclera and iris color influences human gaze perception. We recorded the eye movements of human participants as they performed a visual search task with human faces exhibiting directed or averted gaze. The faces either exhibited light or dark irises. In addition, the faces had sclera that were depigmented (white) or pigmented (matched the color of the iris). We found that participants were quick and accurate in evaluating gaze regardless of iris color in faces with depigmented sclera. When the sclera were pigmented, participants were slower to evaluate the gaze of faces with both light and dark irises but these effects were most pronounced in the faces with dark irises. Furthermore, participants were generally less accurate in assessing faces with pigmented sclera when the irises were dark rather than light. Our results suggest that depigmented sclera are especially important for gaze perception in faces with dark irises. Because depigmented sclera likely evolved at a time when ancestral humans exhibited dark irises, the depigmented sclera may have been crucial for efficient and accurate gaze perception in ancestral humans.

Why Care: Complex Evolutionary History of Human Healthcare Networks

One of the striking features of human social complexity is that we provide care to sick and contagious individuals, rather than avoiding them. Care-giving is a powerful strategy of disease control in human populations today; however, we are not the only species which provides care for the sick. Widespread reports occurring in distantly related species like cetaceans and insects suggest that the building blocks of care for the sick are older than the human lineage itself. This raises the question of what evolutionary processes drive the evolution of such care in animals, including humans. I synthesize data from the literature to evaluate the diversity of care-giving behaviors and conclude that across the animal kingdom there appear to be two distinct types of care-behaviors, both with separate evolutionary histories: (1) social care behaviors benefitting a sick individual by promoting healing and recovery and (2) community health behaviors that control pathogens in the environment and reduce transmission within the population. By synthesizing literature from psychology, anthropology, and biology, I develop a novel hypothesis (Hominin Pathogen Control Hypothesis) to explain how these two distinct sets of behaviors evolved independently then merged in the human lineage. The hypothesis suggests that social care evolved in association with offspring care systems whereas community health behaviors evolved as a type of niche construction. These two types of behaviors merged in humans to produce complex, multi-level healthcare networks in humans. Moreover, each type of care increases selection for the other, generating feedback loops that selected for increasing healthcare behaviors over time. Interestingly, domestication processes may have contributed to both social care and community health aspects of this process.

Intentionally distracting: Working memory is disrupted by the perception of other agents attending to you - even without eye-gaze cues

Of all the visual stimuli you can perceive, perhaps the most important are other people's eyes. And this is especially true when those eyes are looking at you: direct gaze has profound influences, even at the level of basic cognitive processes such as working memory. For example, memory for the properties of simple geometric shapes is disrupted by the presence of other eyes gazing at you. But are such effects really specific to direct gaze per se? Seeing eyes is undoubtedly important, but presumably only because of what it tells us about the "mind behind the eyes" - i.e., about others' attention and intentions. This suggests that the same effects might arise even without eyes, as long as an agent's directed attention is conveyed by other means. Here we tested the impact on working memory of simple "mouth" shapes - which in no way resemble eyes, yet can still be readily seen as intentionally facing you (or not). Just as with gaze cues, the ability to detect changes in geometric shapes was impaired by direct (compared to averted) mouths - but not in very similar control stimuli that were not perceived as intentional. We conclude that this disruption of working memory reflects a general phenomenon of "mind contact," rather than a specific effect of eye contact.

The Eyes as Windows Into Other Minds

Eyes have been shown to play a key role during human social interactions. However, to date, no comprehensive cross-discipline model has provided a framework that can account for uniquely human responses to eye cues. In this review, I present a framework that brings together work on the phylogenetic, ontogenetic, and neural bases of perceiving and responding to eyes. Specifically, I argue for a two-process model: a first process that ensures privileged attention to information encoded in the eyes and is important for the detection of other minds and a second process that permits the decoding of information contained in the eyes concerning another person's emotional and mental states. To some degree, these processes are unique to humans, emerge during different times in infant development, can be mapped onto distinct but interconnected brain regions, and likely serve critical functions in facilitating cooperative interactions in humans. I also present evidence to show that oxytocin is a key modulator of sensitive responding to eye cues. Viewing eyes as windows into other minds can therefore be considered a hallmark feature of human social functioning deeply rooted in our biology.

Put a (Limbal) Ring on It: Women Perceive Men's Limbal Rings as a Health Cue in Short-Term Mating Domains

Limbal rings are dark annuli encircling the iris that fluctuate in visibility based on health and age. Research also indicates their presence augments facial attractiveness. Given individuals' prioritization of health cues in short-term mates, those with limbal rings may be implicated as ideal short-term mates. Three studies tested whether limbal rings serve as veridical health cues, specifically the extent to which this cue enhances a person's value as a short-term mating partner. In Study 1, targets with limbal rings were rated as healthier, an effect that was stronger for female participants and male targets. In Study 2, temporally activated short-term mating motives led women to report a heightened preference for targets with limbal rings. In Study 3, women rated targets with limbal rings as more desirable short-term mates. Results provide evidence for limbal rings as veridical cues to health, particularly in relevant mating domains.

Gorillas with white sclera: A naturally occurring variation in a morphological trait linked to social cognitive functions

Human eye morphology is considered unique among the primates in that humans possess larger width/height ratios (WHR), expose a greater amount of visible sclera (SSI; width of exposed eyeball/width of visible iris), and critically, have a white sclera due to a lack of pigmentation. White sclera in humans amplifies gaze direction, whereas the all-dark eyes of apes are hypothesized to conceal gaze from others. This study examines WHR and SSI in humans (N = 13) and gorillas (N = 85) engaged in direct and averted gazes and introduces a qualitative assessment of sclera color to evaluate variations in sclera pigmentation. The results confirm previous findings that humans possess a larger WHR than gorillas but indicate that humans and gorillas display similar amounts of visible sclera. Additionally, 72% (N = 124) of gorilla eyes in this sample deviated from the assumed all-dark eye condition. This questions whether gaze camouflage is the primary function of darkened sclera in non-human primates or whether other functional roles can be ascribed to the sclera, light or dark. We argue that white sclera evolved to amplify direct gazes in humans, which would have played a significant role in the development of ostensive communication, which is communication that both shows something and shows the intention to show something. We conclude that the horizontal elongation of the human eye, rather than sclera color, more reliably distinguishes human from great ape eyes, represented here by gorillas.

Binocular Symmetry/Asymmetry of Scleral Redness as a Cue for Sadness, Healthiness, and Attractiveness in Humans

Among primates, only humans have the white sclerae that provide the ground necessary to display their own color and that of the overlying conjunctiva. Scleral color, primarily redness, provides cues of socially and biologically significant information about an individual. The present study examines the effect of the asymmetry of binocular scleral redness on perceived sadness, healthiness, and attractiveness by contrasting ratings of images of individuals who had one, both, or neither sclera reddened by digital editing. Building upon previous research, this study further defines the details of the scleral color display and contributes to the more general issue of facial and body symmetry, predictors of phenotypic condition and genotypic quality that are of interest to evolutionary theorists. Individuals with binocular and monocular redness were rated as sadder, less healthy, and less attractive than those with untinted control sclerae, with ratings corresponding to the degree of redness or whiteness. Bilaterally symmetrical (binocular) scleral redness or whiteness provided anchorage points for ratings, with ratings for bilaterally asymmetrical (monocular) redness or whiteness falling between them; there was no unique effect of asymmetry.