When the Whites of the Eyes are Red: A Uniquely Human Cue

Redness of the human eye is produced primarily by vasodilation of blood vessels of the conjunctiva, a membrane positioned over the sclera, the eye’s tough, white outer layer. Reddened eyes are a uniquely human cue because other primates lack the background of white sclera necessary to make the reddened conjunctiva visible. This study evaluates red eyes as a social and biological cue by contrasting the perception of eyes with normal ‘whites’ with copies of those eyes whose sclera were reddened by digital editing. Individuals with reddened sclera were perceived as sadder, less healthy, and less attractive than individuals with normal (white) sclera. Scleral whiteness joins such cues as smooth skin, long, lustrous hair, symmetry, averageness and sexually dimorphic traits as signs of health, beauty and reproductive fitness.     

The Emotional Eye: Red Sclera as a Uniquely Human Cue of Emotion

The sclera, the eye's tough white outer layer, provides the ground necessary for the display of its own color and that of the overlying membrane, the conjunctiva. This study evaluated the sclera as a cue of emotion by contrasting the ratings of 38 subjects for the level of anger, fear, sadness, disgust, happiness, or surprise of normal (untinted) eye images with copies of those images that were reddened by digital editing. Subjects rated individuals with reddened sclera as having more anger, fear, disgust, and sadness, and less happiness than those with normal, untinted sclera. Surprise was the only emotion unaffected by scleral redness. Humans, but not other primates, have evolved the white sclera necessary to display the blood flow in the overlying conjunctiva that produces the redness associated with certain emotional states.     

Dog Pups’ Attractiveness to Humans Peaks at Weaning Age

The domestic dog (Canis lupus familiaris) is characterized by greatly reduced parenting investment compared with the wild type wolf (C. l. lupus) from which it is descended. Unlike wolf pups, which are reared by both parents into their second year of life, dog pups are abandoned by their mother at weaning around eight weeks of age. This relatively small parental involvement may contribute to the high pup mortality observed in dogs not living as pets. We hypothesized that people would find dog pups most attractive around weaning age when conspecific parental care is significantly reduced and pup mortality rate is high. Younger and older pups would benefit less from human intervention because in the former case the mother is providing care, and in the latter their survival is already compromised. To test this hypothesis, 51 participants rated the attractiveness of 39 black and white headshot photographs presented on a computer screen of dog pups from three breeds (Jack Russell Terrier, Cane Corso, and White Shepherd), from birth to 7 months old. In line with our hypothesis, attractiveness of Cane Corsos peaked at 6.3 weeks of age; Jack Russell Terriers’ attractiveness peaked at 7.7 weeks; and White Shepherds were most attractive at 8.3 weeks. There were also differences in attractiveness between the breeds, with Cane Corsos rated less attractive than the other two breeds. If this attractiveness motivates humans to care for the dog pups and thereby improves pup survival, this could confer significant advantages to dogs, and may contribute to our understanding of the process of domestication.     

Categorical perception of human female physical attractiveness and health

Using realistic three-dimensional female body models, we found evidence for a categorical perception of female physical attractiveness and health in male and female Caucasian observers. In a rating task, we showed that these bodies were rated for attractiveness or health in the same way as real bodies. In a two-alternative forced-choice task, we showed that these bodies were categorized into attractive vs. unattractive or healthy vs. unhealthy nonlinearly, which allowed us to estimate the position of a categorical boundary between attractive and unattractive or healthy and unhealthy bodies. In a delayed match-to-sample task, we measured the sensitivity of discrimination between pairs of bodies. We found significantly better discrimination for pairs that crossed the attractive/unattractive or healthy/unhealthy boundary than pairs that did not, even though the physical changes in both conditions were identical. Thus, categorical perception enhances the perception of physical changes that cross the boundary between discrete perceptual categories of important judgments such as attractiveness or health, which can be a cue for mate selection.     

Sex,Race/Ethnicity,Socioeconomic Status,and BMI in Relation to Self‐Perception of Overweight

Objective: To compare the self‐perception of overweight in the study population according to sex, race/ethnicity, and socioeconomic status and to compare the self‐perception of overweight among individuals classified as normal weight, overweight, and obese. Research Methods and Procedures: Data from 5440 adults who participated in the 1994 to 1996 Continuing Survey of Food Intakes by Individuals and the Diet and Health Knowledge Survey conducted by the U.S. Department of Agriculture were analyzed. Data for analysis included self‐perceived weight status, self‐reported weight and height, and demographic and socioeconomic data. Underweight individuals, defined as those with a body mass index <18.5 kg/m², were excluded from the analysis. Results: Self‐perception of overweight was more common in women compared with men and in whites compared with blacks or Hispanics. Both the correct and incorrect perception of overweight was more common in normal weight and overweight white women compared with black women. More overweight and obese white men correctly perceived their overweight status compared with black men. Multiple logistic regression showed that the odds ratio of perceived overweight was significantly higher in women, whites, and individuals with higher body mass index, higher income, and higher education. Discussion: Self‐perceived overweight varied by sex, race/ethnicity, and socioeconomic status. Erroneous perception of body weight may have important health and behavioral implications. In particular, a considerable proportion of overweight men may be at risk of obesity if they continue to perceive themselves as having normal weight.     

Reliance on head versus eyes in the gaze following of great apes and human infants: the cooperative eye hypothesis

As compared with other primates, humans have especially visible eyes (e.g., white sclera). One hypothesis is that this feature of human eyes evolved to make it easier for conspecifics to follow an individual's gaze direction in close-range joint attentional and communicative interactions, which would seem to imply especially cooperative (mututalistic) conspecifics. In the current study, we tested one aspect of this cooperative eye hypothesis by comparing the gaze following behavior of great apes to that of human infants. A human experimenter "looked" to the ceiling either with his eyes only, head only (eyes closed), both head and eyes, or neither. Great apes followed gaze to the ceiling based mainly on the human's head direction (although eye direction played some role as well). In contrast, human infants relied almost exclusively on eye direction in these same situations. These results demonstrate that humans are especially reliant on eyes in gaze following situations, and thus, suggest that eyes evolved a new social function in human evolution, most likely to support cooperative (mututalistic) social interactions.     

Whole body bone,fat and lean mass in children: Comparison of three ethnic groups

We measured whole body bone, fat and lean mass, by dual-energy x-ray absorptiometry, of third-grade children in a suburban public school district adjacent to Detroit. Of 1,340 eligible children, 773 participated. Using U.S. Census categories, parents identified their children as black/African-American (57%), white (38%), or one of several other categories (5%). Some of the participants also identified with a relatively large Middle Eastern subgroup (Chaldeans). Of the 773 participants, 734 are included in this report (71 Chaldeans, 226 whites, and 437 black/African-Americans; other categories are omitted). We describe body size, body composition, and physical activity levels in the three groups. The Chaldean and black children have significantly higher average whole body bone mineral content (BMC) than whites (P > 0.05), but are not different from each other. Lean mass and height are significantly greater for Chaldeans and blacks than for whites. The ratio of BMC to height was also significantly greater in Chaldeans and blacks compared with whites. Chaldeans have a significantly higher weight and fat mass than either the black or white children, and report significantly less physical activity than either the white or the black children. The higher bone mass among the Chaldean children may be partially explained by their greater body mass, but there is no readily apparent explanation for the observed ethnic differences in body size. We cannot exclude genetic or environmental factors not evaluated in this observational study. Our unexpected finding that Chaldean children, when analyzed as a separate group, are more similar in body composition to black/African-American than to white children contributes to a growing body of literature indicating that the uncritical use of “race” categories may obscure rather than facilitate the identification of population differences. Am J Phys Anthropol 103:157–162, 1997. © 1997 Wiley-Liss, Inc.     

Beautiful fruits taste good: the aesthetic influences of fruit preferences in humans

Although diversity of fruit/seed colouration has received a great amount of attention since Darwin, little is known about its role in eating preferences in humans. We have determined that humans prefer certain fruits/seeds over others and that their willingness to eat them has been significantly influenced by the perceived aesthetic of the presented fruits and seeds. Participants were unable to discriminate between edible and poisonous fruits/ seeds based on their colour. Females rated all the groups of fruits/seeds as more attractive than males with this supporting the role of females in picking fruit in our evolutionary past. Red fruits were rated as more attractive than green or brown fruits. The results support the idea that fruit/seed colouration plays an important role in plant--disperser coevolution and that aesthetic judgment in humans have been shaped by natural selection.     

The role of facial hair in women's perceptions of men's attractiveness,health, masculinity and parenting abilities

Facial hair strongly influences people's judgments of men's socio-sexual attributes. However, the nature of these judgments is often contradictory. The levels of intermediate facial hair growth presented to raters and the stage of female raters' menstrual cycles might have influenced past findings. We quantified men's and women's judgments of attractiveness, health, masculinity and parenting abilities for photographs of men who were clean-shaven, lightly or heavily stubbled and fully bearded. We also tested the effect of the menstrual cycle and hormonal contraceptive use on women's ratings. Women judged faces with heavy stubble as most attractive and heavy beards, light stubble and clean-shaven faces as similarly less attractive. In contrast, men rated full beards and heavy stubble as most attractive, followed closely by clean-shaven and light stubble as least attractive. Men and women rated full beards highest for parenting ability and healthiness. Masculinity ratings increased linearly as facial hair increased, and this effect was more pronounced in women in the fertile phase of the menstrual cycle, although attractiveness ratings did not differ according to fertility. Our findings confirm that beardedness affects judgments of male socio-sexual attributes and suggest that an intermediate level of beardedness is most attractive while full-bearded men may be perceived as better fathers who could protect and invest in offspring.     

Female health,attractiveness, and desirability for relationships: Role of breast asymmetry and waist-to-hip ratio

Fluctuating asymmetry (FA) of ordinarily bilaterally symmetrical traits in humans has been proposed to indicate developmental anomaly. Recent research has shown that individuals with minimal FA are judged to be attractive, and preferred as sexual partners (Thornhill and Gangestad 1993). Waist-to-hip ratios (WHR) have been also shown to reflect health and reproductive capability of woman and those with low WHRs are judged more attractive and healthy (Singh 1993a,b). The present study examines the relative roles of WHR and FA in female breasts in judgments of female attractiveness, health, and desirability for short-term and long-term relationships. Male college students were asked to judge attractiveness of female figures that differed in WHR (high and low) and breast symmetry (none, low, or high). In the first test, paired comparison method was used in which each figure was paired one at a time with all other figures. In the second test, subjects examined all figures simultaneously, estimated their age, and rated each figure for attractiveness, feminine looks, health, overall degree of body symmetry, and willingness to engage in short- and long-term relationship. Results from both tests show that figures with low WHRs were judged to be more attractive than figures with high WHRs, regardless of their degree of breast asymmetry. The figure with low WHR and symmetrical breasts was judged to be most attractive and youngest of all other figures. It appears that men use both WHR and breast asymmetry in judging attractiveness and being willing to develop romantic relationships. It is proposed that WHR and breast asymmetry may signal different aspects of overall female mate quality.     

Ethnic differences in body composition and other markers of cardiovascular disease risk: study in matched Haitian and White subjects from Quebec

Objectives: People of African descent may be at greater risk of metabolic syndrome (MS) compared with whites. We examined the associations among MS markers, body composition, and resting metabolic rate (RMR) in black Haitians and in white subjects living in Quebec, Canada. Research Methods and Procedures: Forty randomly selected Haitians were matched with 40 white subjects for age, sex, and BMI. Glycemic status and insulin resistance were assessed based on a 3‐hour glucose tolerance test. Blood lipids, blood pressure, abdominal fat (computed tomography), and waist circumference (WC) were measured. RMR was estimated by indirect calorimetry. Results: Triglycerides were significantly correlated with blood pressure only in Haitians and with the area under the curve for insulin only in whites. Haitians had significantly (p < 0.05) lower triglycerides and higher high‐density lipoprotein‐cholesterol concentrations but higher blood pressure than whites at any given WC value. General linear models showed that Haitians had less visceral adipose tissue than whites for the same WC. RMR was lower among Haitians for any given value of BMI or WC than in whites. Also, WC was more strongly associated with glucose area under the curve and to log‐homeostasis model assessment in white than in Haitian subjects. Discussion: The MS may be ethnospecific in its features and etiology. The standard anthropometric indices of obesity may not be as effective in populations of African descent compared with whites, unless appropriate cut‐off values are defined.     

Fitness Alters the Associations of BMI and Waist Circumference with Total and Abdominal Fat

Objective: We tested the following hypotheses in black and white men and women: 1) for a given BMI or waist circumference (WC), individuals with moderate cardiorespiratory fitness (CRF) have lower amounts of total fat mass and abdominal subcutaneous and visceral fat compared with individuals with low CRF; and 2) exercise training is associated with significant reductions in total adiposity and abdominal fat independent of changes in BMI or WC. Research Methods and Procedures: The sample included 366 sedentary male (111 blacks and 255 whites) and 462 sedentary female (203 blacks and 259 whites) participants in the HERITAGE Family Study. The relationships between BMI and WC with total fat mass (determined by underwater weighing) and abdominal subcutaneous and visceral fat (determined by computed tomography) were compared in subjects with low (lower 50%) and moderate (upper 50%) CRF. The effects of a 20‐week aerobic exercise training program on changes in these adiposity variables were examined in 86% of the subjects. Results: Individuals with moderate CRF had lower levels of total fat mass and abdominal subcutaneous and visceral fat than individuals with low CRF for a given BMI or WC value. The 20‐week aerobic exercise program was associated with significant reductions in total adiposity and abdominal fat, even after controlling for reductions in BMI and WC. With few exceptions, these observations were true for both men and women and blacks and whites. Discussion: These findings suggest that a reduction in total adiposity and abdominal fat may be a means by which CRF attenuates the health risk attributable to obesity as determined by BMI and WC.     

Common HLA alleles associated with health, but not with facial attractiveness

Three adaptive hypotheses have been proposed to explain the link between the human leucocyte antigen (hla) genes, health measures and facial attractiveness: inbreeding avoidance, heterozygote advantage and frequency-dependent selection. This paper reports findings that support a new hypothesis relating HLA to health. We suggest a new method to quantify the level of heterozygosity. HLA heterozygosity did not significantly predict health measures in women, but allele frequency did. Women with more common HLA alleles reported fewer cold and flu bouts per year, fewer illnesses in the previous year and rated themselves healthier than women with rare alleles. To our knowledge, this is the first study to show a positive correlation between HLA allele frequency and general health measures. We propose that certain common HLA alleles confer resistance to prevalent pathogens. Nevertheless, neither HLA heterozygosity nor allele frequency significantly predicted how healthy or attractive men rated the female volunteers. Three non-mutually exclusive explanations are put forward to explain this finding.     

MHC-assortative facial preferences in humans

Individuals tend to choose mates who are sufficiently genetically dissimilar to avoid inbreeding. As facial attractiveness is a key factor in human mate preference, we investigated whether facial preferences were related to genetic dissimilarity. We asked female volunteers to rate the attractiveness of men from photographs and compared these results with individual genotypes at the major histocompatibility complex (MHC). In contrast to previously reported preferences based on odour, we found a non-significant tendency for women to rate MHC-similar faces as more attractive, suggesting a preference for cues to a self-similar MHC in faces. Further analysis revealed that male faces received higher attractiveness scores when rated by women who were MHC-similar than by MHC-dissimilar women. Although unexpected, this MHC-similar facial preference is consistent with other studies documenting assortative preferences in humans, including for facial phenotype.     

Ambient odor's effect on creativity, mood, and perceived health

In two sessions held 1 week apart, subjects completed a performancetask involving creativity, four personality tests, and questionnairesconcerning their mood, perceived health, and perceptions ofthe testing environment. In one session the testing room wasscented with lemon, lavender or dimethyl sulfide (DMS); in theother session it was unscented. There were 15 women and 15 menin each odor conditon. Fewer health symptoms were reported inthe lemon conditon on scented compared to unscented days. Subjectsin the DMS group were in a less pleasant mood than those inthe lavender group on both scented and unscented days; the orderin which subjects were exposed to DMS played a role in the moodfindings. There were no significant differences in feelingsof arousal or control. The room was rated as smelling less pleasanton scented compared to unscented days by subjects in the DMScondition. Differences in creativity performance were not significant,but relationships emerged between personality traits and theeffect of odor on task performance. These findings are discussedin terms of how associations and expectations concerning odorsmay play a role in odor's effect on humans.     

The effect of race on the incidence of low birth weight: persistence of effect after controlling for socioeconomic, educational, marital, and risk status

The purpose of this study was to determine whether the elevated risk for low birth weight (LBW) infants among black mothers would persist when biologic, behavioral, and socioeconomic factors (as measured by socioeconomic status, level of education, and marital status) were controlled. It was found that the odds ratios for the risk of LBW for blacks/whites persisted above 1.5, regardless of what subgroups were used and what factors were controlled. The black/white odds ratios were, however, less than 2.0 when cigarette smoking was not a risk factor and higher than 2.0 when it was. In fact, the highest odds ratios, up to 2.65, occurred among the smoking group. These data suggest that smoking may have a more strongly negative effect among black than white pregnant mothers. In general, the effect of race on the LBW risk was much less strong than that of risk factors that can be influenced, such as adverse maternal practices.     

Preferences for Infant Facial Features in Pet Dogs and Cats

A set of infant features (large forehead, large and low‐lying eyes, and bulging cheeks), were described in classical ethology as social releasers, simple stimuli that evoke a stereotyped response, in this case nurturing. We assessed the attractiveness of such features in the faces of dogs or cats (adults and young) or teddy bears or human infants, and also related these preferences to the degree of attachment to a pet. Overall, faces with the infant features were rated as more attractive than those without. Human infant faces were no more attractive than those of kittens or puppies. Pet faces were rated as more attractive by pet owners than non‐pet owners, regardless of whether the faces had infant features. A preference was also found for infant features in teddy bear faces. Women showed higher ratings than men for pets with infant features, but not for human infants or pets without infant features. Parents found human infants’ faces more attractive than did non‐parents, but there were no differences for other faces with infant features. Preferences were to some extent specific to the participant’s preferred pet species. Owners who were more strongly attached to their pets showed stronger preferences for photographs with infant features. The findings are discussed in terms of the concept of social releaser, and its part in the development of attachment to a pet species.     

The masculinity paradox: facial masculinity and beardedness interact to determine women's ratings of men's facial attractiveness

In many species, male secondary sexual traits have evolved via female choice as they confer indirect (i.e. genetic) benefits or direct benefits such as enhanced fertility or survival. In humans, the role of men's characteristically masculine androgen‐dependent facial traits in determining men's attractiveness has presented an enduring paradox in studies of human mate preferences. Male‐typical facial features such as a pronounced brow ridge and a more robust jawline may signal underlying health, whereas beards may signal men's age and masculine social dominance. However, masculine faces are judged as more attractive for short‐term relationships over less masculine faces, whereas beards are judged as more attractive than clean‐shaven faces for long‐term relationships. Why such divergent effects occur between preferences for two sexually dimorphic traits remains unresolved. In this study, we used computer graphic manipulation to morph male faces varying in facial hair from clean‐shaven, light stubble, heavy stubble and full beards to appear more (+25% and +50%) or less (?25% and ?50%) masculine. Women (N = 8520) were assigned to treatments wherein they rated these stimuli for physical attractiveness in general, for a short‐term liaison or a long‐term relationship. Results showed a significant interaction between beardedness and masculinity on attractiveness ratings. Masculinized and, to an even greater extent, feminized faces were less attractive than unmanipulated faces when all were clean‐shaven, and stubble and beards dampened the polarizing effects of extreme masculinity and femininity. Relationship context also had effects on ratings, with facial hair enhancing long‐term, and not short‐term, attractiveness. Effects of facial masculinization appear to have been due to small differences in the relative attractiveness of each masculinity level under the three treatment conditions and not to any change in the order of their attractiveness. Our findings suggest that beardedness may be attractive when judging long‐term relationships as a signal of intrasexual formidability and the potential to provide direct benefits to females. More generally, our results hint at a divergence of signalling function, which may result in a subtle trade‐off in women's preferences, for two highly sexually dimorphic androgen‐dependent facial traits.     

Increased aggrecan (cartilage proteoglycan) production in the sclera of myopic chicks

A previously characterized chick model of myopia was used to evaluate biochemical changes in the sclera which are associated with ocular enlargement and myopia. Chicks were monocularly occluded for 10 days and the DNA, hydroxyproline, and glycosaminoglycan contents of the sclera were compared between the normal and the myopic eyes. No significant differences could be detected in total DNA or hydroxyproline content. There was, however, a 34% increase in glycosaminoglycans and a 20.7% decrease in cell density within the posterior sclera of myopic eyes. The biosynthesis of scleral proteoglycans was determined by measuring 35SO4 incorporation in the sclera of chicks visually occluded for 5, 10, and 15 days. No differences could be detected in 35SO4 incorporation into the cornea or the anterior sclera. However, 35SO4 incorporation was significantly increased in the posterior sclera of myopic eyes by 64% at Day 5, 39% at Day 10, and 49% at Day 15. When fractionated on Sepharose CL-4B, scleral proteoglycans were resolved into two peaks which were identified by Western blot analysis as aggrecan (cartilage proteoglycan) and decorin. Furthermore, Western blot and dot blot analyses indicated that significantly more aggrecan core protein was present in the sclera of myopic eyes compared with equivalent amounts of sclera from control eyes. These results indicate that increased synthesis and accumulation of aggrecan, which increases the volume of extracellular matrix in the posterior sclera, are responsible for the ocular enlargement observed in this model of myopia.     

Chimpanzees and Humans Mimic Pupil-Size of Conspecifics

Group-living typically provides benefits to individual group members but also confers costs. To avoid incredulity and betrayal and allow trust and cooperation, individuals must understand the intentions and emotions of their group members. Humans attend to other''s eyes and from gaze and pupil-size cues, infer information about the state of mind of the observed. In humans, pupil-size tends to mimic that of the observed. Here we tested whether pupil-mimicry exists in our closest relative, the chimpanzee (P. troglodytes). We conjectured that if pupil-mimicry has adaptive value, e.g. to promote swift communication of inner states and facilitate shared understanding and coordination, pupil-mimicry should emerge within but not across species. Pupillometry data was collected from human and chimpanzee subjects while they observed images of the eyes of both species with dilating/constricting pupils. Both species showed enhanced pupil-mimicry with members of their own species, with effects being strongest in humans and chimpanzee mothers. Pupil-mimicry may be deeply-rooted, but probably gained importance from the point in human evolution where the morphology of our eyes became more prominent. Humans'' white sclera surrounding the iris, and the fine muscles around their eyes facilitate non-verbal communication via eye signals.