A darker shade of pale

 

Subjects identified the left-hand image as a woman and the right-hand one as a man. Yet the two images differ only in skin tone. Study by Richard Russell, Sinha Laboratory for Vision Research, MIT.

 

Skin color differs by sex: women are fairer and men browner and ruddier. Women also exhibit a greater contrast in luminosity between their facial skin and their lip and eye areas. These differences arise from differing concentrations of three skin pigments: melanin (brown); hemoglobin (red); and carotene (yellow). The cause is ultimately hormonal, as shown by studies on castrated and ovariectomized adults, on boys and girls during puberty, and on digit ratios (Edwards and Duntley, 1939; Edwards and Duntley, 1949; Edwards et al., 1941; Frost, 2010; Kalla and Tiwari, 1970; Manning et al., 2004; Mesa, 1983; Omoto,1965; Porcheron et al., 2013). Women are fairer than men in all human populations. The difference is greatest in people of medium color and least in very dark- or very fair-skinned people, apparently because of "floor" or "ceiling" effects (Frost, 2007). 

This sex difference is used by the human mind for sex recognition. In fact, it's more important for this purpose than other visual cues, like face shape. When subjects are shown an image of a human face, they can tell whether it is male or female even if blurred and differing only in hue and luminosity. Hue provides a "fast channel" for sex recognition. If the facial image is too far away or the lighting too dim, the mind switches to the "slow channel" and relies on luminosity (Bruce and Langton, 1994; Dupuis-Roy et al., 2009; Frost, 2011; Hill et al., 1995; Russell and Sinha, 2007; Russell et al., 2006; Tarr et al., 2001; Tarr et al., 2002).

Age differences

Skin color also differs by age. It can be used to distinguish younger from older women, since the contrast in luminosity between facial skin and the lip/eye areas decreases with age (Porcheron et al., 2013). It can also be used to recognize infants. All humans are born with very little melanin, and the resulting pinkish-white skin is often remarked upon in different cultures.

This is especially so where adults are normally dark-skinned, in striking contrast to newborns. In Kenya, the latter are often called mzungu ('European' in Swahili), and a new mother may ask her neighbors to come and see her mzungu (Walentowitz, 2008). Among the Tuareg, children are said to be whitened by the freshness and moisture of the womb (Walentowitz, 2008). The situation in other African peoples is summarized by a French anthropologist: "There is a rather widespread concept in Black Africa, according to which human beings, before 'coming' into this world, dwell in heaven, where they are white. For, heaven itself is white and all the beings dwelling there are also white. Therefore the whiter a child is at birth, the more splendid it is" (Zahan, 1974, p. 385). A Belgian anthropologist makes the same point: "black is thus the color of maturity [...] White on the other hand is a sign of the before-life and the after-life: the African newborn is light-skinned and the color of mourning is white kaolin" (Maertens, 1978, p. 41). 

This infant/adult difference is evolutionarily old, being present in nonhuman primates. In langurs, baboons, and macaques, the newborn's skin is pink while the adult's is black. This visual cue not only helps adults to locate a wayward infant but also seems to induce a desire to defend and provide care (Jay, 1962). Humans may respond similarly to the lighter color of infants and women. This would be consistent with a tendency by the adult female body to mimic the newborn body in other ways: face shape; pitch of voice; amount of body hair; texture and pliability of the skin; etc. Over time, women may have come to resemble this 'infant schema' because it is the one that can best reduce aggressiveness in a male partner and induce him to assume a provider role.

 

The sun-tanning fad: An aesthetic revolution

The sex-specific aspects of skin color have influenced the development of cosmetics in many cultures. Even in ancient times, women would use makeup to increase the natural contrast in luminosity between their facial skin and their lip/eye areas (Russell, 2009; Russell, 2010). They would also make their naturally fair complexion even fairer by avoiding the sun and applying white powders or bleaching agents.

This feminine aesthetic changed radically in the 1920s with the sudden popularity of sun-tanning throughout the Western world, initially as a health fad. Tanned skin then entered women's fashion and became part of the flapper image, along with bobbed hair, broad shoulders, a relatively flat chest, narrow hips, and long legs. This fashion image was intended to be hermaphroditic, the aim being to energize the erotic appeal of the female body by masculinizing some of its key features (Bard, 1998; Segrave, 2005). 

Has the tanning fad un-gendered skin color? Not wholly, to judge by the above studies on sex recognition. There still seems to be a tendency to prefer a lighter skin tone for women than for men. This was the conclusion of a recent study on how people perceive two skin pigments: melanin (brown) and carotene (yellow). When shown facial images with varying concentrations of melanin and carotene, the subjects had a greater preference for carotene than for melanin. This preference was stronger for female faces than for male faces, irrespective of the observer's sex. Nonetheless, for both male and female faces, the preferred skin color was much darker than it would have been a century ago (Lefevre and Perrett, 2014).

Again, this aesthetic norm has darkened only in the Western world. The tanned look had some popularity among Japanese women in the postwar era up to the 1970s, but it has since gone out of fashion (Ashikari, 2005). It never did catch on elsewhere in Asia (Li et al., 2008). In North America and Europe, the tanned look seems much more persistent, and this persistence suggests that it is locked into place by something else in our cultural environment.

Such as ...? One factor may be the conscious effort to promote images of dark-skinned people in advertising and, more generally, in popular culture. Another factor may be a half-conscious desire in popular culture for more aggressive, "harder" forms of eroticism. This is something that fair skin is less effective at delivering, having been originally part of the infant schema and thus less conducive to that kind of emotional response.

 

References

Ashikari, M. (2005). Cultivating Japanese whiteness. The 'whitening' cosmetics boom and the Japanese identity, Journal of Material Culture, 10, 73-91. 

Bard, C. (1998). Les Garçonnes. Modes et fantasmes des Années folles, Flammarion, Paris. 

Bruce, V., and S. Langton. (1994). The use of pigmentation and shading information in recognising the sex and identities of faces, Perception, 23(7), 803-822.

Dupuis-Roy, N., I. Fortin, D. Fiset, and F. Gosselin. (2009). Uncovering gender discrimination cues in a realistic setting, Journal of Vision, 9(2), 10, 1-8.

http://journalofvision.org/9/2/10/

Edwards, E.A. and S.Q. Duntley. (1949). Cutaneous vascular changes in women in reference to the menstrual cycle and ovariectomy, American Journal of Obstetrics and Gynecology, 57, 501-509.

Edwards, E.A. and S.Q. Duntley. (1939).The pigments and color of living human skin, American Journal of Anatomy, 65, 1-33.

Edwards, E.A., J.B. Hamilton, S.Q. Duntley, and G. Hubert. (1941). Cutaneous vascular and pigmentary changes in castrate and eunuchoid men, Endocrinology, 28, 119-128. 

Frost, P. (2011). Hue and luminosity of human skin: a visual cue for gender recognition and other mental tasks, Human Ethology Bulletin,

http://www.researchgate.net/publication/256296588_Hue_and_luminosity_of_human_skin_a_visual_cue_for_gender_recognition_and_other_mental_tasks/file/72e7e5223eb2c3eb3b.pdf

Frost, P. (2010). Femmes claires, hommes foncés. Les racines oubliées du colorisme, Québec: Les Presses de l'Université Laval, 202 p.

Frost, P. (2007). Comment on Human skin-color sexual dimorphism: A test of the sexual selection hypothesis, American Journal of Physical Anthropology, 133, 779-781.

Hill, H., Bruce, V., and S. Akamatsu. (1995). Perceiving the sex and race of faces: The role of shape and colour, Proceedings of the Royal Society B: Biological Sciences, 261, 367-373. 

Jay, P.C. (1962). Aspects of maternal behavior among langurs, Annals of the New York Academy of Sciences, 102, 468-476.

http://onlinelibrary.wiley.com/doi/10.1111/j.1749-6632.1962.tb13653.x/abstract

Kalla, A.K. and S.C. Tiwari. (1970). Sex differences in skin colour in man, Acta Geneticae Medicae et Gemellologiae, 19, 472-476. 

Lefevre, C.E. and D.I. Perrrett. (2014). Fruit over sunbed: Carotenoid skin colouration is found more attractive than melanin colouration, The Quarterly Journal of Experimental Psychology,

http://www.tandfonline.com/doi/full/10.1080/17470218.2014.944194#tabModulehttp://www.tandfonline.com/doi/full/10.1080/17470218.2014.944194#tabModule

Li, E.P.H., H.J. Min, R.W. Belk, J. Kimura, and S. Bahl. (2008). Skin lightening and beauty in four Asian cultures, Advances in Consumer Research, 35, 444-449.

http://www.acrwebsite.org/volumes/v35/naacr_vol35_273.pdf

Maertens, J-T. (1978). Le dessein sur la peau. Essai d'anthropologie des inscriptions tégumentaires, Ritologiques I, Paris: Aubier Montaigne. 

Manning, J.T., P.E. Bundred, and F.M. Mather. (2004). Second to fourth digit ratio, sexual selection, and skin colour, Evolution and Human Behavior, 25, 38-50.

http://www.ehbonline.org/article/S1090-5138(03)00082-5/abstract 

Mesa, M.S. (1983). Analyse de la variabilité de la pigmentation de la peau durant la croissance, Bulletin et mémoires de la Société d'Anthropologie de Paris, t. 10 series 13, 49-60.

http://www.persee.fr/web/revues/home/prescript/article/bmsap_0037-8984_1983_num_10_1_3882 

Omoto, K. (1965). Measurements of skin reflectance in a Japanese twin sample, Journal of the Anthropological Society of Nippon (Jinruigaku Zassi), 73, 115-122.

http://ci.nii.ac.jp/naid/130003726811/http://ci.nii.ac.jp/naid/130003726811/ 

Porcheron, A., E. Mauger, and R. Russell (2013). Aspects of facial contrast decrease with age and are cues for age perception, PLoS ONE 8(3): e57985

http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0057985 

Russell, R. (2010). Why cosmetics work. In R. Adams, N. Ambady, K. Nakayama, and S. Shimojo. (eds.) The Science of Social Vision, New York: Oxford.

http://public.gettysburg.edu/~rrussell/Russell_SocialVision_cosmetics_chapter.pdf 

Russell, R. (2009). A sex difference in facial contrast and its exaggeration by cosmetics, Perception, 38, 1211-1219

http://public.gettysburg.edu/~rrussell/Russell_2009.pdf

Russell, R. (2003). Sex, beauty, and the relative luminance of facial features, Perception, 32, 1093-1107.

Russell, R. and P. Sinha. (2007). Real-world face recognition: The importance of surface reflectance properties, Perception, 36, 1368-1374.

Russell, R., P. Sinha, I. Biederman, and M. Nederhouser. (2006). Is pigmentation important for face recognition? Evidence from contrast negation, Perception, 35, 749-759.

Segrave, K. (2005). Suntanning in 20th Century America, Jefferson (North Carolina), McFarland & Co. 

Tarr, M. J., Kersten, D., Cheng, Y., and Rossion, B. (2001). It's Pat! Sexing faces using only red and green, Journal of Vision, 1(3), 337, 337a

http://journalofvision.org/1/3/337/

Walentowitz, S. (2008). Des êtres à peaufiner. Variations de la coloration et de la pigmentation du nouveau-né, in J-P. Albert, B. Andrieu, P. Blanchard, G. Boëtsch, and D. Chevé (eds.), Coloris Corpus, (pp. 113-120), Paris: CNRS Éditions, 2008.

Zahan, D. (1974). White, Red and Black: Colour Symbolism in Black Africa, in A. Portmann and R. Ritsema (eds.) The Realms of Colour, Eranos 41 (1972), 365-395, Leiden: Eranos.

Facial contrast and femininity

 

How to hyperstimulate a sex-recognition algorithm. Women have higher luminous contrast between their facial skin and their lip/eye color. This contrast effect is influenced not only by degree of lightness but also by degree of redness (source).

Women are fairer in complexion because their skin has less melanin and less blood (Edwards and Duntley, 1939). Male castrates similarly look pale (Edwards et al., 1941). For this reason, relative lightness or darkness of skin seems to be key to sex recognition. People especially focus on the luminous contrast between facial skin and the color of the lips and eyes (Russell, 2009; Russell, 2010; see also Dupuis-Roy et al., 2009):

The luminance contrast between the eyes and the surrounding skin and the lips and the surrounding skin has been termed ‘facial contrast’. Female faces have greater facial contrast than male faces, and facial contrast plays an important role in sex classification and the perception of masculinity and femininity and also attractiveness (Porcheron et al., 2013)

This contrast effect has influenced the development of cosmetics. Different societies, sometimes independently of each other, have invented ways to increase facial contrast by darkening the lips and eye area and, conversely, by lightening facial skin (Russell, 2009; Russell, 2010).

Ancient Egypt was an early center for the development of cosmetics (Dayagi-Mendels, 1989). Indeed, the Egyptians “had most of the cosmetic aids which have ever been devised” (Corson, 1972, p.8), including rouge for the lips and cheeks, eyeliner (kohl), eyeshadows, and foundation […] In Mesopotamia, pots of colored paints for the eyes, and rouges for the lips have been found in Sumerian tombs near Ur from 5,000 years ago.

[…] Excavations at Harappa and Mohenjo-daro have found kohl pots and sticks for lining the eyes, as well as red iron oxides and white lead-based compounds that have been surmised to be rouge for the lips and cheeks and foundation for lightening the skin […] Evidence for ancient uses of received cosmetics in the East Asia is less clear, though there is a long history of the use of white face paint and rouge for the lips in China and Japan (Corson, 1972). Overall, there is evidence for the idea that the received style of cosmetics developed in multiple centers of early technology development, and spread outward to peripheral areas such as Europe and Southeast Asia, analogous to the spread of other technologies like agriculture and writing. (Russell, 2010)

This contrast effect may have likewise influenced evolution within our species. The most common human phenotype combines relatively light skin with dark hair and dark eyes. The opposite pattern is biologically possible but rare. In addition, the whitening of European skin might have provided a sufficient degree of luminous contrast for hair and eye color to diversify.

Not just luminous contrast

Facial contrast involves differences not only in luminosity, i.e., lightness/darkness, but also in hue, particularly the redness of the lips versus the lack of ruddiness of adjacent facial skin. Contrast in luminosity and hue declines with age and may provide a means for distinguishing younger from older women:

Several aspects of facial contrast – the luminance and color differences between the facial features and the skin surrounding those features – were found to decrease with age. These included the luminance contrast around the eyes and eyebrows, the red-green (a*) contrast around the mouth and eyes, and the yellow-blue (b*) contrast around the eyes. (Porcheron et al., 2013)

Hue contrast explains why redness—lipstick, red clothing, etc.—seems to enhance femininity even though women are the less ruddy sex. Red colors highlight this lack of ruddiness, thereby stimulating the mental algorithm that people use for sex recognition:

Stephen & McKeegan [26] found that people increase the redness (a*) of the lips to make a female face appear more feminine and attractive. Elliot & Niesta [46] found that pictures of women are perceived by men as more attractive and sexually desirable when they are associated with the color red–whether by the placement of a red border around the picture, or the presence of a red colored shirt on the woman. (Porcheron et al., 2013)

Some questions remain unanswered. Why, for instance, do many women redden their cheeks? Are the cheeks a secondary contrast zone? And what about women from racial groups with dark facial skin?

This last question seems to have given the study’s authors some trouble with the reviewers. Evidently, darker-skinned women still have some facial contrast. They just don’t have as much, probably because they don’t need as much. Dark skin tends to characterize highly polygynous societies; in fact, the correlation between darkness of skin and polygyny rate is greater than the one between lightness of skin and latitude (Manning et al., 2004). In such societies, women are almost sure to get mated and remain so as long as they are fertile. There is thus only weak selection for those who can better attract potential mates and more strongly stimulate sex-recognition algorithms. 

On a final note, it’s no coincidence that the polygyny rate correlates inversely with latitude. In simple tropical societies, especially those with year-round farming, women can more easily provide for themselves and their children without male assistance. Since women cost so little to support, men will tend to seize all mating opportunities. A man will have as many wives as he can get, and not as many as he can support.

 

References

Dupuis-Roy, N., I. Fortin, D. Fiset, and F. Gosselin. (2009). Uncovering gender discrimination cues in a realistic setting, Journal of Vision, 9(2), 10, 1–8. http://journalofvision.org/9/2/10/ 

Edwards, E.A. and S.Q. Duntley. (1939). The pigments and color of living human skin, American Journal of Anatomy, 65, 1-33.

http://onlinelibrary.wiley.com/doi/10.1002/aja.1000650102/abstract;jsessionid=43509EAC6110062BBF5676995DA16C6B.d01t01?deniedAccessCustomisedMessage=&userIsAuthenticated=false 

Edwards, E.A., J.B. Hamilton, S.Q. Duntley, and G. Hubert. (1941). Cutaneous vascular and pigmentary changes in castrate and eunuchoid men, Endocrinology, 28, 119-128.

http://endo.endojournals.org/content/28/1/119.short 

Manning, J.T., Bundred, P.E., and Mather, F.M. (2004). Second to fourth digit ratio, sexual selection, and skin colour, Evolution and Human Behavior, 25, 38-50.

http://www.ehbonline.org/article/S1090-5138(03)00082-5/abstract 

Porcheron, A., E. Mauger, and R. Russell (2013). Aspects of facial contrast decrease with age and are cues for age perception, PLoS ONE 8(3): e57985

http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0057985 

Russell, R. (2010). Why cosmetics work. In Adams, R., Ambady, N., Nakayama, K., & Shimojo, S. (eds.) The Science of Social Vision, New York: Oxford.

http://public.gettysburg.edu/~rrussell/Russell_SocialVision_cosmetics_chapter.pdf 

Russell, R. (2009). A sex difference in facial contrast and its exaggeration by cosmetics, Perception, 38, 1211-1219

http://public.gettysburg.edu/~rrussell/Russell_2009.pdf 

Gender/face recognition: hue and luminosity

Averaged female face (left) and averaged male face (right). The key facial regions for gender recognition, in terms of either response time or accuracy, seem to be where facial skin borders the lips or the eyes.

The human face is a special visual object. We do not learn to recognize it. Instead, it is processed by the brain via a hardwired mechanism. There seems to be an evolutionary tendency to hardwire recognition of objects that appear often enough while being significant enough to human existence.

One task of this mechanism is to tell a female face from a male face. To this end, we unconsciously focus on several visual cues, including skin tone. It is well established that skin pigmentation visibly differs between men and women. This sexual dimorphism reflects differences in both constitutive pigmentation (untanned skin) and facultative pigmentation (tanning capacity). In comparison to women, men have higher concentrations of melanin and hemoglobin in their skin and lower concentrations of carotene. Men also tan more readily than women do for equal exposure times.

The psychologist Richard Russell argues that this visual cue has two components: (1) the absolute luminosity of facial skin and (2) the contrast between this luminosity and that of the lips and the eyes. Now a study from the Université de Montréal points to a third component: differences in hue, i.e., the degree of ruddiness and yellowness. Hue assists gender recognition particularly where facial skin borders the mouth region. In contrast, luminosity is most helpful where facial skin borders the eye/eyebrow region.

This gender recognition works faster with hue than with luminosity. If the observer is too distant or the lighting too dim, the brain falls back on the “slow channel” of luminosity:

This suggests that humans do use chromatic cues for discriminating face gender: When it’s informative, they use it and respond rapidly (for evidence that color is perceived faster than shape, see Holcombe & Cavanagh, 2001; Moutoussis & Zeki, 1997a, 1997b); when it’s not, they have to rely on the more robust and more sluggish luminance cues.
(Dupuis-Roy et al., 2009)

Interestingly, the authors conclude that this mechanism may be located in the infero-temporal cortex, since this brain region is involved in both face perception and color perception.

H/T to Eugene

References

Dupuis-Roy, N., I. Fortin, D. Fiset, and F. Gosselin. (2009). Uncovering gender discrimination cues in a realistic setting. Journal of Vision, 9(2), 10, 1–8.
http://journalofvision.org/9/2/10/, doi:10.1167/9.2.10.

Russell, R. (2003). Sex, beauty, and the relative luminance of facial features, Perception, 32, 1093-1107.

Russell, R., B. Duchaine, and K. Nakayama. (2009). Super-recognizers: People with extraordinary face recognition ability. Psychonomic Bulletin & Review, 16(2), 252-257.

Russell, R. and P. Sinha. (2007). Real-world face recognition: The importance of surface reflectance properties, Perception, 36, 1368-1374.

Russell, R., P. Sinha, I. Biederman, and M. Nederhouser. (2006). Is pigmentation important for face recognition? Evidence from contrast negation, Perception, 35, 749-759.

Facial color and sex recognition

Upper left: average of 22 Caucasian female faces
Upper right: average of 22 Caucasian male faces
Lower left: white pixels are where the female average is lighter than the male average
Lower right: white pixels are where the male average is lighter than the female average

To a large degree, we do not learn to recognize whether a human face is male or female (Bestelmeyer et al., 2008; Little et al., 2005). This mental task is mainly performed by a hardwired algorithm that uses certain visual cues, one of them being facial color (Frost, 1994). Men are more reddish-brown in complexion because their skin has more melanin and hemoglobin (Edwards & Duntley, 1939). Women are paler and show greater contrast between the color of their face and that of their lips and eyes (Russell, 2009). This algorithm is used not only for visual recognition but also for tasks apparently related to sexual attraction and social dominance (Feinman & Gill, 1978; Ioan et al., 2007).

Richard Russell has investigated the way we use facial color to identify male and female human faces. In one experiment, he morphed together 22 photos of Caucasian female faces and then 22 photos of Caucasian male faces. The participants were clean-shaven and did not wear make-up. As we can see from the above composites, the visually average face is noticeably lighter when it is female than when it is male. There is also greater contrast between facial color and lip/eye color on the female face than on the male one.

Russell (in press) argues that the human mind uses lip and eye color as a benchmark for visual processing of facial color:

If female skin is lighter than male skin, but female eyes and lips are not lighter than male eyes and lips, there should be greater luminance contrast surrounding female eyes and lips than male eyes and lips. This would be important, because the visual system is sensitive to contrast rather than to absolute luminance differences. Indeed, luminance contrast is the cue to which most neurons in the early visual cortex respond. Moreover, contrast internal to the face would be robust to changes in illumination. The black ink of this text under direct mid-day sun reflects more light than does the white page under dim indoor lighting, yet in both contexts the text appears black and the page appears white because the contrast between the two is constant. In the same way, a sex difference in contrast could be a particularly robust cue for sex classification. If there is a sex difference in contrast it would also mean that the femaleness of the face could be increased by lightening the skin or by darkening the eyes and lips—either change would increase the contrast. (Russell, in press)

He goes on to argue that this sex difference in facial color is reflected in the development of women’s cosmetics.

The received style of cosmetics involves darkening the eyes and lips while leaving the rest of the face largely unchanged. This is one of two patterns of cosmetic application that could increase facial contrast (the other being to significantly lighten the entire face, except for the eyes and lips). (Russell, in press)

This same pattern has appeared in a wide range of culture areas (ancient Egypt, Mesopotamia, South Asia, East Asia, Mesoamerica), in some cases independently of influence from other culture areas.

References

Bestelmeyer, P.E.G., B.C. Jones, L.M. DeBruine, A.C. Little, D.I. Perrett, A. Schneider, L.L.M. Welling, & C.A. Conway. (2008). Sex-contingent face aftereffects depend on perceptual category rather than structural encoding, Cognition, 107, 353-365.

Edwards, E.A., & Duntley, S.Q. (1939). The pigments and color of living human skin. American Journal of Anatomy, 65, 1-33.

Feinman, S. & G.W. Gill. (1978). Sex differences in physical attractiveness preferences, Journal of Social Psychology, 105, 43-52.

Frost, P. (1994). Preference for darker faces in photographs at different phases of the menstrual cycle: Preliminary assessment of evidence for a hormonal relationship, Perceptual and Motor Skills, 79, 507-514.

Ioan, S., Sandulache, M., Avramescu, S., Ilie, A., & Neacsu, A. (2007). Red is a distractor for men in competition. Evolution and Human Behavior, 28, 285-293.

Little, A.C., L.M. DeBruine, & B.C. Jones. (2005). Sex-contingent face aftereffects suggest distinct neural populations code male and female faces, Proceedings of the Royal Society of London, Series B, 272, 2283-2287.

Russell, R. (in press) Why cosmetics work. In Adams, R., Ambady, N., Nakayama, K., & Shimojo, S. (Eds.) The Science of Social Vision. New York: Oxford University Press

Russell, R.( 2009). A sex difference in facial contrast and its exaggeration by cosmetics, Perception, 38, 1211-1219

Russell, R. (2003). Sex, beauty, and the relative luminance of facial features, Perception, 32, 1093-1107.

Russell, R. & P. Sinha. (2007). Real-world face recognition: The importance of surface reflectance properties, Perception, 36, 1368-1374.

Russell, R., P. Sinha, I. Biederman, & M. Nederhouser. (2006). Is pigmentation important for face recognition? Evidence from contrast negation. Perception, 35, 749-759.