Differences in Human Pigmentation: Measurement, Geographic Variation, and Causes* G
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Vol. 60. o. 6 TilE J OU RNA L OF I NVESTIGATIVE DEilMATOLOGY Printed in U.S.A. Copyri ght© 1973 by The Williams & Wilkins Co. DIFFERENCES IN HUMAN PIGMENTATION: MEASUREMENT, GEOGRAPHIC VARIATION, AND CAUSES* G. AINS WORTH HARRISON, PH.D. INTRODUCTION affected b y o nly a s ingle pair of ge nes. Since most of the genetic studies of population differences in T he racial differences in pi gmentation t hat have pigmentation relate to skin color, I shall , for the been studied by ant hropolog ists are t he readily most part, confine my d isc ussion to it. observable o nes t hat occur in skin, ha ir, a nd t he eye. T he co lor of t hese structures is due to a THE MEASUREM ENT OF SKIN CO LOR number of factors, but variation in them, espe For a long time, t he study o f skin co lor variation cia lly as it occurs between popu l at i m~s, ~p pe ~ r s to. in fi eld sit uations was beset with difficult problem be largely due to the amount and dtstn butwn of of m easurement. At first, measurements involved t he pigment melanin. This was first _most firml y_ visual co mpari son with sets of co lor standards such established fo r skin in t he now classtcal work of as the Va n Luschan t iles or the colored papers used Edwards and Duntley (1939) and has subsequently by Gates ( 1949) . These were hi ghly unsatisfactory been demonstrated fo r hair and the iris diaphragm not onl y because of the subjectivity of visual of the eye. Hair color, however, may be influenced matching, but also because of t he necessity of by a totally different pigment, t ri chosiderin, which imposing discrete units upon a natural continuum. is concerned with the red shades (Barnicot, 1956a, Furthermore, it was a lmost impossible to m atch b). Histologic (Gates and Zimmermann, 1953; t he texture of the skin with the texture of eit her Gates, 1961) a nd electron microscopic studies tiles or papers; t hose who have used t hese tech (Barnicot et a l. , 1955; Birbeck et al. , 1956) have niques know t he difficul ty of fin ding a n_y corre show n h ow t he mela nin pigment is organized into spondence between the standa rds a nd s ~m color. discrete encapsul ated gra nules in skin and ha ir. H arrison and Salza no (1966) , workmg wtth South Little attention has been given recently to t he basis American Indian populations, fou nd li ttle corre of eye color differences, but it has been established spondence between measurements made with Van t hat much of t he color vari ation in the iris is due to Luschan tiles and t hose made with a s pectropho different a moun ts of phys ical refraction which tometer. depend upon how t he melanin is di stributed in t he The sit uation was somewhat improved by the t issues (Coon et a l. , 1950). use of the Milton Bradley color top in which In the development o f skin color, melanin is segments of four colored papers-black, white, red. synthesized solely by melanocytes in the basal and yellow-were arra nged in differin g amounts so layer of t he epid ermis from which pigment e nters that wh en t he top was spun t he blending of the t he neighboring epidermal cell s of t he stratum colors produced a visual range varying fr om dark to Malpighi (Bi llingha m and Medawar, 1948). There li cr ht brow n . This at least presented a continuous fore, in a ny consideration of racial va ri ation and s;ale of measurements, but it still retain ed the the ge netics of skin color, the ul timate concern ts fundamental disadvantage of visual subjectivity. t he hereditary fa ctors that regulate melanin sy n H ow to co mbine t he amounts of t he four color t hesis a nd distribution by the skin melanocytes. discs, which corresponded to any particula r skin However, the activit ies of these melanocytes them color, into a s ingle measurement o f t hat color wa selves are a ffected by numerous external factors, never adequately solved . At first, t he extent of especia ll y exposure to ultraviolet radiation . Hence expos ure of t he black disc was taken as the skin color in man i s a typical, quantitatively measurement, but it was later show n that the red varying character dependent wit hin and between disc itself contained black pi gment a nd t hat allow populations upon both hered itary an? environ ance had to be made for t his. mental factors and poss tbly o n an mteractton The fi eld of study o f skin color vari ation on a between them. geogra phic basis has been revo~ut i o ni ze d in t he last Al t hough ha ir a nd eye color appear to be less decades by the development of t he portable reflect affected by norma l enviro nmental factors, t hey are ance spectrophotometer. This instrument oYer in£1 uenced by age and sex differences; and ha tr came practicall y a ll the earli er problems by provid color, of course, is often artifi cia lly c hanged. Even ing a measure o f color on a continuous_ and after a ll owance has been made for t hese factors, objective scale of measurement. A vane_ty of such both tend to show quantitative variations wit hin instruments have been used in [i eld stud1 es, but all populations. Red ha ir (Reed, 1952) a nd i he more work on essentiall y t he sa me principle. marked differences in eye color such as blue versus Li ght is e mitted from a standa rd source and brown (Brues, 1946) behave as t hough t hey were passes on to the surface of the skin t h1:ough any one of a series of co lor fil ters, wh1ch typtcall y samp l~ • From the An thropology Laboratory , Depa rtment of t he visual spectrum. The re11 ected light is the11 Human Anatomy, Un iversity of Oxford, Oxford, Eng picked up by a photocell a nd measured on a land . 418 DIFFERENCES IN HUMAN PIGMENTATION 419 galvano meter unit . The a mount of li ght refl ected melanin concentration, it probably arises from from t h e skin is compared with t hat ref1 ected from different li fe styles and different exposure to sun a pure white standard such as magnesium oxide or light. Age differences t hat have been reported magnesium carbonate, or in some cases with a (some rather complicated ) have been assumed to more robust standard which itself has been ca li be related to hormonal balance. brated against what is regarded as 100% ref1 ect The use of spectrophotometers hi ghlights some ance. T he two photometers most commonly used of the problems inherent in the measurement of in field studies are t he EEL (Weiner, 1952), large ly a ny continuously varin g t rait, for one seldom has used by British, Cont in ental, a nd Indian investiga t he opportunity to measure t he same c haracter tors, and the P hotovolt (Lasker, 1954), used by directly on such totally different scales as t hose orth Ameri cans. These instruments are obtaina afforded b y t he various wavelengths of t he differ ble with many different fi lters. The EEL is nor ent fi lters. Within most popul ations, t he variation mally s upplied with a standard set of nine. Photo in skin color tends to be sli ght and when graph volt offers a considerable variety from which to ically plotted does not detectably depart from a choose. Unfortunately, because t he filters differ Gaussian distribution. Where t he within-popula not only in their dominant wavelength but also in tion variabili ty is high (e.g., in hybrid groups and many other optical properties, it is difficul t to between p arental populations differing markedly mak e direct comparisons between t he two instru in color), a non-Gaussia n distribution may be due ments. Filters have been designed for many spec i to other factors tha n the scale of measurements, fic purposes; for example, for color analys is a nd, such as the differing contributions of t he parental like t h e set of nine provided wi th the EEL instru populations o r assortative mating. men t, for m easuring ref1ecta nce over a narrow Only when populations differing markedly in waveband. In using t hese instruments, one must skin color a re studied on t he diffe rent scales of never forget exactly what is being measured. wavelength d oes the problem of scale itself become Typical refle ctance curves measured with t he clear. For example, Figure 2 compares a European EEL instrument are shown in Figure 1. Refl ectance whi te population, an African black population, values depend not only on t he particul ar instru and an F 1 hybrid population in terms of t he ment a nd fi lter u sed, bu t a lso body s ite. Most reflectance of blue light with that of red li ght. Note investigators measure at one or more of t hree that one wou ld have to come to totally diffe rent sites- t h e fo rehead, t he inner aspect of t he fore conclusions about t he potence of t he genes on the arm, a nd t he inner aspect of the upper a rm two scales.