Harris 2003.4 84 85 If one measures the conventional mesiodistal and buccolingual crown diameters of the 32 permanent teeth in the human dentition, there are 64 variables, which is comparable to an extensive battery of craniometrics or anthropometrics (e.g., Davenport, 1927; Martin, 1928). One might suppose that there is a lot of statistical information—several axes of variation—in the odon- tometrics based on tooth types, side, arcade, a tooth’s position in its morphogenetic field, sex, race, and so on. However, the morphological and statistical redundancy among tooth types has long been recognized, and this redundancy sharply diminishes the information con- tent of batteries of crown dimensions. Bateson (1894) included teeth in his anatomic examples of meristic series that included phalanges, vertebrae, and ribs. This phenomenon of multiple analogous skeletodental units that develop clinally along a growth axis also is termed polyisomerism (Gregory, 1934). The supposition is that the shared morphologies are controlled by common control mechanisms (genes, gene products), but verifi- cation has only recently been provided (e.g., Yamaguchi, 1997; Green, 2002). Weiss (1990), Jernvall (2000; Jernvall and Jung, 2000; Jernvall and Thesleff, 2000), and others suggest that polyisomerism is a conservative, efficient mechanism for increasing (or, occasionally, decreasing) the anatomic units, which is more obvious phylogeneti- cally, but occurs ontogenetically as well. The “several” Where’s the Variation? Variance Components in Tooth Sizes of the Permanent Dentition Edward F. Harris* Department of Orthodontics, The Health Science Center, University of Tennessee 38163 *Correspondence to: Edward F. Harris, Department of Ortho- dontics, The Health Science Center, University of Tennessee, Memphis 38163. E-mail eharris@utmem.edu A version of this paper was presented at the Annual Meet- ing of the American Association of Physical Anthropologists, Tempe, AZ, 2003. ABSTRACT Studies have shown that there are only a few canonical axes of tooth size variation in the perma- nent dentition. Despite the numerous measurements that might be taken (e.g., crown length and breadth of 32 teeth = 64 variables), most of the canonical structure is explained by 3 or 4 overarching axes of variation. This study used maximum likelihood components of vari- ance analysis to determine where the major sources of statistical variation are among the crown dimensions in the permanent dentition. Mesiodistal and buccolingual crown dimensions were measured on all permanent teeth (excluding M3s and averaging sides) in 100 Ameri- can whites and 100 American blacks, evenly divided by sex. The SAS program varcomp estimated the sources of variation across 7 aspects of the dentition, namely race, sex, arcade, tooth (incisor, canine, premolar, molar), po- sition (mesial, distal), dimension (MD, BL), and a residu- al term. Most variation is shared; residual variance was just 21.8% of the total. Considering the six components of shared variance, the greatest (82.8%) was due to tooth type (I, C, P, M). In contrast, only 4.9% was attributable to the black-white race difference, which confirms re- sults of other biological data that the preponderance of variation is within groups, not among them. More strik- ing is the lack of variation between males and females (1.2%)—confirming the insensitivity of tooth crown dimensions for forensic purposes. Very little shared variance (0.6%) was due to tooth position, indicating that the mesial “pole” tooth that is metrically and mor- phologically more stable does not possess much more informational content statistically. Whether the tooth was maxillary or mandibular accounted for 6.9%. In a practical sense, the large variance due to tooth type im- plies that dental anthropologists commonly will want to include variables from all tooth types (I, C, P, M) rather than multiple measurements within a tooth type, since tooth type is the canonical axis of variation. Dental An- thropology 2003;16(3):84-94. canonical axes of variation expected from a battery of tooth dimensions do not actually occur because tooth crown dimensions are invariably positively intercorre- lated (e.g., Moorrees and Reed, 1964; Potter et al., 1968; Henderson, 1975; Townsend, 1976; Harris and Bailit, 1988). Genetic covariance among continuous-scale variates like crown dimensions arises from pleiotropic effects of the contributing genes (e.g., Falconer, 1989). Indeed, the principal theme of Butler’s seminal studies of morpho- genetic fields (1939, 1956, 2001) is the developmental dependencies (covariance) of tooth morphologies and dimensions of teeth within the three major fields in mammals, namely incisors, canines, and postcanine tooth types. The consequences for the dental anthro- pologist are that much of the informational content of many tooth crown dimensions are statistically redun- dant. Measuring more teeth or measuring more dimen- sions of the same teeth does not proportionately increase 84 85 the researcher’s ability to discriminate between sexes, or populations, or races, or species. Falk and Corruccini (1982) have shown this quite simply: the discrimina- tory power among groups was much better using cra- niometric variables (with less covariance among traits; Solow, 1966) than an equivalent battery of tooth crown dimensions. MATERIALS AND METHODS The data analyzed here consist of maximum me- siodistal (MD) and buccolingual (BL) tooth crown dimensions of all 14 permanent tooth types, omitting M3s. Measurements were made on the full-mouth dental casts of 100 American whites and 100 American blacks using electronic-readout sliding calipers with the beaks machined to fit well into the embrasures between teeth. Measurement technique followed that described by Selmer-Olson (1949). There was an equal number of males and females in each race, and the subjects were contemporary American adolescents with all 28 teeth fully erupted without any restorations that would affect the measurements. Teeth on just one side of the mouth were measured (either left or right, on an individual ba- sis), but numerous studies have shown that the variance attributable to side is meager and due to just bilateral asymmetry plus technical error and may safely be ig- nored without biasing the other effects (e.g., Lundström, 1948; Potter and Nance, 1976). Statistical analysis It is implausible from what is known about odon- tometrics (e.g., Kieser, 1990; Hillson, 1996) to suppose that either genetic or environmental variation in tooth size is distributed across the dentition in even a vaguely uniform manner. Instead, some of the axes of variation will account for appreciably more than other sources of variation. Six axes of variation were estimated in the present study. Variation was compared by (1) race, (2) sex, (3) arcade (maxilla or mandible), (4) tooth type (in- cisors, canines, premolars, and molars), (5) dimension (mesiodistal versus buccolingual crown diameters), and (6) position (the mesial or distal tooth within a morpho- genetic field). To find out how the variance in tooth size is appor- tioned across these six axes, model II, maximum-likeli- hood estimates of variance components were estimated (Hartley et al., 1978) using the SAS procedure varcomp (SAS, 1989). Two “races” were contrasted, American blacks and whites, but the perspective is to view these as random samples from the “universe” of possible races (e.g., Coon, 1965). Similarly, any number of crown dimen- sions could be measured on a tooth (cf. Corruccini, 1977, 1978; Black, 1979; FitzGerald and Hillson, 2002), the conventional two assessed here (i.e., the standard MD and BL dimensions) are best viewed as a sample of two picked from a population of dimensional possibilities. RESULTS AND DISCUSSION Multivariate analysis Most of the total variance for odontometrics is shared (common) rather than unique variance. This has long been recognized (e.g., Garn et al., 1965, 1968; Moorrees and Reed, 1964) insofar as all MD and BL crown dimen- sions are positively intercorrelated with one another throughout the dentition. This is true for the present data set (Table 1) where every one of the 378 pairwise comparisons is positively and significantly correlated at P < 0.001 (n = 200 for each comparison). This means that “size” is a pervasive controlling factor throughout the dentition. It also means that (1) tooth size can be pre- dicted with some accuracy from other tooth sizes (e.g., Moyers, 1988; Tanaka and Johnston, 1974) but that (2) since all dimensions are intercorrelated, they all tend to estimate the same thing (namely “overall size”) rather than carrying unique, nonredundant information. De- velopmentally, these statistical intercorrelations appear to reflect the communalities of a few rather than many axes of ontogenetic control (Weiss, 1990; Salazar-Ciudad and Jernvall, 2002). Statistical redundancy also has been illustrated in the several studies of human tooth size using factor analy- sis (e.g., Potter et al., 1968; Harris and Bailit, 1988). For the present data, there are just three orthogonal axes of shared variation among the 28 crown dimensions, with “overall size” accounting for most (83%) of this (Fig. 1). The other two axes of variation are (1) BL breadths of the anterior teeth contrasted with MD lengths of the cheek teeth (premolars and molars), accounting for 10% of the shared variation, and (2) MD lengths of the incisors con- trasted with BL breadths of the posterior teeth (canines, premolars and molars) accounting for 7%. Collectively, these three axes of shared variation (i.e., variation not unique to a single crown dimension) is 73% of the total variation. PCA has been performed across a broad range of human samples, showing concordant results and, thus, the nature of the covariance matrices probably is essentially independent of the population under study. It is obvious that these three canonical axes of metric control of the dentition are far fewer than the 28 variables measured for the dentition, and this “reduction” is due to statistical (and developmental) redundancy among crown sizes. Variance components Results from the SAS program varcomp disclosed that, taking total variance as 100%, the shared variance accounted for by the six variables in the model was 79.2% while the residual variance, unique to individual measurements accounted for the other 21.8% (Fig. 2). This is about a four-to-one ratio of explained to residual variances, suggesting that the six factors listed above do, VARIANCE COMPONENTS IN TOOTH SIZE 86 87 I1 I2 C P 1 P 2 M 1 M 2 I1 I2 C P 1 P 2 M 1 M 2 U I1 M D .8 06 . 33 9 .0 99 - .0 10 .0 12 .1 29 .0 73 .4 13 .1 84 .0 12 - .1 10 - .1 28 .0 65 - .0 56 - .0 71 .0 23 - .1 38 - .0 17 .2 07 .0 02 .0 33 - .1 47 .1 28 .1 44 - .1 03 .0 32 .0 32 .0 04 U I2 M D .7 37 . 69 0 -.0 26 .1 15 .0 02 - .1 76 - .0 20 .0 29 .0 72 .1 73 .0 71 .0 22 .0 26 .0 09 .0 94 .2 58 - .0 02 - .0 30 .0 29 - .0 81 -.1 04 .1 12 - .1 43 - .0 98 .0 41 -.1 09 .0 62 .0 70 U C M D .6 62 . 58 2 .7 22 .1 43 - .0 71 - .0 27 .0 89 .0 71 - .0 49 .3 81 - .0 25 .0 97 .1 35 - .0 93 .0 69 - .1 05 .2 32 .0 61 - .0 67 .0 95 -.0 25 .0 56 - .0 38 .0 20 - .0 75 -.0 45 .0 27 - .0 86 U P1 M D .6 73 . 66 2 .7 03 .8 84 .4 07 .0 84 .0 54 - .0 58 .1 41 - .0 12 .4 75 - .0 19 - .0 71 - .0 16 - .0 34 - .0 26 .0 17 .2 27 - .0 41 - .0 58 -.0 45 - .0 91 .1 03 .0 18 - .0 40 -.0 55 - .1 23 .1 88 U P2 M D .5 64 . 52 9 .5 84 .8 16 .7 48 .0 61 .0 32 - .0 27 - .0 04 - .0 43 - .1 08 .2 91 - .0 14 .0 32 .0 95 - .0 55 .0 26 - .0 81 .1 85 - .0 22 .0 89 .0 21 - .0 06 - .0 43 - .0 43 .0 56 .0 19 - .1 21 U M 1 M D .6 37 . 49 1 .6 17 .6 97 .6 59 .7 88 .4 18 .1 21 - .0 21 .0 20 - .1 25 .1 97 .3 18 .0 20 - .0 29 .2 31 - .1 35 - .0 75 .0 07 .1 89 -.1 11 - .1 01 - .0 21 .0 34 - .0 19 -.0 29 .0 79 - .0 69 U M 2 M D .5 78 . 49 3 .6 20 .7 23 .6 71 .7 71 .7 68 - .1 36 - .0 38 - .0 77 .0 94 - .0 84 - .0 31 .2 64 .0 47 - .0 92 .1 11 .0 30 .0 23 - .1 87 .2 13 .1 73 - .0 72 - .0 58 .0 57 .0 39 - .0 14 - .0 18 L I1 M D .8 14 . 66 3 .6 10 .6 35 .5 17 .5 72 .5 02 .7 87 .4 08 - .0 45 .1 24 - .0 16 - .0 77 .0 44 .0 83 - .1 22 .0 95 - .0 36 - .0 12 .0 08 .0 18 .1 38 .0 36 - .1 87 .0 46 .0 37 - .1 32 .0 82 L I2 M D .7 77 . 68 3 .6 20 .7 03 .5 62 .5 91 .5 32 .8 23 .7 86 .1 92 .0 45 .0 43 - .0 41 .1 03 - .0 07 - .0 13 - .0 72 .0 23 - .2 16 .0 51 -.0 29 .0 34 .0 59 - .0 42 .0 45 .0 35 .0 67 - .0 64 L C M D .7 05 . 66 3 .7 87 .7 39 .6 18 .6 35 .6 17 .6 44 .7 08 .7 83 .0 38 .0 42 .1 03 .0 41 - .0 73 - .0 18 - .0 27 - .0 07 .0 43 .0 08 .0 98 - .0 83 .0 33 .2 21 .0 91 .1 00 - .0 57 - .0 14 L P1 M D .6 40 . 64 6 .6 67 .8 91 .7 45 .6 87 .7 15 .6 33 .6 94 .7 34 .8 66 .3 00 .0 64 .1 73 - .0 67 .1 33 - .1 15 .0 13 .0 61 .0 18 .0 39 - .0 82 .0 06 .0 08 .1 05 -.0 14 .0 32 - .1 44 L P2 M D .5 86 . 55 6 .6 59 .8 06 .7 84 .7 28 .6 83 .5 66 .6 27 .7 06 .8 34 .8 11 .0 87 .0 29 - .0 73 - .0 54 - .0 38 .0 05 .0 57 - .0 32 -.0 12 .0 11 .0 79 - .0 46 .0 48 .1 65 .0 76 - .0 54 L M 1 M D .6 18 . 54 8 .6 69 .6 95 .6 26 .7 94 .7 07 .5 53 .5 98 .6 87 .7 21 .7 30 .7 90 .3 51 .0 65 - .0 06 .0 43 .1 20 - .0 96 .0 38 -.0 98 - .0 10 - .0 45 - .1 71 .0 46 -.0 86 .2 11 .0 00 L M 2 M D .6 19 . 56 4 .6 19 .7 44 .6 64 .7 30 .7 83 .5 99 .6 42 .6 81 .7 71 .7 22 .7 90 .8 08 - .1 30 .0 50 .0 41 - .0 96 .0 27 - .0 79 .2 28 .0 81 .0 04 - .0 05 - .0 84 -.0 32 - .1 12 .2 28 U I1 B L .4 22 . 44 9 .4 68 .4 06 .3 78 .3 61 .3 93 .4 20 .3 79 .4 20 .3 60 .3 28 .3 70 .3 54 .6 04 .3 25 .1 48 .0 63 - .0 42 .1 69 .0 44 .0 93 .1 74 - .0 35 .0 16 -.0 27 - .0 77 - .0 38 U I2 B L .4 99 . 57 7 .4 90 .5 29 .4 48 .5 03 .4 90 .4 33 .4 47 .5 17 .5 24 .4 53 .4 78 .4 97 .6 63 .6 54 .2 13 .1 32 - .0 64 .0 21 .0 07 .1 70 - .0 19 .0 92 .0 12 -.0 13 - .0 68 - .1 02 U C B L .4 51 . 43 4 .6 07 .5 27 .4 81 .4 20 .5 34 .4 15 .3 81 .5 61 .4 73 .4 66 .4 62 .5 08 .5 91 .6 04 .6 75 - .0 38 .1 52 - .0 59 .0 94 - .1 02 .1 33 .2 63 - .0 51 .1 25 .0 24 .0 72 U P1 B L .6 44 . 59 2 .6 87 .8 10 .7 10 .6 46 .6 94 .5 66 .5 92 .7 30 .7 78 .7 46 .6 86 .6 83 .4 94 .5 83 .6 23 .8 67 .4 94 - .0 08 .1 55 - .1 23 .0 93 - .0 49 .1 92 .0 10 .0 46 .0 33 U P2 B L .6 39 . 56 5 .6 42 .7 52 .7 23 .6 21 .6 77 .5 38 .5 23 .7 00 .7 31 .7 29 .6 23 .6 54 .4 55 .5 38 .6 39 .8 84 .8 55 .0 99 -.0 16 .1 29 - .1 24 .0 65 .1 22 .2 25 - .0 40 - .0 23 U M 1 B L .6 02 . 48 2 .6 38 .6 07 .5 52 .6 43 .5 81 .5 52 .5 59 .6 52 .5 99 .6 07 .6 57 .6 28 .5 30 .5 19 .5 68 .7 10 .6 86 .7 92 .4 12 .0 57 .0 11 - .0 03 .0 19 -.0 21 .3 09 .1 39 U M 2 B L .5 97 . 49 7 .6 38 .6 89 .6 33 .6 31 .7 14 .5 49 .5 58 .6 80 .6 80 .6 49 .6 57 .7 46 .4 87 .5 21 .6 17 .7 64 .7 27 .8 02 .8 00 - .0 26 - .0 49 .0 00 .0 09 -.0 85 - .0 40 .1 89 L I1 B L .4 75 . 46 6 .4 80 .4 34 .3 92 .3 78 .4 61 .4 94 .4 61 .4 73 .4 06 .3 95 .3 80 .4 69 .6 02 .5 89 .5 58 .4 80 .4 97 .5 30 .5 05 .7 05 .5 10 .1 94 - .0 56 .0 14 .0 07 .0 46 L I2 B L .5 91 . 51 3 .5 83 .5 89 .5 00 .4 69 .5 03 .5 80 .5 76 .6 03 .5 43 .5 24 .4 73 .5 31 .6 29 .5 99 .6 36 .6 20 .5 84 .6 06 .5 85 .7 87 .7 70 .1 85 .1 04 -.0 65 .0 36 .0 35 L C B L .4 08 . 34 8 .4 70 .3 96 .3 29 .3 01 .3 61 .3 02 .3 18 .5 15 .3 54 .3 32 .2 78 .3 46 .4 79 .5 19 .6 32 .4 82 .5 08 .4 70 .4 66 .6 04 .6 50 .6 08 .1 33 -.0 82 .0 48 .0 03 L P1 B L .5 92 . 55 4 .6 16 .7 16 .6 33 .5 92 .6 30 .5 54 .5 81 .7 10 .7 23 .7 07 .6 22 .6 27 .4 50 .5 24 .5 78 .8 15 .7 95 .6 68 .6 90 .4 87 .6 18 .5 10 .7 64 .2 63 .0 05 .0 83 L P2 B L .5 38 . 44 9 .5 50 .6 30 .6 13 .5 41 .5 72 .4 96 .5 05 .6 31 .6 28 .6 80 .5 48 .5 63 .3 40 .4 03 .5 40 .7 21 .7 61 .5 99 .6 08 .3 98 .4 91 .3 91 .7 49 .6 89 .1 29 .1 23 L M 1 B L .5 59 . 46 7 .5 84 .5 78 .5 31 .6 25 .5 70 .4 77 .5 21 .6 09 .5 81 .6 26 .6 87 .6 09 .3 74 .4 12 .5 09 .6 69 .6 39 .7 75 .6 81 .4 42 .5 31 .4 08 .6 48 .6 33 .7 46 .3 80 L M 2 B L .5 95 . 51 6 .5 84 .6 47 .5 44 .5 80 .6 13 .5 56 .5 55 .6 36 .6 08 .6 01 .6 43 .6 98 .4 16 .4 42 .5 76 .7 08 .6 78 .7 61 .7 67 .5 08 .5 90 .4 48 .6 86 .6 48 .7 83 .7 75 T A B L E 1 . P ar ti al c or re la ti on c oe ffi ci en ts a bo ve t he m ai n d ia go n al , c ov ar ia n ce s on t he d ia go n al , a n d fu ll c or re la ti on c oe ffi ci en ts b el ow t he d ia go n al M ax il la M es io d is ta l B u cc ol in gu al M an d ib le M an d ib le M ax il la I1 I2 C P 1 P 2 M 1 M 2 I1 I2 C P 1 P 2 M 1 M 2 E.F. HARRIS 86 87 Fig. 1. Results of principal components analysis on the 200 cases in the present study (28 crown dimensions). Top: Distribution of eigenvalues showing how most of the variation is in the first canonical component and how quickly the subsequent values descend, so that just the first three are larger than 1.0. The other three panels are graphs of the variables’ weights on each of the three principal components. collectively, account for most of the variability in this data set in the statistical sense. Variance components of the six factors tested here are expressed as percentages of the explained variance (Table 2). Caveat Partitioning total phenotypic variance into the relative fractions due to the six sources (listed above) is done to disclose differences in the relative contributions of these contributors to anatomic variation. So, for ex- ample, variations among the four tooth types (58.8% of total) is found to be enormously greater than variations between the MD and BL crown diameters at 2.5% (i.e., between the two conventional axes used to reflect size PC I PC II PC III VARIANCE COMPONENTS IN TOOTH SIZE 88 89 TABLE 2. Estimates of the proportion of variance for each of the 7 parameters in the model Source Estimate Percentage Tooth Type 2.47247 58.76 Arcade 0.20484 4.87 Race 0.14707 3.49 Dimension 0.10735 2.55 Sex 0.03633 0.86 Position 0.01656 0.39 Residual 0.83461 19.84 Total 100.00 Fig. 2. Pie chart showing the apportionment of tooth size variation based on the six variables in the model (see text for details). variation). Whether large or small, these components do not address whether there are statistically significant differences between groups within one of these six ca- nonical dimensions. For example, the smallest source of variation in the present analysis is “position”—whether a tooth is the mesial, stable tooth or the distal, vari- able tooth within a morphogenetic field (I, P, M). Even though position only accounts for 0.4% of the total vari- ance, there still are highly significant statistical differ- ences in mean size and in variance between mesial and distal teeth within a field (Kieser, 1990). Consequently, these two issues (source of variation versus statistical significance) are unrelated issues. Tooth type By far, the largest variance component (82.8%) is tooth type, namely whether the tooth is an incisor, ca- nine, premolar, or molar (Fig. 3). This finding has an in- tuitive appeal because heterodonty—the segmentation of the dentition into functionally specialized tooth types (incisors for nipping, canines for piercing, premolars for trituration, and molars for crushing)—is the fundamen- tal arrangement of the primate dentition (Todd, 1918; Butler 1939, 1956; Swindler, 2002). The other anatomic effects in the present analysis simply involve duplica- tion within the fields: duplication across the upper and lower arch producing structurally similar antagonists; duplication of a distal tooth creating the short meristic series that Weiss (1990), Jernvall (2000; Jernvall and Jung 2000), and others point out is an efficient method of in- creasing the number of structures, essentially by dupli- cating existing ones. The other sort of duplication (not included here) is tied to the ontogeny of bilateral sym- metry, where left and right paired structures develop, apparently using the same genetic information, sym- metrically across the midline. It would seem, then, that the four morphogenetic fields (one for each tooth type) constitute the basic organizing theme—with most of the variation among fields—and that, within fields, teeth enumerated front-to-back (the “pole” and the “variable” tooth; Dahlberg, 1945, 1951), side-to-side (bilateral sym- metry), and craniocaudally (creating analogous tooth morphologies in the two jaws) consume comparatively little variation. In a practical sense, this large variance due to tooth type implies that dental anthropologists commonly will want to include variables from all tooth types (I, C, P, M) rather than multiple measurements within a tooth type, since tooth type is the canonical axis of variation. Arcade While it is a distant second in terms of absolute variance, arcade (Fig. 4) counts for the next-largest component of variance (6.9%), which is in concert with the results of factor analysis of dental metrics showing that, aside from an overall size effect, most factors or principal components (i.e., intercorrelated multivariable dimensions of teeth) typically are arcade-specific (e.g., Potter et al., 1968; Brown and Townsend, 1979). Perhaps this has been shown most clearly by Potter et al. (1976) who characterized the few axes of genetic variation in the dentition. One genetic factor was bilateral sym- Fig. 3. Graph of mean tooth size by tooth type. E.F. HARRIS 88 89VARIANCE COMPONENTS IN TOOTH SIZE metry; every genetic factor identified for a dimension on one side included the antimeric dimension on the other. Secondly, Potter disclosed a buccolingual crown size factor that extended throughout the maxillary (but not the mandibular) teeth. Thirdly, a genetic factor in- fluenced both MD and BL dimensions of the mandibular anterior teeth. It is noteworthy that these genetic factors control regions of the dentition, not specific teeth. Recent computer modeling (Salazar-Ciudad and Jernvall, 2002) shows comparable results, namely that controlling just a few parameters can account for both the ontogenetic and phylogenetic variations within and among tooth types, both metrically and morphologically. In the present study, Figure 4 displays the arcade differences graphed across the 14 tooth crown dimen- sions. Race The estimate of variance for “race” in this study might be criticized because only two groups were in- cluded and because American blacks and whites have experienced several generations of low level gene flow, primarily from whites to blacks (literature reviewed in Pollitzer, 1999). On the other hand, Subsaharan Africans and Ameri- can whites are at either end of the contemporary spec- trum of human tooth sizes (Harris and Rathbun, 1991), except of course for the megadont native Australians (e.g., Smith et al., 1981). Odontometric studies of Ameri- can blacks and whites routinely find that blacks possess significantly larger teeth (Richardson and Malhotra, 1975; Macko et al., 1979; Vaughan and Harris, 1992). In the future, it may be informative to increase the mix of ethnic samples in this assessment of the sources of tooth size variation. The critical issue, however, is recognition of the small component of variance attributable to the black-white difference, estimated here at 3.5% (Fig. 5). The minor contribution of “race” is no longer surprising (Lewon- tin, 1972), but these data are confirmatory, using quite a different tissue system, that races have been defined historically using very superficial criteria, whereas the great preponderance of variation is among individuals within groups, not among them. Dimension Dimension of the tooth crown—whether the crown is measured mesiodistally or buccolingually—accounted for 3.6% of the total variance. This is interesting because it shows that these geometrically orthogonal axes of a Fig. 4. Top: Graphs of the mean crown sizes by tooth and arcade and plot of the maxillary-minus-mandibular size differences (bottom). 90 91 crown are largely coupled in terms of their ontogeny and genetic control. If these two commonly-measured axes of crown size (MD and BL) were not strongly related, one would expect appreciably more variance to be due to this contrast of measured dimensions. Researchers who have studied the genetic control of tooth size (e.g., Sofaer et al., 1971; Potter et al., 1976; Townsend and Brown, 1978) have commented on differences between MD and BL dimensions, but the results often are inconsistent among studies, suggesting that sampling fluctuations may be at work. The suggestion has been advanced that MD dimen- sions have lower heritabilities than BL dimensions be- cause teeth compete for size of the dental lamina in the dental arch. In contrast, BL dimensions do not. This scenario seems to be insufficient as concerns a couple of developmental points. Teeth do not develop from the dental lamina—like beads on a string—instead, they develop from projections of condensed mesenchyme (i.e., the presumptive dental papilla) that extend away from the presumptive occlusal plane, with considerable space between them (Arey, 1965; Slavkin, 1974; Ooë, 1981). The tooth buds develop in a three-dimensional array such that, while their bony crypts may overlap mesiodistally, they are offset mediolaterally and cranio- caudally (van der Linden and Duterloo, 1976; Duterloo, 1991). Teeth do not compete for space until their fully formed crowns erupt into the oral cavity where under- developed arch size may cause an arch-size to tooth– size discrepancy (Little, 1975). The high prevalence of crowding in contemporary westernized populations is a recent epidemiological problem that seems to be pre- dominately acquired rather than inherited (Corruccini and Potter, 1980; Harris and Smith, 1980). Sex It is well documented that males have bigger teeth than females as statistical averages (e.g., Mijsberg, 1931; Gonda, 1959; Garn, 1966; Garn et al. 1964, 1967; Harris and Bailit, 1987), though the amount of sex difference is specific to a population, not a fixed effect (Hanihara, 1978). It is a bit surprising, then, that variance due to sex accounted for just 1.2% of the total variation in the pres- ent study (Fig. 6). On the other hand, humans are char- acterized by their trivial sexual dimorphism in tooth size compared to the great apes (e.g., Harvey et al., 1978; Swindler, 2002). Garn et al. (1967) showed that the ca- nine was the most dimorphic tooth in humans, at 4-6% depending on the group studied, which pales against such nonhuman primates as Papio and Pan, where the canine is more than half again as large in males as in fe- males. The issue should also be considered that univari- ate analysis tends to exaggerate sex differences because redundant male-female differences are included in each test (Potter, 1972). Ditch and Rose (1972) used discriminant functions analysis to correctly determine sex in an average of Fig. 5. Plot of crown dimensions by race (top) and black-minus-white differences in mean size showing that black have larger means throughout the dentition (bottom). E.F. HARRIS 90 91VARIANCE COMPONENTS IN TOOTH SIZE 93% of their cases (depending on the set of variables analyzed), and Garn and coworkers (1977, 1978) ar- rived at similar success rates. Brown and Townsend (1979) reported lower correct allocations (ca. 75% or less) using data from aboriginal Australians—the same as reported by Hanihara (1979)—indicating that the de- gree of sexual dimorphism is not tied to the tooth sizes of a group per se. In passing, researchers also have provided discrimi- nant functions based on crown sizes of the primary teeth that correctly identify sex better than expected from chance (DeVito and Saunders, 1990; Tsutsumi et al., 1993) even though the primary teeth are much less dimorphic (Harris, 2001). Position Depending on their position within a morphogenetic field (I, P, M), teeth are labeled as “stable” or “variable” (Butler, 1939; Dahlberg, 1945). This dichotomy refers to the metric and morphological variation exhibited by a tooth. A stable, early-forming tooth is larger, possesses more and larger cusps and other crown features, and is less likely to be reduced in size or congenitally absent. These and other considerations led Dahlberg (1945, 1951, 1986) and others to characterize the “fields” of the human dentition (Fig. 7). Several studies have shown that the increased variability of distal “variable” teeth is due to diminished genetic control (e.g., Lundström, 1948; Alvesalo and Tigerstedt, 1974). (As an aside, this study did not account for the ap- parent field reversal, where LI1 is more variable than LI2, which Kjaer (1980) attributes to the weak vascular supply in the mandibular midline because of the sym- physis menti.) The present study shows that position is a compara- tively small axis of variation, estimated at 0.4%, making it the most trivial of the factors studied in this model. This also emphasizes the caveat (above) that estimating the relative sources of variation in the dentition is a dif- ferent issue than whether particular teeth exhibit statis- tically significant differences. A key metrical attribute of a pole tooth within a field is its relative metric stability (Townsend and Brown, 1981). Coefficients of variation are graphed in Figure 8, where it is seen that it is not a foregone conclusion that the later-forming tooth pos- sesses significantly great variance statistically. For the six contrasts in Figure 8, just three achieved significance (α = 0.05 for one-tail tests). Just the maxillary incisors (I2 > I1) and the upper and lower molars (M2 > M1) exhibit significantly more variance in one tooth vis-à-vis the other. In all these instances, the distal tooth is always the more variable tooth. Fig. 6. Plot of tooth crown dimensions by sex (top) and plot of the male-minus-female differences (bottom) showing that, characteristically, males have larger mean crown dimensions. 92 93 OVERVIEW What are the major axes of variation in the perma- nent dentition in terms of tooth size? Results of the present study show that the canonical axis is among tooth types, which accounts for more than half of the variation (59%). There is a dramatic drop-off after tooth type is accounted for. Arcade (4.9%), race (3.5%), and crown dimension (2.6%) have only minor but com- paratively intermediate values. Least influential are sex (0.9%) and tooth position within a field (0.4%). None of these axes of variation hinges on any one tooth, and the fundamental lack of more and more-prominent axes of variation is assumed to be due to the strong, pervasive statistical and developmental correlations among crown dimensions. 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The annual meeting of the Canadian Association for Physical Anthropology will be held in Edmonton, Alberta, October 23-25 of 2003. Contributed papers and posters for a symposium on Dental Anthropology are welcome. For further information, contact Dr. Nancy Lovell, Department of Anthropology, University of Alberta, Edmonton, T6G 2H4 Canada. E-mail: nancy.lovell@ualberta.ca CAPA Meeting E.F. HARRIS Decoding Your Subscription Want to know when your subscription to Dental Anthropology expires? Membership in the Association and, thus, your subscription to Dental Anthropology is on an annual basis coinciding with the calendar year. Have a look at the mailing label on the evelope that this issue arrived in, and you will see the year for which your dues have been paid. The year is located in parentheses to the right of your name. So, if the mailing label says “(2003)” you are paid to the end of this calendar year. 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