Hemphill 2013.3


16  

 

A fool’s mission? A test of three common assumptions in dental 
metric analyses 
Brian E. Hemphill 
Department of Anthropology, 405A Bunnell Hall, University of Alaska, Fairbanks, Fairbanks, AK 99775-7720 
 

Keywords: Morphogenetic Fields, Ontogenetic Canalization, Sex Dimorphism 

ABSTRACT  Three aspects of metric variation in the 
permanent dentition of humans are often simply 
accepted as true. The first is that formation of the 
permanent dentition occurs within morphogenetic 
fields broadly associated with tooth type and jaw. 
The second is that dental development of among 
females is characterized by a higher degree of on-
togenetic buffering relative to males. The third is 
that expression of sex dimorphism in permanent 
tooth size is expressed uniformly among well-
nourished human populations. This study tests 
these assumptions through an examination of 
mesiodistal and buccolingual dimensions of all 
non-canine permanent teeth, except third molars, 
among 2,709 living individuals of 15 ethnic groups 
from South Asia. With sexes pooled, only one in 
four contrasts of variance among key versus distal 
teeth within dental fields are significantly hetero-
geneous, while one in four contrasts yield higher 
levels of variance among key teeth relative to their 
distal counterparts within a dental field. Such re-
sults weaken considerably orthodox applications 
of Butler’s dental field theory. When samples are 

the unit of analysis, male samples are marked by 
fewer dental fields with significantly heterogene-
ous levels of variance between key and distal 
members, while males and females are affected 
equally by significantly heterogeneous variation 
between key and distal members when dental 
fields are the unit of analysis. Such results suggest 
males and females are equally buffered against 
environmental perturbations that affect odon-
tometric variation. One-way ANOVA indicates 
that a tooth’s position within a dental field ac-
counts for 15.5% to 23.1% of the observed varia-
tion in tooth size, while two-way ANOVA reveals 
that when sex is added as a second factor, the per-
centage of variance in tooth size explained increas-
es from 16.7% to 30.8%, an improvement of 27.2%. 
Such results indicate sex dimorphism in tooth size 
varies in both patterning and in magnitude  
among these samples, thereby explaining why 
discriminant functions developed for one popula-
tion often perform more poorly when applied to 
other populations. 

 Over the last 70 years a consensus has 
emerged that dental development in humans is 
characterized by a series of developmental fields 
that correspond broadly to tooth type by jaw 
(Butler 1939; Dahlberg 1945, 1951), that odonto-
genesis is marked by a greater degree of develop-
mental buffering, or “canalization,” among fe-
males relative to males (Garn et al. 1965, 1966; 
Nichol et al. 1984; Niswander & Chung 1965), and 
that expression of sex dimorphism is uniformly 
expressed across adequately nourished human 
populations (Kieser et al. 1985). This study tests 
these assumptions through assessment of mesi-
odistal and buccolingual dimensions of all non-
canine permanent teeth except third molars 
among 2,709 living individuals of 15 ethnic groups 
from South Asia. 
 
 
 

MATERIALS AND METHODS 
 

 Dental casts were collected from 2,709 living 
individuals with informed consent of 15 ethnic 
groups from the Hindu Kush/Karakoram High-
lands of northern Pakistan, the northern periphery 
of the Indus Valley of Pakistan, Gujarat State of 
northwestern peninsular India, and Andhra Pra-
desh State of southeastern India (Fig. 1). Of these, 
some 2,455 individuals (1,087 Females, 1,368 
Males) are represented by casts for both upper and 
lower dentitions. Mesiodistal tooth lengths and 
buccolingual tooth breadths were measured for all 
teeth, except third molars using standard oden-
tometric procedures (Moorrees, 1957). Kolmogo-
rov-Smirnov tests were used to determine whether 

Correspondence to: Brian Hemphill, Department of Anthro-

pology, 405A Bunnell Hall  

University of Alaska, Fairbanks, AK 99775-7720 

bhemphill@alaska.edu 



17  

 

variable distributions by sex and by sample depart 
significantly from normality. Antemortem tooth 
loss, dental pathology and casting defects pre-
clude some measurements from being collected. 
EM estimation (Dempster et al., 1977) was used to 
estimate missing values by sex and by sample. No 
more than three of the 28 variables (10.7%) were 
estimated by individual. Teeth within incisor, pre-
molar and molar dental fields were separated into 
“key” and “distal” members by jaw. Standard de-
scriptive statistics were calculated for each varia-
ble. Heterogeneity of variance between key and 
distal members was tested with Bartlett’s chi-
square (Snedecor and Cochran, 1989) and vari-
ances were compared to test for the expected pat-
tern of higher variance for the distal member with-
in each morphogenetic field, except for the man-
dibular incisors for which Dahlberg (1945, 1951) 

maintained that the morphogenetic field was re-
versed, such that LI2 is considered the key tooth 
and LI1 the distal tooth. One-way ANOVA was 
used to test for the impact of position within a 
dental field upon tooth size in both sex-pooled 
and sex-segregated samples by ethnic group. Sex-
pooled samples were further tested with two-way 
ANOVA to determine the impacts of position and 
sex by ethnic group. Relative contributions of sex 
to position were rank ordered to illustrate differ-
ences between samples in the expression of sex 
dimorphism.  
 

RESULTS 
 

 Kolmogorov-Smirnov tests reveal that mesi-
odistal lengths and buccolingual breadths for 
males and females of all 15 samples are distributed 
normally. Of the 2,455 individuals represented by 

Fig. 1. Location of the samples used in the study. Abbreviations are from Table 1.  



18  

 

   

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19  

 

casts for both dentitions only 1,198 (48.8%) are rep-
resented by all 28 variables. Estimation of missing 
values improved the number of individuals with 
complete data from 1,595 (65.0%), to 1,923 (78.3%), 
to 2,128 (86.7%) when 1, 2, and 3 variables were 
estimated, respectively (Table 1).  
 Bartlett’s chi-square reveals that just over one-
fourth (47/180= 26.11%) of contrasts of variance 
between key and distal members of a dental field 
exhibit significant heterogeneity of variance. The 
number of significant differences by sample aver-
ages 3.13 out of the 12 fields (26.08%) and ranges 
from a high of six fields among Awans and Gara-
sias to a low of zero among the Yashkuns of As-
tore. When instances of significant heterogeneity 
of variance within dental fields are examined to 
determine whether this heterogeneity is driven by 
higher variance in key teeth versus higher vari-
ances in distal teeth, expectations of dental field 
theory are resoundingly confirmed. As expected, 
the vast majority (42/47= 89.36%) of cases involve 
higher variance for the distal member of a dental 
field (Fig. 2). In fact, instances of significantly 
higher variances among key teeth occur among 
members of only three of the ethnic groups con-
sidered here. These include Awans,  Bhils, and  
Rajputs. 
 A situation in which the amount of variance 
among key members of a dental field exceed that 
found among their distal counterparts represents a 

reversal of dental field theory expectations. Exami-
nation of levels of variance reveals some 46 in-
stances of reversal, accounting for just over one-
fourth of all comparisons (46/180= 25.56%). The 
number of reversals runs from a high of seven 
(58.33%) among Shinas from Gilgit (SHIg) to lows 
of a single reversal among Garasias (GRS) and 
Gompadhomptis Madigas (GPD) (Fig. 3). Further 
examination indicates that while all non-canine 
dental fields of both jaws are affected, reversals 
are by far most common among the mandibular 
incisors (LI2>LI1) where two-thirds of all contrasts 
yielded reversals (20/30= 66.7%). Reversals are 
also common among mandibular molars (9/30= 
30.0%), are less common among maxillary incisors 
(6/30= 20.0%) as well as among mandibular 
(5/30= 16.67%) and maxillary premolars (5/30= 
16.67%), and are rarest among maxillary molars 
(1/30= 3.33%). 
 Analysis of variance indicates that position 
within a dental field contributes substantially to 
the percentage of variance explained in tooth size 
(Fig. 4). Across all 15 samples position alone ac-
counts for nearly 20% of the variance in tooth size 
within a dental field, ranging from highs of 23.08% 
and 22.92% among Bhils and Chenchus to a low of 
15.47% among the Wakhis of Gulmit.  
 Bartlett’s chi-square (Fig. 5) reveals that males 
are marked by a fewer number of dental fields 
with significantly heterogeneous levels of variance 

Fig. 2.  Number of significant differences in variance between key and distal members of a dental field 
by position with sexes pooled. 



20  

 

between key and distal member, for significant 
heterogeneity occurs in only three of the 15 sam-
ples (20.0%), while females are marked by equiva-
lent or higher numbers of reversals in 12 of the 15 
samples (80.0%). When heterogeneity of variance 
is considered by dental field across all samples, 
Bartlett’s chi-square identifies 68 of 360 (18.89%) 
contrasts as exhibiting significantly heterogeneous 
levels of variance. Of these, 35 occur among males 
and 33 occur among females, indicating that males 
and females are marked by nearly identical num-
bers of significantly heterogeneous contrasts with 
regard to variance. 
 Examination of the patterning of variance 
among key and distal teeth within dental fields 
reveals that somewhat more than one-fourth 
(99/360= 27.5%) are marked by a reversal in which 
variance is greater among key teeth than their dis-
tal counterparts (Fig. 6). When considered by sex, 
males are more often affected by reversals 
(31.11%) than females (23.89%). In fact, males ex-
hibit a marked increase (30.23%) relative to that 
observed among females. When considered by 
sample, reversal prevalence is greater among 
males for only six of the 15 samples. This means 
that, contrary to expectations, males more often 
exhibit variance reversals than females overall, 
while in marginal support of expectations, females 

have a higher or equivalent number of dental 
fields marked by variance reversals than males in 
nine (60%) of the 15 samples. 
 Analysis of variance has already indicated that 
a tooth’s position within a dental field accounts for 
15.5% to 23.1% of the variance in size across the 15 
samples (Fig. 4). When this relationship is further 
explored by sex it is clear the influence of sex on 
the relative size of key and distal members within 
dental fields differs markedly (Fig. 7). In 11 sam-
ples, the average contribution of position is greater 
among females, while in the remaining four the 
contribution is greater among males. In some sam-
ples, such as the Awans (4.82%) Swatis (6.3%) and 
Baltis (4.74%) this difference is well-marked, but in 
others, such as the Bhils (0.01%), the greater contri-
bution of position among females is minimal. In 
fact, the opposite pattern may also be discerned, 
where among some samples the difference be-
tween the sexes is well-marked, but is greater 
among males than females, such as among the 
Wakhis of Gulmit (5.3%), or is but minimal as is 
the case for Pakanatis (0.14%). Such findings indi-
cate that sex contributes substantially, but differ-
ently by sample, to relative tooth size between key 
and distal members of the same morphogenetic 
field.  

Fig 3. Number of reversals in relative variance between key and distal members of a dental field 
with sexes pooled. 



21  

 

 As noted above, one-way ANOVA has already     
demonstrated that a tooth’s position within  a 

Fig. 4. Average contribution by position in accounting for variance in tooth size between key and distal 
members of a dental field with sexes pooled. 

Fig. 5.  Number of dental fields in which there are significantly different levels of variance between the 
key and distal member.  



22  

 

Fig. 6.  Number of dental fields in which there is a reversal in the amount of variance expressed by key 
and distal members.  

dental field contributes substantially (15.5%-
23.1%) to the determination of tooth size (Fig. 4), 
but when considered by sex across the 15 samples 
it is also clear this contribution differs markedly in 
both magnitude and polarity (Fig. 7). A two-way 
analysis of variance by sample indicates that when 
sex is added as a second factor, the percentage of 
variance explained increases between 16.7 to 
30.8%, an improvement of 27.2% over when posi-
tion is considered alone. The improvement in ac-
counting for the variance in tooth size between 
key and distal members of a dental field varies 
widely, from a low of 0.6% among Awans, to a 
high of 13.2% among Wakhis from Sost. Neverthe-
less, a paired-samples t-test indicates this im-
provement is statistically significant (t= 2.764; p= 
0.015). Clearly, then, sex, in addition to position, is 
influential in the determination of relative tooth 
size between key and distal members within a 
dental field. However, that influence appears to 
differ markedly across samples. 
  Rank ordering is used to illustrate differences 
among samples in the relative contributions 
played by sex and by position in the relative size 
of key and distal members of the same morphoge-
netic field. Ranks were assigned such that those 

variables in which sex provides a relatively great 
contribution to the determination of relative size 
receive high ranks, while those variables in which 
sex plays a relatively lesser role receive low ranks. 
Ranks are plotted for maxillary variables in Figure 
8, while ranks are plotted for mandibular variables 
in Figure 9. 
  Two-way ANOVA reveals that the contribu-
tion of sex to relative tooth size of key and distal 
members of dental fields is greatest for the bucco-
lingual breadths of the premolars and molars in 
the maxillary dentition, as well as the buccolingual 
breadths of the incisors and mesiodistal lengths of 
the premolars in the mandibular dentition. By con-
trast, the contribution of sex is low for the mesi-
odistal lengths of both maxillary and mandibular 
incisors. Nevertheless, despite these overall 
trends, there is considerable variation among the 
15 samples in the contribution of sex for the re-
maining variables. Indeed, variation in the relative 
contribution of sex appears especially well-
marked for buccolingual breadths of incisors and 
mesiodistal lengths of premolars in the maxillary 
dentition, as well as the buccolingual breadths of 
the incisors and premolars in the mandibular den-
tition. 



23  

 

Fig 7.  Average contribution by position in accounting for variance in tooth size between key and distal 

members of a dental field by sex. 

Fig 8.  Average relative contribution of sex to position in determination of relative tooth size between 
key and distal maxillary teeth within a dental field by rank order (ranked by contribution from sex). 



24  

 

  When considered by jaw, variation in the con-
tribution of sex to relative tooth size of key and 
distal members of the same morphogenetic field 
among the maxillary teeth varies most among the 
15 samples for the mesiodistal lengths of the pre-
molars (sd= 2.274), followed by the buccolingual 
breadths of the premolars (sd= 1.988) and incisors 
(sd= 1.397). By contrast, variation in rank order is 
rather low for the mesiodistal lengths (sd= 1.223) 
and buccolingual breadths (sd= 1.060) of the mo-
lars, while variation among samples is lowest of 
all for the mesiodistal lengths of the incisors (sd= 
0.743). Looked at another way, the rank order 
score for the relative contribution by sex to posi-
tion for mesiodistal dimension differences be-
tween the key and distal members of this morpho-
genetic field ranges from one among the Awans 
(where sex contributes the most among the 12 var-
iables considered) to 10 among the Wakhis of Gul-
mit (where the sex contributes third lowest among 
the 12 variables considered). 
  Turning to the mandibular teeth, variation in 
the contribution of sex to relative tooth size of key 
and distal members of the same morphogenetic 
field among the mandibular teeth varies most 
among the 15 samples for the buccolingual 
breadths of the premolars (sd= 2.000), followed by 
the buccolingual breadths of the incisors (sd= 
1.668) and the mesiodistal lengths of the molars 
(sd= 1.624). Variation in rank order is rather low 
for the buccolingual breadths of the molars (sd= 
1.397) and the mesiodistal lengths of the premolars 
(sd= 1.187), while as in the maxillary arcade, varia-
tion is lowest for the mesiodistal lengths of the 
incisors (sd= 1.183). When the dispersion in rank 
order scores across samples is considered, the rela-
tive sex contribution versus the contribution by 
position for differences in buccolingual breadths 
between the key and distal members of the premo-
lars ranges from two among the two Wakhi sam-
ples (WAKg, WAKs) to a high of nine among 
Chenchu tribals of southeastern peninsular India. 
By contrast, dispersion in mesiodistal lengths of 
the incisors only ranges from one in three samples 
(CHU, GPD, SHIg) to five (WAKg). 
   

DISCUSSION 
 

Question 1: Do Developmental Fields exist such 
that Variance is Less among “Key” Teeth Rela-

tive to “Distal” Teeth? 
 

  It has often been maintained that the earlier 
developing members within a morphogenetic field 
are less affected by environmental factors than 
later developing members (Alvesalo and Ti-
gerstedt, 1974; Townsend and Brown, 1980) and 
this has led some researchers who focus on dental 
morphology to limit considerations of differential 
trait frequencies found on key teeth only (Scott 
and Dahlberg 1982; Scott et al 1983; Sofaer et al 
1972; Turner 1976). A recent review by Townsend 
and co-workers (2009) observes that later develop-
ing teeth within a morphogenetic field spend a 
relatively longer period of time in the soft tissue 
stage prior to calcification during which epigenetic 
and environmental factors can influence the shape 
and size of the crown. A similar observation was 
made by Keene (1982), whose concept of the mor-
phogenetic triangle emphasized the dynamism in 
the formation of the individual cusps until coales-
cence among the cusps fuses them in place. Not 
surprisingly, given these expectations, it has been 
widely assumed that the key tooth within each 
morphogenetic field ought to possess the highest 
heritabilities, while the non-key teeth ought to be 
marked by lower heritabilities. Indeed, Alvesalo 
and Tigerstadt (1974) reported such patterning in 
their data, but other researchers have been unable 
to confirm such results (Dempsey and Townsend, 
2001). 
  With sexes pooled, only one out of four con-
trasts of variance between key and distal members 
within dental fields are significantly heterogene-
ous, but the overwhelming majority that are sig-
nificant are due to much higher variance among 
distal members. While such findings corroborate 
dental field theory and the findings of other re-
searchers (Harris & Nweeia 1980; Herskovitz et al. 
1993; Kieser & Groeneveld 1998; Mayhall & Saun-
ders 1986), it is also the case that one in four con-
trasts yields higher variance for the key tooth than 
for the distal tooth within a dental field. A large 
number of these reversals occur among the man-
dibular incisors, suggesting that Dahlberg’s (1945, 
1951) insistence on a reversal of the dental field 
among mandibular incisors is incorrect. In contrast 
to expectations of the theory of compensatory 
tooth size effect (Sofaer 1973; Sofaer et al, 1972a,b), 
as well as the findings of some researchers with 
regard to bilateral asymmetry (Harris & Nweeia 
1980; Townsend & Brown 1980), no predilection 
for increased variance was found for mesiodistal 



25  

 

over buccolingual dimensions or vice versa. In-
deed, one-way ANOVA indicates that position 
within a dental field only contributes about one-
fifth of the percentage of variance explained in 
tooth size. 
 Taken together, such results weaken consider-
ably an orthodox application of Butler’s field theo-
ry. As noted by Townsend et al. (2009), a compli-
cated array of epigenetic and morphogenetic 
events appears to be involved at different times 
and to various degrees in crown formation. Fur-
ther, given more recent research which indicates 
that secondary enamel knot formation determines 
the location of cusp tips (Jernvall et al., 1994; Mata-
lova et al., 2005), that knot positioning relative to 
the margin of the occlusal surface (Moorman et al., 
2013) and overall crown size are related to such 
morphological features of the permanent tooth 
crown as Carabelli’s trait (Harris, 2007), it is clear 
that crown size and shape are phenomena whose 
interrelatedness are poorly captured by simplistic 
developmental models that rely upon morphoge-
netic fields with key and distal members.  
 

Question 2: Are Females more Genetically  
Canalized than Males? 

 

 The assertion that among humans males are 
less buffered against environmental stress than 
females can be traced to Greulich’s (1951) study of 
growth and development among children on the 
island of Guam who suffered from nutritional 
stress and other deprivations during World War 
II. Greulich found than Guamanian boys suffered 
greater shortfalls in height, weight, weight for 
height and skeletal maturation than girls when 
compared to well-nourished U.S. children. Similar 
results were found among children who survived 
the atomic bombing of Hiroshima and Nagasaki 
(Gruelich et al., 1953), as well as children exposed 
to radiation caused by nuclear testing in the Mar-
shall Islands (Sutow et al., 1965). 
 In 1969, Stini examined the impacts of malnu-
trition upon growth and development among boys 
and girls of Helconia, Colombia. He found skeletal 
maturation to be delayed in all malnourished chil-
dren early in life. However, skeletal age among 
girls was closer to U.S. standards in the earliest 
years of life and the differences in skeletal maturi-
ty between boys and girls increased throughout 
adolescence such that girls experienced a form of 
“catch-up” growth to U.S. standards while similar-

ly malnourished boys failed to do so resulting in a 
reduction of “blunting” of sex dimorphism 
(Dettwyler, 1992; Eveleth, 1975; Leonard, 1991; 
Stini, 1972; Tobias, 1972). Similar results have been 
obtained in studies of the impact of high altitude 
upon growth and development among Andean 
populations (Frisancho and Baker, 1970; Pawson, 
1977; Stinson, 1980), as well as sex differences in 
response to infectious diseases (Stini, 1985), para-
site loads (Brabin, 1990), and famine (Grayson, 
1990). Stini (1975, 1982, 1985) suggested that such 
sex differences may be the consequence of selec-
tion for better environmental buffering in females 
because of their greater investment in reproduc-
tion in supporting pregnancy, lactation and child 
rearing. 
 Turning to odontometric variation within the 
permanent dentition and given the expectations of 
dental field theory, males ought to express a lesser 
degree of genetic canalization by exhibiting great-
er variance among distal members of a morphoge-
netic field relative to key members. That is, the 
lesser degree of buffering against environmental 
perturbations ought to more often result in levels 
of variance among key and distal teeth that are 
statistically different. Further, because of lesser 
buffering and hence greater variation among distal 
teeth within a morphogenetic field, reversals in 
levels of variance among key and distal members 
of the same morphogenetic field ought to be few. 
By contrast, among females the greater amount of 
buffering should reduce the relative amount of 
variance found among the distal members of a 
morphogenetic field and thereby result in fewer 
instances in which the levels of variance between 
key and distal members of a morphogenetic field 
are significantly heterogeneous. A secondary con-
sequence of greater buffering among females is 
that greater parity in variance among key and dis-
tal members of a morphogenetic field is that rever-
sals ought to be more common due to random 
chance. 
 Running contrary to expectations, Bartlett’s 
chi-square indicates that males are marked by a 
fewer number of dental fields with significant het-
erogeneous levels of variance between key and 
distal members, for significant heterogeneity oc-
curs in only three of the 15 samples (20.0%), while 
females are marked by equivalent or higher num-
bers of reversals in 12 of the 15 samples (80.0%). 
When heterogeneity of variance is considered by 



26  

 

dental field across all samples, Bartlett’s chi-
square identifies 68 contrasts as exhibiting signifi-
cantly heterogeneous levels of variance. Once 
again running contrary to expectations, males do 
not exhibit a pattern in which they are affected far 
more often than females. Instead, with 35 and 33 
significant differences affecting males and females, 
respectively, it appears that members of both sexes 
are equally buffered against environmental pertur-
bations that affect odontogenesis. 
 As noted above, an examination of the pat-
terning of variance among key and distal teeth 
within dental fields finds that a little more than 
one-fourth (99/360= 27.5%) are marked by a rever-
sal in which variance is greater among key teeth 
than their distal counterparts. Males are more of-
ten affected than females, but when considered by 
sample, reversal prevalence is equivalent or great-
er among females than males in nine of the 15 
samples. Taken together, these results offer only 
tepid support for the contention that females are 
more highly genetically canalized and hence 
odontogenesis is less affected by environmental 
factors among females than are males. These find-
ing corroborate those of other researchers who 
find similar levels of postnatal variability in 
growth and development among members of both 
sexes (Frisancho et al., 1980; Martorell et al., 1975, 
1984; Stinson, 1985; Yarborough et al., 1975) as 
well in linear enamel hypoplasia prevalence 
(Angel et al., 1987; Goodman et al., 1987, 1991; 
Manzi et al., 1999; May et al., 1993; Santos and 
Coimbra, 1999; Zhou and Corruccini, 1998). How-
ever, as noted by Guatelli-Steinberg and Lukacs 
(1999), indicators of postnatal stress offer a mixed 
signal concerning sex differences in response to 
stress. This is because cultural factors may out-
weigh and obfuscate the actual levels of stress ex-
perienced. Thus, the evidence found here for 
equivalent levels of variability for males and fe-
males may be the consequence of cultural factors 
that favor care, treatment and feeding of boys over 
girls. Thus, with regard to greater developmental 
canalization of females over males, it is clear that if 
such canalization exists it is not of a sufficient de-
gree to be expressed consistently across the sam-
ples analyzed here. Consequently, one cannot as-
sume that females will be less variable odontomet-
rically than their male counterparts.  
  

Question 3: Is Sex Dimorphism Uniformly Ex-
pressed across Adequately Nourished Human 

Populations? 
 

 Teeth are considered a useful means for deter-
mination of sex (Ghose and Baghdady, 1979; Har-
ris and Nweeia, 1980; Potter et al., 1981; Iscan and 
Kedici, 2003), especially in cases where remains 
are highly fragmentary (Anuthama et al., 2011; 
Prabhu and Acharya, 2009; Vodanovic et al., 2006). 
 It is usually the case that the canines are the 
most dimorphic teeth in the permanent dentition 
(Acharya and Mainali S., 2007; Garn et al., 1967; 
Iscan and Kedici, 2003; Lund and Mörnstad, 1999; 
Potter et al., 1981; Townsend and Brown, 1979), 
but some studies report that other teeth are either 
the most dimorphic (Garn et al., 1966; Shrestha, 
2005) or nearly as dimorphic as the canine in cer-
tain populations (Iscan and Kedici, 2003; Kieser 
and Groeneveld, 1989; Perzigian, 1976; Potter 1972; 
Potter et al., 1981; Sharma 1983). Indeed, some 
studies have reported the presence of “reverse 
dimorphism” in which females possess larger av-
erages for certain variables than males (Acharya 
and Mainali, 2007; Ghose and Baghdady, 1979; 
Harris and Nweeia, 1980; Prabhu and Acharya, 
2009). In fact, Ghose and Baghdady (1979) report 
that fully one-third of the variables they examined 
among Yemenites exhibit such “reverse dimor-
phism.” 
 Numerous studies report population differ-
ences in both the patterning (Anuthama et al., 
2011; Ates et al., 2006; Iscan and Kedici, 2003; 
Prabhu and Acharya, 2009) and magnitude 
(Anuthama et al., 2011; Iscan and Kedici, 2003; 
Prabhu and Acharya, 2009) of sex dimorphism in 
odontometric variables. Such differences also ex-
tend to the relative size of key versus distal mem-
bers of the same morphogenetic field. Designating 
such differences as “tooth size crown gradients,” 
Harris and Harris (2007) found marked differences 
between major human groups in which some are 
marked by “steep” gradients of sharp reductions 
in size from the key to distal teeth, while others 
possess “shallow” gradients with similar dimen-
sions across the members of a field. 
  One-way ANOVA demonstrated that among 
the 15 samples considered here a tooth’s position 
within a dental field accounts for 15.5% to 23.1% of 
the observed variation in tooth size within mor-
phogenetic fields. Yet, it is also the case that when 



27  

 

variation within dental fields is considered by sex 
it is clear the contribution from sex differs marked-
ly with regard to both magnitude and polarity. A 
two-way analysis of variance by sample revealed 
that when sex is added as a second factor, the per-
centage of variance explained increases to 16.7%-
30.8%, which is an improvement of 27.2% when 
consideration is limited to position within a mor-
phogenetic field. In accordance with the observa-
tions of Harris and Harris (2007), the improvement 
in accounting for the variance in tooth size be-
tween key and distal members of a dental field 
varies widely. Thus, not only does it appear that 
sex, in addition to position, is influential in the 
determination of relative tooth size between key 
and distal members within a dental field, it is also 
the case that this influence differs markedly across 
samples. Such differences in the expression of sex 
dimorphism were found to mirror differences in 
tooth size allocation as a whole (Hemphill, 1991) 
and also explain why discriminant functions de-
veloped for determination of sex in one popula-
tion often predict sex with much lower accuracy 
when applied to members of other populations 
(Wright and Hemphill, 2012). 

CONCLUSION 
 

 Viewed as a whole, this “fool’s mission” ap-
pears not to have been at all foolish. Dental field 
theory offers an inaccurate picture of the true pat-
tern of variation among key and distal members of 
morphogenetic fields. For while it is the case that 
key teeth are often less variable than their distal 
counterparts, reversals are common. Dahlberg’s 
(1945, 1951) alleged reversal of polarity among 
mandibular incisors is not supported, nor is So-
faer’s (1973; Sofaer et al, 1972a,b) notion of com-
pensatory tooth size effect. The notion that females 
tend to be more highly genetically canalized than 
males and hence are more resistant to environ-
mental perturbations is not confirmed. Males and 
females were found to exhibit similar levels of rel-
ative variability between key and distal members 
of morphogenetic fields. However, since much of 
the development of the permanent tooth crown 
occurs post-natally, potential mitigating cultural 
factors that favor males over females cannot be 
ruled out. There is abundant evidence that sex di-
morphism is expressed differently, both with re-
gard to patterning and to magnitude across hu-

Fig 9.  Average relative contribution of sex to position in determination of relative tooth size between 
key and distal mandibular teeth within a dental field by rank order (ranked by contribution from sex).  



28  

 

man populations. Drawing from Harris and Har-
ris’ (2007) notion of tooth crown size gradients 
within morphogenetic fields it is clear that among 
the South Asian ethnic groups considered here, 
there is considerable variation in the expression of 
sex dimorphism. Indeed, the very low expression 
of sex dimorphism among the relatively well-
nourished Awans of Mansehra District coupled 
with the marked expression of sex dimorphism 
among the isolated high altitude Wakhis of Sost, 
suggest strongly that these differences cannot be 
attributed to mere environmentally induced 
“blunting” of sex dimorphism. Instead, these dif-
ferences in the degree and patterning of sex di-
morphism in permanent tooth size are the conse-
quence of the same population-specific differences 
in the array of genes that control the apportion-
ment of overall tooth size throughout the perma-
nent dentition. Given that population differences 
in the expression of sex dimorphism in permanent 
tooth size are even less likely to be subject to the 
impacts of natural selection than overall tooth size, 
patterning in the expression of sex dimorphism in 
permanent tooth size offers an additional avenue 
for unraveling the complex histories of human 
populations on local, regional and continental lev-
els.         

 

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