Hassett 2006.1

Torus mandibularis is a non-metric trait found
in varying frequencies among human populations
past and present, and as such is commonly recorded
along with a battery of other such traits during the
archeological assessment of skeletal remains. Non-
metric traits are largely used in analyses of biological
distance within and among archeological samples due
to the assumption of a high heredity quotient in their
occurrence. Berry and Berry (1967) established the utility
of several skeletodental traits in biodistance analyses,
but several traits commonly included in such analyses
lack substantive evidence of genetic involvement.

Torus mandibularis was chosen to test the utility of the
trait in establishing biological relationships specifically
because of the debate surrounding its etiology. Lacking
a clear pattern of genetic inheritance, the trait has been
seen by a number of researchers to relate instead to the
osteological response of the masticatory complex to
mechanical stresses. There has been nearly 100 years
of contention as to whether the trait might represent
a phenomenon related either to genetic heritability on
a population level or to functional stress. This study
was intended to provide direct evidence of either a
correlation between occurrence and functional stresses
on the masticatory complex, or a conclusive lack of
correlation. Additionally, the degree to which this data
set corresponds to other researchers’ assessments of
population-level variation is addressed. A total of 498
individuals from 8 archeological collections was assessed
here on factors that have been posited to play a role in

Torus Mandibularis: Etiology and Bioarcheological Utility

Brenna Hassett

University College London Institute of Archaeology, London, United Kingdom

ABSTRACT: Torus mandibularis is a non-metric trait
commonly recorded in bioarcheological investigation
and often included in the battery of non-metric traits
used to analyse biological distance among populations.
However, there is considerable debate regarding the
etiology of the trait, with genetic and environmental
factors both having been posited as the primary factor
in torus development. This study of 498 individuals,
drawn from eight archeological samples, investigates
the variation in torus frequency in different groups as

defined by sample, age, sex, and measures of functional
stress. Frequencies varied significantly among both
samples and dental attrition categories, supporting
the idea that mandibular tori are a threshold trait,
influenced by both genetic and environmental factors.
Results of this study suggest the utility of mandibular
tori in bioarchaeology may lie outside of biodistance
analyses that rely on the high heritability quotient of
non-metric traits to establish population distances.
Dental Anthropology 2006;19:1-14.

Correspondence to: Brenna Hassett, 5 Oakworth Court,
160 Nelson Road, London N8 9RP, UK
E-mail: brennaryan_1@hotmail.com

torus development, namely population group, age, sex,
and evidence of functional stress.

Torus mandibularis is recognized as a bony ridge
or series of bony nodules or lumps appearing on the
lingual surface of the alveolar margin of the mandible,
generally in the premolar region (Hauser and DeStefano,
1989). These tori may be completely absent or present in
varying degrees, and may present a variety of forms.

Mandibular tori are not associated with any
pathological condition and can be easily distinguished
from instances where osteological activity is the result of
a pathological condition causing abnormal growth, such
as trauma or tumor. Torus mandibularis is generally
manifested bilaterally, though it may be present just
on one side of the mandible. There is often a degree
of asymmetry between sides, with the right side most
commonly presenting a more pronounced torus than
the left (Haugen, 1990; Seah, 1995).

ETIOLOGICAL DEBATE

The question of etiology is vital in assessing whether
use of oral tori in biodistance analysis is appropriate. If
tori are assumed to be solely under genetic control, then
mandibular exostoses are accepted as useful estimators
of population distance along with the other traits
commonly used as part of the battery of non-metric
traits established by Berry and Berry (1967). If, however,
environmental factors play a larger role in determining
trait frequency, then their use as estimators of population
affinity is not acceptable. The relative importance
of environmental compared to genetic factors in the

Editor’s note: Ms. Hassett’s paper was awarded First
Prize for 2005 in the Albert A. Dahlberg student
research competition sponsored by the Dental
Anthropology Association.

development of facial tori has been widely debated,
with arguments for the functional basis of tori being
contradicted by arguments for a higher genetic factor in
increasing tori incidence. The trend has been to observe
an increasing role for genetic causality, but it remains to
be seen whether genetic inheritance fully explains the
development of mandibular exostoses. The most recent
studies have suggested that the tori arise in response
to both genetic and environmental factors (Haugen,
1990; Seah, 1995). There are several particular aspects
of tori distribution that have served as foci for debate,
and multiple hypotheses that have been constructed to
address the significance of population, sex, age, evidence
of functional stress, robusticity of mandible, symmetry,
and trait interaction in the formation of the mandibular
tori.

Multiple studies attempting to assess the heritability
of torus mandibularis have been conducted, coming
to divergent conclusions—autosomal recessive (Krahl,
1949; Alvessalo and Kari, 1972), autosomal dominant
(Suzuki and Sakai, 1960), or polygenetic in origin
(Johnson et al., 1965; Sellevold, 1980)—that there is a
strong argument against a simplistic assumption of
genetic transmission of the trait. However, it has been
commonly found to occur in family groupings, and
children whose parents exhibited the trait were found to
be more likely to exhibit the trait themselves in a study
of modern Thais (Kerdpon and Sirirungrojying, 1999).

The explanation for these diverse findings may lie
in the work undertaken by researchers to establish the
heritability of non-metric traits in studies of mice and
non-human primates. Grüneberg (1963) established
the concept of quasi-continuous variation in non-
metric traits, positing that the size or rate of formation
of a given trait may be the inherited factor, which he
demonstrated by using genetically isolated strains of
mice. Wright (1968) followed this by suggesting the
idea of a “threshold” trait that appears only once a
certain point determined by environmental factors has
been crossed; what the individual inherits is a liability
towards developing a trait which environmental factors
act upon. Berry and Berry (1967) undertook to study the
genetic origins of a large battery of non-metric traits in
mice, and it is to this work that most researchers utilizing
non-metric traits in estimating biological distance refer.
Observing the analogues of several human skeletal
traits in generations of mice, they proposed that most of
the human non-metric traits also originate from normal
genetic variation. The degree to which the environment
influences mandibular torus prevalence, and the degree
to which genetic inheritance does, should be understood
in order for this particular non-metric trait to be included
in the battery of traits assembled by Berry and Berry
(1967) commonly used to establish biological distance
among populations.

Factors affecting this debate include the difference

in age and sex. Variation between age classes in tori
frequency and degree of expression has not been found
by all researchers (Hauser and De Stefano, 1989:9), but
the largest studies suggest that there is some degree
of variation (Eggen, 1954; Korey, 1980; Haugen, 1990;
Jainkittivong and Langlais, 2000; Ruprecht et al., 2000).
Researchers have found in some cases that males are
more likely to exhibit tori than females (Haugen, 1990;
Seah, 1995; Hjertstedt et al. 2001), whilst in others there
is no significant sex difference (e.g., Bernaba 1974), or
that females have higher frequencies of tori (Corruccini,
1974). A brief summation of the disparate results of
some of these studies may be found in Table 1. The
consensus appears to suggest that there is some degree
of sexual dimorphism (Trinkaus, 1978; Haugen, 1990;
Seah, 1995), but it is difficult to assess whether this
difference is significant in a statistical sense, particularly
in archeological samples where sample sizes may be
small.

Variation in tori prevalence and expression with
increasing evidence of functional stress to the masticatory
complex has been the cornerstone of arguments for the
primacy of environmental factors in the development of
oral tori. Proponents of this view suggest that differences
among groups may be accounted for either entirely
or partially by non-hereditary factors, in which case
mandibular tori are unsuitable for assessing biological
distance without consideration of complicating
environmental factors. Instead, they would be of
greatest utility in assessing differences in environmental
factors such as diet or parafunctional use of the jaws.
Patterns of dental attrition, as a result of masticatory
hyperfunction related to diet or conditions such as
bruxism, have been seen to co-occur with mandibular
tori in a statistically significant way in several instances.
This has not been a universal observation (e.g., Scott
et al., 1991). Sirirungrojying and Kerdpon (1999)
found that torus mandibularis was significantly (P <
0.005) more common in dental patients suffering from
temporomandibular disorder (TMD), perhaps due to
high levels of parafunctional activity, such as clenching
and grinding of the teeth (bruxism). This led them to
suggest that torus mandibularis could be viewed as
an early indication of risk for TMD. Larsen (1997) also
comments that the general robusticity of the masticatory
complex may be closely correlated with the amount of
stress placed on the jaws; for example, a smaller, more
gracile jaw would be correlated with a softer diet.

Johnson (1959) studied the mechanical stress of the
jaws in conjunction with bone histology. The conclusions
of his research were published posthumously, however,
and the specific details of his findings are not provided;
there is just the supposition that tori may be interpreted
from histologiy to be the result of functional stress
(Johnson, 1959). No subsequent study has found any
evidence to support this hypothesis (Haugen, 1990; Seah,

B. HASSETT

�

1995). An earlier study by Van den Broek (1943), who had
formerly supported the hypotheses of functional stress
acting to produce the tori, investigated tori histology
and found that the structure of the tori did not represent
an obvious bony response to mechanical stress.

The symmetry of mandibular torus formation has also
been called on to account for trait etiology. Ossenberg
(1981) ascribes overall tori frequency to environmental
factors, but maintains that the degree of expression and
any resulting asymmetricality is a function of genetics.
Korey (1980) suggests that genetic factors would be
more likely to act equally on both sides of the mandible.
McGrath et al. further emphasize the importance of
assessing asymmetry in measuring non-metric traits,
suggesting that asymmetrical development, insofar as
it is correlated to environmental causes, may be a clue
to an individual’s ability to buffer stress (McGrath et al.,
1984, p 401).

Another suggested explanation of the development of
the tori is local inflammation of the periosteum, leading

to torus formation (Schreiner, 1935). This was followed
by Van den Broek after his (1943) investigation of torus
histology failed to support the hypothesis that the tori
are laid down to strengthen the structural integrity of
the jaw. Little evidence has thus far been provided to
further this hypothesis.

HYPOTHESES TESTED

Having examined previous approaches to
understanding the etiology of torus mandibularis,
several questions remain. Of primary interest here is the
suitability of this non-metric trait for use in biodistance
analyses; that is, whether there is sufficient genetic
control of the tori to warrant its use as a marker of
family or population group membership. The prevailing
opinion in the most recent summaries of the issue is that
mandibular tori are a quasi-continuous or threshold
trait (Haugen, 1990; Seah, 1995), having both a genetic
and environmental component. If this is accurate, then
questions arise concerning what degree of influence

Sample Males(%) Females (%) Citation

Poundbury 16.3 10.9 Farwell and Molleson 1993
Cannington 11 15 Brothwell et al. 2000
Ukranian 0.0 1.5 Cesnys and Kundruktova 1982a
Lapps 26.8 38.8 Schreiner 1935
North American Whites 6.5 8.1 Corruccini 1974
Eskimo 58.1 35.2 Dodo and Ishida 1987a
Canadian Eskimo 85.3 80.0 Dodo and Ishida 1987a
Aleuts 71.7 75.9 Dodo and Ishida 1987a
Brazilian Indians 0.5 0.5 Bernaba 1977
Blacks 6.1 6.2 Corruccini 1974
Japanese 26.7 33.3 Mouri 1976a
Ainu 44.3 21.1 Dodo and Ishida 1987a
Iglooik Eskimo 38.7 40.8 Mayhall and Mayhall 1971
Hall Beach Eskimo 41.5 32.1 Mayhall and Mayhall 1971
Norwegian 6.36 8.53 Haugen 1990
aCited in Hauser and DeStefano (1996).

TABLE 1. Frequencies of torus mandibularis in various groups, by sex

Sample n Males Females Unknown Location Period

Chumash 47 19 23 6 California Coast,
Channel Islands Prehistoric
Abingdon 103 39 43 21 Oxfordshire, UK Medieval
Cannington 101 47 37 3 Somerset, UK Dark Ages and
Late Roman (7-8th c. AD)
Spitalfields 100 48 31 12 London, UK 17th-19th century
Poundbury 71 29 32 9 Dorchester, Dorset, UK Roman 4th c. AD
Hawara 50 28 17 5 Hawara, Fayum, Egypt Roman 2nd-3rd c. AD
Egypt 23 10 9 4 Abydos, Egypt Pre- to Dynastic Period
Lachish 13 4 8 1 Lachish, Israel Mixed Bronze, Iron Age

TABLE 2. Characteristics of the samples used in the study.

TORUS MANDIBULARIS

each factor has on trait prevalence. To address these
questions and the impact of the major factors on torus
development posited by previous research, the following
hypotheses have been formulated.

Firstly, if there is a genetic component to trait
prevalence, different ethnic or population groups should
show different frequencies of trait development. This
does not, of course, rule out any environmental influence,
but merely establishes the possibility that genetics could
play a role. To firmly establish the genetic etiology of the
tori would require a carefully controlled study covering
generations, which was not feasible here.

Secondly, significant variation between the sexes
would show that there is a level of sexual dimorphism
to tori development. If the rates of sexual dimorphism
differ significantly from sample to sample, this would
indicate that the major force acting on dimorphism for
these traits is environmental, rather than genetic, as the
mode of genetic transmission of the trait is assumed
to not vary between populations, while culturally
differentiated sexual labor roles may differ.

Thirdly, variation between age groups in tori
prevalence would reflect a difference in frequency caused
by either progressive development of the exostoses or
by a dynamic process related to functional stress. If the
occlusal attrition and robusticity of the mandible are
strongly correlated with age and tori prevalence, then
the latter hypotheses may be supported. If there is little
correlation between indicators of masticatory stress,
age, and prevalence, then progressive development
would be supported by significant variation between
age classes.

Finally, the indications of masticatory hyperfunc-
tion—particularly tooth wear—would be greater in
individuals with mandibular tori if masticatory hyper-
function is a large factor in determining tori prevalence.

MATERIALS AND METHODS

The materials used in this study come from the
large collection of human skeletal material held by the
Department of Anthropology of the Natural History
Museum of London. The collection comprises material
from over 20,000 individuals, gathered throughout
the 19th, 20th, and 21st centuries from archeological
excavations, anthropological fieldwork, and donated
private collections (R. Kruzynski, pers. comm.). The
larger coherent samples included here were largely
archeological samples, groups that were spatially and
chronologically limited to the time and location of
excavated burials. The ideal sample size, based on both
statistical and temporal concerns, was established as 100
individuals for each population. However, this was not
possible in all cases. Some samples were comprised of
fewer than 100 individuals, such as the Egypt group,
which remains in the final analysis despite the smaller
size in an effort to broaden the regional scope of the

study. The Lachish sample was dramatically limited by
the disassociation of mandibles from crania and was not
considered sufficient to be included as anything other
than an ancillary note in the population-based analysis,
though it is included in the overall analyses. A summary
of the samples included here is given Table 2.

On the individual level, inclusion was dictated by
several criteria. As mentioned above, and particularly in
the case of the Lachish sample, all groups were of course
limited by the number of individuals with associated
mandibles. Additionally, inclusion occasionally
depended on the level of preservation encountered, as
the most fragmented remains could not be scored for all
points of interest. In order to avoid biasing the samples
in favor of more robust crania less likely to suffer a high
degree of fragmentation, however, every effort was
made to include both fragmented and non-fragmented
remains where scores could be taken. Exclusion of
individuals only occurred where it was not possible to
take the scores of presence or absence of the tori.

For the collected data, a total of 41 measurements or
scores were taken and used to create the final scores that
are used in analysis. The aim behind the selection of these
measurements was to provide standardized and, thus,
easily comparable, data on age, sex, trait expression and
frequency, mandibular thickness, temporomandibular
joint remodeling, and tooth macrowear. Where
published figures on the samples investigated here were
available, they were compared with the assessments of
this study. Age and sex assignments were established
solely on cranial material due to the time constraints on
the present study. Assignments were analyzed against
previous published results drawn from both cranial
and postcranial material, and in the exceptional case
of the Spitalfields material, drawn from individuals of
known age and sex. Insufficient discrepancy was found
to warrant any adjustment of my own assessments,
and because a standardized method was applied to
all samples under discussion, the study is certainly
internally coherent.

Age and sex were established after the Chicago
Standards (Buikstra and Ubelaker, 1990). Not used here
as a criteria for age estimation was the degree of dental
attrition. Attrition, or the wear on the occlusal (chewing)
surface of teeth, progresses with age, though at variable
rates and is commonly used to assess biological age
among archeological samples (Brothwell, 1986; Hillson,
1996). Samples may differ widely in their rates of attrition,
as the amount of abrasive material in the diet and
functional stress acting to wear the teeth differ among
groups and even between the sexes (Walker et al., 1997,
p 174). Lacking a comparable population with known
age, sex, diet, and functional stresses, age estimates
based on tooth attrition may be seriously distorted by
a number of variables depending on the population in
question. As attrition is also used here to examine the

B. HASSETT

functional stresses placed on the masticatory complex,
the age estimates garnered from scoring tooth wear
were excluded. Sutural closing of the cranium was used
instead, though this method is rightfully criticized for its
unreliability (Masset, 1976). Therefore, the age categories
used here are very broad—young adult (17-34 years of
age), adult (35-54 yr), and older adult (55+). This study
counts trait presence in three categories, namely absent,
unilateral, and bilateral. Metric equivalents of these
categories (measured as maximum tori breadth on the
transverse plane) are delineated in 2 mm increments with
less than 2 mm being “slight,” 2-4 mm “moderate,”and
greater than 4 mm “pronounced.” Examples of the
slight and moderate categories are provided in Figures
1 and 2. An example of the trait as it appears in a living
individual can be found in Figure 3.

Attrition was scored at the left first and second molars,
both upper and lower, and the “edentulous” category
represents individuals who had premortem loss of the
teeth. Remodeling activity at the temporomandibular
joint (TMJ) was assessed on morphological changes to the
joint surface both on the mandibular and the temporal

bones (e.g., excessive porosity, osteophyte activity).
Robusticity was of the mandible was assessed as

the variable BODTH after Humphrey et al. (1999). The
measurement is of mandibular body thickness, taken
from the left side of the mandible by spreading calipers
placed parallel to the occlusal plane at M1 in the center
of the mandibular body. This measurement was seen to
correlate strongly with overall metric measures of size
in the mandible (Humphrey et al., 1999).

The presence of maxillary tori—a similar non-metric
trait that is nearly identical to torus mandibularis in
manner of expression and the degree of understanding
regarding its etiology—was also recorded according to
the same methodology as mandibular tori. As a second
example of exostoses of the dental arch, this trait was
considered likely to have similar factors affecting its

TORUS MANDIBULARIS

Sample Absent Unilateral Bilateral Total

Chumash 47 1 48
Lachish 13 13
Hawara 43 1 6 50
Cannington 65 2 24 91
Abingdon 98 2 � 10�
Spitalfields 96 2 2 100
Poundbury 56 5 9 70
Egypt 20 1 2 2�
Total 438 13 47 498

1Chi-square = 56.639; df = 14; P value < 0.000; chi square for unilateral and bilateral categories grouped as “present”
= 41.348; df = 7; P value < 0.000.

TABLE 3. Counts of unilateral and bilateral forms of torus mandibularis by sample1

Fig. 1. Example of slight expression of torus
mandibularis in a skull (SK34) from Cannington
Cemetery (courtesy Natural History Museum, London).

Fig. 2. Example of moderate expression of torus man-
dibularis in a skull (SK112) from Cannington Cemetery
(courtesy Natural History Museum, London).

development.
There were two primary areas in which the

functional stresses placed on the masticatory complex
were assessed in this study. The best skeletal evidence
of stresses associated with excessive chewing and/or
grinding motions necessitated by either a tough diet or
a pathological condition (i.e., bruxism) in an individual
is found in morphological changes in the masticatory
apparatus (Eggen and Natvig, 1986). Representing these
changes in the skeleton are, firstly, the degree of occlusal
wear of the molar dentition of the upper and lower jaws,
and, secondly, osteological activity in the TMJ.

ANALYSIS

All of the data collected for this study were entered
into a computerized database to facilitate statistical and
comparative analysis. This database was created in SPSS
Version 11.0, a statistical processing application authored
by Norusis (1994), which was used to produce the final
analyses. The variables of interest were cross-tabulated
in SPSS to provide the tables for this article. Chi-square
tests were conducted for each cross-tabulation by SPSS,
as well as correlation statistics. Generally, an alpha level
of 0.05 was used to distinguish significant results, though
trends that were discernable by direct observation of
visual representations of data but fell slightly outside
of the significant range are noted on occasion. Due to
the small sample size provided by some of the groups,
it is possible that some trends would be found to be
significant if larger samples were available.

RESULTS

The tables included here give the results of the
statistical cross-tabulation of the factors discussed above
in tori frequency. All P-values given in the text are the
result of Pearson’s chi-square test unless otherwise
noted. As with all archeologically derived data sets, it
is important to remember that the standard statistical
procedures used to test significance and correlations
of variables assume a random sample of a normally
distributed population, which is very rarely the case
with archeological material. Therefore, where trends
were observed in the distribution of the data but not
deemed to be significant at the P < 0.05 level, they are

B. HASSETT

Age Group

Less than 352 35-54 Years� 55 and Over4

Sample A U B T A U B T A U B T

Chumash 11 1 12 1� 1� 2� 2�
Lachish 6 6 4 4 3 3
Hawara 18 1 19 15 3 18 10 1 2 13
Cannington 34 11 45 12 1 6 19 19 1 7 27
Abingdon 43 43 34 2 2 38 21 1 22
Spitalfields 17 1 1 19 41 1 42 29 1 30 30
Poundbury 7 1 8 24 1 3 28 25 4 5 34
Egypt 8 1 9 9 9 3 1 1 5

Total 144 1 16 161 152 4 15 171 133 8 16 157

TABLE 4. Occurrences of torus mandibularis by age group and by sample.

1Trait expression codes: Absent (A), Unilateral (U), Bilateral (B), and Total (T).
2Pearson chi square = 24.455; df 14; P value = 0.040; combining categories of presence X2 = 15.634; df = 7; P value =
0.029.
�Chi-square = 23.945; df 14; P value = 0.047; combining categories of presence X2 = 16.788; df = 7; P value = 0.019.
4Chi-square = 25.636; df 14; P value = 0.029; combining categories of presence X2 = 19.805; df = 7; P value = 0.006.

Fig. 3. Example of torus mandibularis in a living
individual (courtesy S. Gill, Laguna Hills, CA).

�

still described, but all results should be viewed with this
caveat in mind.

Populations

Highly significant variability in frequency of
occurrence of torus mandibularis was found among the
samples in this study (P < 0.000). Table 3 provides the
frequencies by sample of mandibular tori, along with
the results of chi-square tests for significance. Variation
in frequency of torus mandibularis among age groups
defined by degree of sutural closing, as shown in Table
4, was also found to be significant among samples. The
variability in prevalences between the sexes in different
samples is greater for males (P < 0.09) than females (P
< 0.11) or those of unknown sex (P < 0.19), as shown in
Table 5.

Also found to be highly significant was the variability
of mandibular torus expression (torus size) among
groups (P < 0.000), as seen in Table 6.

Sex

The degree of expression of the torus (whether
slight, moderate, or pronounced) did not vary signifi-
cantly between sexes. As mentioned above, however,
variability between the sexes is evident among samples,
particularly in males.

Age

Variation among age groups for mandibular tori, as
defined by sutural closing, was not significant unless,
as with sex, “group” is added as a variable. However,

TORUS MANDIBULARIS

Unknown2 Females� Males4

Sample A U B T A U B T A U B T

Chumash 6 6 22 1 2� 19 19
Lachish 1 1 8 8 4 4
Hawara 5 5 14 3 17 24 1 3 28
Cannington 3 3 6 27 2 8 37 35 12 47
Abingdon 20 1 21 38 1 39 40 3 43
Spitalfields 12 12 29 1 1 31 46 1 1 48
Poundbury 7 2 9 26 3 3 32 23 2 4 29
Egypt 4 4 6 1 2 9 10 10

Total 58 1 5 64 170 8 18 196 201 4 23 228

TABLE 5. Occurrences of torus mandibularis by sex and sample

1Trait expression codes: Absent (A), Unilateral (U), Bilateral (B), and Total (T).
2Chi-square = 23.514; df = 14; P value = 0.052; combining categories of presence X2 = 16.826; df = 7; P value = 0.019.
�Chi-square = 23.435; df = 14; P value = 0.054; combining categories of presence X2 = 18.307; df = 7; P value = 0.011.
4Chi-square = 27.631; df = 14; P value = 0.016; combining categories of presence X2 = 18.888; df = 7; P value = 0.009.

Torus Mandibularis Expression

Sample Absent Slight Moderate Pronounced Total

Chumash 47 1 48
Lachish 13 13
Hawara 42 7 49
Cannington 65 17 8 1 91
Abingdon 98 5 103
Spitalfields 96 3 1 100
Poundbury 55 10 4 69
Egypt 20 � 2�

Total 436 46 13 1 496

TABLE 6. Occurrence of torus mandibularis expression by sample1

1Chi-square = 55.688; df 21; P value = 0.000; combining trait expressions, X2 = 54,280; df = 14; P value = 0.000.

if age were to be defined by dental attrition classes as
opposed to sutural closing, very significant variation
in torus mandibularis is observed. The results of such
a comparison are given under the Functional Stress:
Attrition heading below. Torus mandibularis expression
was not seen to vary between age classes.

Functional stress: attrition

Torus mandibularis showed significant variation
in frequency between classes of occlusal tooth wear
in the lower first molar (P < 0.001), as shown in Table
7. Significant (P < 0.01) differences between degree of
expression of the mandibular tori in the different attrition
classes was observed and may be seen in Table 8.

Functional stress: mandibular robusticity

No significant variation was found between
size categories of mandible and occurrence of torus
mandibularis as measured by maximum mandibular
breadth and defined in millimetre increments. Nor was
any significance seen in the variation in degree of torus
expression between size categories.

Functional stress: activity at TMJ

There was no significant difference in levels of osteo-
phyte activity or porosity at the temporomandibular
joint and degree of torus mandibularis expression or the
incidence of mandibular tori.

Trait interaction

No significance was attached to the co-occurrence
of mandibular and maxillary tori or to the degree of
expression of either tori with co-occurrence or the degree
of expression of the co-occurring tori.

DISCUSSION

“Group” appears to be a significant variable in
prevalence of torus mandibularis and torus maxillaris.
The results of this study are not surprising in confirming
what is already an observed trend in the literature on the
subject; that tori incidence rates vary widely according
to group. The trait is seen here to occur in different
frequencies and to different degrees in geographically
and chronologically separated groups. No previous
investigations of tori incidence have contested this
variability, yet there remains a multitude of hypotheses
as to why this variation occurs. It is useful to look to
the reasons why multiple studies have reached such
divergent conclusions in the light of the results of the
present study.

Perhaps the largest factor in the disparity of
conclusions arises from the different variables assessed
among investigations. The variables tested in other
works may have been chosen based on assumptions
on the investigators part as to what factors could be
involved in tori development. Thus, in those studies that
began with an assumption of genetic inheritance acting
as the sole factor (Suzuki and Sakai, 1960; Gould, 1964),

B. HASSETT

Torus Grade of LM1 Attrition1
mandibularis 1 2 3 4 5 6 7 8 9 Total

Absent 1 14 51 67 43 31 17 29 149 402
Unilateral 1 2 2 1 2 � 11
Bilateral 1 2 16 4 5 9 4 4 45

Total 1 16 55 83 49 37 26 35 156 458
1Chi-square = 55.688; df = 21; P value < 0.000; combining grades of presence X2 = 27.430; df = 8; P value = 0.001.

TABLE 7. Occurrence of torus manibularis by grade of occlusal attrition on LM11

Torus Grade of LM1 Attrition1
mandibularis 1 2 3 4 5 6 7 8 9 Total

Absent 1 14 51 67 42 31 17 29 148 400
Slight 2 2 11 5 5 7 4 6 42
Moderate 2 5 1 1 2 1 1 13
Pronounced 1 1

Total 1 16 55 83 48 37 26 35 155 456

TABLE 8. Grade of torus manibularis tabulated against grade of occlusal attrition on LM11

1Chi-square = 42.059; df = 24; P value = 0.013.

9TORUS MANDIBULARIS

it is unsurprising that family relationships were the only
variables considered. Unfortunately, family relationships
are very difficult to ascertain in archeological samples,
and these could not be considered here. However, the
disparity of modes of inheritance posited by familial
studies and the concurrence of environmental factors
with the frequency of tori suggest that a simple
Mendelian-mode of inheritance is not adequate to
explain the variation in tori incidence among samples.
Other factors seem to play a role in determining the
prevalence of these traits.

As significant differences were found between the
sexes in only some of the samples, the conclusions of
previous researchers of an actual difference in prevalence
rates between males and females, regardless of group,
appear unsupported. However, as sexual dimorphism
is known to differ in degree among groups (Brothwell,
1981), it is possible that the variation observed here is a
product of this difference, acting to influence robusticity
of the masticatory complex. Additionally, culturally
defined sexual roles may include different functional
stresses for men and women, further skewing any
evidence of an actual tendency in tori prevalence should
functional stress be a factor in development of the trait.
It is worth noting here that Haugen (1990) observed
greater frequency of mandibular tori in Eskimo men,
though the ethnographic accounts of Pederson (1944)
make it clear that Eskimo women had greater functional
stresses placed on their jaws.

As with variability between sexes, the variability
among age groups in development of the tori of the jaws
is less obvious from the results of this study than that
among groups. “Age” as a variable becomes especially
problematic when it is assessed on skeletal material for
a number of reasons. Because archeological samples
do not generally provide individuals of known ages,
the categorization of individuals into age groups must
be done on the basis of morphological changes in the
skeleton that are normally associated with ageing. These
techniques are only accurate to the degree that other
factors influencing the morphology of the skeleton can
be controlled for. Walker (1978) points out that attrition
rates depend not only on the age of an individual but

also on the abrasiveness of the diet. The possibility that
tori develop in response to the changing functional
pressures on the jaw and teeth, along with this caveat,
is why dental wear is not used here as an indication
of age. This leaves the closing of cranial and palatine
sutures as a basis for ageing the material studied here.
An additional issue in using age as a variable arises,
however, with the realization that the morphological
changes to the cranium associated with age may be
part of other skeletal processes affecting an individual’s
pattern of sutural closing (e.g., trauma, etc.). An overall
tendency towards early or late ossification due to
genetic or nutritional factors, or several other conditions
(e.g., general tendency to robusticity, etc.) that affect
the skeleton may act to conflate or deflate evidence
of a relationship between chronological age and the
development of tori. In the first case, all care was taken
to remove individuals with obvious disruptions to
the normal pattern of sutural closing. The possibilities
associated with the second case are discussed further
below, while the remaining multitude of possible factors
acting to influence skeletal morphology must remain as
a caveat in assessing the import of age in development
of torus mandibularis and torus maxillaris.

The results of this investigation do show a significant
variation between the age groups in development of
mandibular and maxillary tori when “group” is added
as a variable. This possibly reflects population-level
differences in the morphological characteristics on
which age was assigned, but other investigations have
suggested that age is indeed a factor in tori development.
Mandibular torus is very rare in juveniles, excepting
those samples that normally have a high frequency of
the trait (Haugen, 1990). Development of tori is generally
agreed to begin within the first 30 years of life, though
may occasionally occur later (Seah, 1995). The contention
that tori development is not a slow, progressive process
but rather a dynamic one (Seah, 1995, contra Haugen
1990) is perhaps supported by the evidence of this
study, as degree of expression was not found to vary
significantly among age classes. This has not been a
universal finding; Halffman et al. (1992) and Eggen
and Natvig (1986) found that tori were more frequent
in the middle-aged, and Eggen (1989) also found no
significant increase in frequency after ca. 30 years of age.
Johnson (1959), however, mentions that tori resorption,
or shrinkage, has been observed in both the very old
and those whose teeth have been removed. The results
of this study show that the most significant variation in
age groups between populations occurs within the 55+
age group (P < 0.03). These results suggest that is not
so much a causal factor in development of the tori, but
rather a covariant which is affected by the same factors
acting to effect torus development.

The first and foremost of the variables associated with
age is the degree of occlusal wear of the dentition. Tooth

Torus
mandibularis Dentate Edentulous Total

Absent 366 69 435
Unilateral 1� 1�
Bilateral 45 2 47

Total 424 71 495

TABLE 9. Grade of torus mandibularis partitioned by
whether the cases was dentate or edenulous1

1Chi-square = 6.887; df = 2; P value = 0.032.

wear is strongly correlated with age, and commonly
used in archeological samples to categorize age. In this
study, tooth wear was significantly correlated with the
age assignments (r = 0.233, at a significance of P < 0.000).
Generally, attrition increases with age (Walker, 1978;
Brothwell, 1981; Waldron, 2001). But age is not the only
factor acting to wear the dentition (Walker et al., 1991).
Coarseness of diet or differing levels of functional stress
on the teeth may hasten the normal process, with the
result that individuals with different diets or stresses
may show markedly different wear rates. Wear rates
may be expected, then, to vary among groups (Walker,
1978). One of the most widely observed examples of this
difference is found in the Eskimos of North America,
Iceland, and Greenland, who have very high attrition
rates as well as a high frequencies of chipping and
pitting of the teeth that coincide with the high functional
demands they place on them. In the samples observed
here, there is a definite variability among rates of
attrition, as evidenced by the molars.

The frequency of torus mandibularis is significantly
correlated with the level of attrition recorded at the first
molar (P = 0.000). The distribution of the tori over the
wear classes clearly shows a gradual increase up to a
peak in the number of occurrences around the fourth
stage of wear, with the subsequent pattern of decline
in frequency only occasionally interrupted. This is very
suggestive when the reasons posited for development of
the tori are considered. If the tori arise as a response to
functional stress, evidenced by tooth wear, expectation
is that the frequency of torus mandibularis would be
low in individuals with low levels of wear. Frequency
would be expected to increase as functional forces acting
to wear the teeth increase. Should the tori develop as
a skeletal response to mechanical forces, frequency
would be expected to be highest in individuals with
the most severe wear, with those exerting the most
stress presumably exhibiting the most wear. However,
frequency does not dramatically increase after the
fourth stage of wear. A partial explanation for the lesser
number of tori in the latter stages may be that, as tooth
wear increases, the functionality of the teeth may be
impaired, and the need for functional strengthening of
the jaw may decrease if the jaw is no longer used due
to tooth loss. The resorption of bone from the mandible
in edentulous individuals due to this loss of function
may partially explain the reduction of occurrence with
the most severe wear. In examining torus mandibularis
occurrences in dentulous and edentulous individuals,
as defined by the premortem loss of the first and second
molars, significant variability is found, which lends
strength to this suggestion (Fig. 4).

This study did not find that osteophyte activity at
the temporomandibular joint was significantly varied
between those individuals with torus mandibularis. Nor
was there a significant difference in levels of porosity.

This does not necessarily rule out the possibility of
an association between disorders of the joint and the
development of torus mandibularis; temporomandibular
disorder (TMD) is only generally identifiable in the
most severe cases from skeletal remains (S. Hillson,
pers. comm.). In clinical studies, torus mandibularis has
been seen to correspond very significantly (P < 0.0005)
to TMD as well as to one of the most common causal
factors for TMD, namely parafunctional activity such as
bruxism (Kerdpon and Sirirungrojying, 1999).

Radiographic measures of bone density at multiple
locations in the body taken in a study of torus
mandibularis suggest that higher bone mass density is
significantly associated with development of the trait
(Hjertstedt et al., 2001), but, again, this was something
the present study was unable to assess.

General robusticity of the jaw has been thought
to have some relation to development of torus
mandibularis. Eggen and Natvig (1986) suggested that
individuals with better developed jaws had higher
frequencies of torus mandibularis. Ossenberg (1981)
hypothesized that the development of tori after puberty
is possibly related to the greater food intake necessitated
by growth, which in turn necessitates greater muscular
power in order to process the larger amount of food.
While many of the samples observed in the literature
–with high prevalence rates of tori are generally robust
in terms of skeletal build, the quality of robusticity
is so ill-defined as to make analysis of this variable
nearly impossible. With robusticity here defined by
the thickness of the mandibular body, no significant

B. HASSETT

Fig. 4. Bar graph of the occurrence of torus mandibu-
laris depending on whether the subject was dentate.

relationship between torus mandibularis and general
robusticity was observed.

This brings the discussion to the hypothesis of
trait interaction. The concurrence of the two exostoses
included in this study was insufficient to suggest a
strong correlation. Other researchers have debated the
co-occurrence of torus palatinus, with some finding
a correlation between the traits and some not. The
investigations using the largest sample sizes have not
seen a strong correlation, and the investigations into the
effects of bone mineral density (Hjertstedt et al. 2001)
and parafunctional activities (Eggen, 1954; Eggen and
Natvig, 1986; Kerdpon and Sirirungrojying, 1999) have
shown different prevalence rates for torus palatinus.
In this study, the co-occurrence of torus mandibularis
and torus maxillaris was not found to be significant in
either degree or expression of frequency of incidence.
This finding, taken in conjunction with results showing
no correlation between possible causal factors in
torus mandibularis development and torus maxillaris
development, suggests that the two traits are not the
result of a single causal factor, either environmental or
genetic. The lack of co-occurrence in familial studies of
the traits and in studies of environmental or functional
stress lead to the conclusion that the tori arise due to
separate stimuli. That is not to say absolutely that the
same factors aren’t responsible for development of both
maxillary and mandibular tori, but the inconclusive
efforts to relate the maxillary tori to those factors, which
show a significant relation to mandibular tori, suggest
that the developmental process of the former is not
identical to the latter. Due perhaps to the rarity of torus
maxillaris, less can be said about the possible correlations
of environmental or functional stress. This finding
follows logically in steps of the growing consensus that
the other major tori of the jaw, torus palatinus and torus
mandibularis, are affected by different factors (Kolas et
al., 1983; Haugen, 1990; Seah, 1995).

In relation to the hypotheses surrounding torus
etiology outlined previously, this study has found the
following:
• Frequency of both mandibular and maxillary tori

varies between populations to a significant degree,
suggesting that genetic inheritance could play a role
in torus etiology.

• Sexual dimorphism in tori frequency is found to
be significant within some populations, but not
significantly varied in others, possibly as a result
of the different effects of culturally defined labour
roles on the sexes. Additionally, this may explain the
disparate results of previous work in establishing
whether the traits are more prevalent in males,
females, or neither.

• Age is not found to be a significant factor in torus
development when measured on criteria other than
dental attrition, suggesting a more dynamic, possibly

environmentally induced, pattern of growth.
While robusticity of the mandible was not strongly
correlated with age or tori frequency, age was strongly
correlated with attrition classes. The interaction of
age with tooth wear is well established, which may
explain why previous research has suggested age as
a factor in torus development.

• Masticatory hyperfunction, evidenced by tooth
wear, is seen to be correlated with mandibular torus
prevalence. However, frequency of the trait is greatest
at a lower level of wear and there is a pronounced
difference in trait distribution between dentulous
and edentulous individuals. This may suggest that
mandibular tori are a successful response to functional
stress, as opposed to the result of loss of masticatory
function with increasing dental attrition

CONCLUSIONS

The results of this study conform to a pattern sug-
gested by the most recent research into tori etiology. In the
last 15 years, the general consensus has been that man-
dibular tori at least arise from a combination of genetic
and environmental factors. A more consistent obser-
vation of any correlations, might have been observed,
should they exist, if all investigations included the
multiplicity of variables proposed as affecting the occur-
rence of mandibular torus. However, because research
designs have often been constructed in a dichotomiz-
ing either/or fashion to show the significance of one
particular variable in the development of tori at the
expense of any other factors, there has been a tendency
to include only those factors the investigator wishes to
demonstrate as being either positively or negatively cor-
related with tori incidence. This is unfortunate, because
the widely divergent results produced by such studies
only serve to cloud the issue further. By incorporating
as many variables as possible into an investigation of
tori development, the polarised results of earlier studies
become understandable as partial glimpses of a multi-
factorial etiology only discernable when a wide ranging
investigation is carried out.

The results obtained here suggest that functional
stress plays a large role in the development of mandibular
tori. The correlation of age to torus development remains
unclear, though it is vital to remember that tooth wear
is strongly correlated with age. Had the age assessments
used here relied on attrition categories to define age, as
is common with archeological samples, the variation
between age categories and attrition categories would be
identical. A possible result of this correlation is the over-
emphasis of the importance of age in tori development,
as the passage of time allows increasing amounts of wear
and stress to act on the jaw. Significant variation in tori
development between classes of tooth wear were found
that support the idea of the torus arising as a response
to functional stress acting on the mandible in the form

TORUS MANDIBULARIS

of increased functional demand on the masticatory
complex. However, histological studies have not borne
out the expectation that the bony structure of the tori
themselves would reflect the direction of this mechanical
force. This leads to the conclusion that the exostoses
of the mandible are not purely a skeletal response to
pressure, an argument that also finds support in this
study’s finding a lack of significant correlation between
overall mandibular robusticity and trait incidence.
Further refuting the idea of a single variable, functional
stress, as the sole causal factor in tori development, are
the familial studies carried out in living populations
of known biological relations. Variation among
populations of different origins in torus frequency must
be accounted for, and the most appropriate explanation
may be found in the concept of the threshold trait
as proposed by Wright (1963). If the inherited factor
of torus mandibularis is a liability for development,
an individual tendency towards formation of either
this particular exostosis or exostoses in general, then
etiology must be multi-factorial, with environmental
factors acting to determine whether or not the threshold
for development is surpassed. This model explains both
the variability in frequency among groups and among
dental attrition classes found in this study.

It is hoped that future research into the development
of mandibular tori will address the issues raised by this
study. Paramount of these issues is the establishment
of a standardised method of recording the presence of
the tori, which may only be accomplished by assigning
metric categories to what have been somewhat arbitrary
size distinctions. Additionally, the correlation between
relatively good periodontal conditions and torus
development should be investigated in a broader, cross-
population context. A final direction of considerable
interest is towards a better understanding of trait
interaction, particularly between all exostoses of the face
and skeleton, such as palatine and maxillary tori.

In conclusion, it seems necessary to reconsider the
suitability of torus mandibularis for analysis of biological
distance between populations. Unless environmental
factors can be completely controlled for, population
frequencies may differ or converge without relation
to the degree of genetic relation between groups. This
study has shown the significance of dental attrition in
variance of torus mandibularis frequency, suggesting
that environmental factors should be carefully weighed
when assessing the genetic component of tori etiology.
While the entire battery of non-metric traits is beyond
the scope of this study, the findings related here suggest
that careful consideration of trait etiology is a necessary
step in choosing variables for biodistance analyses. Not
all non-metric traits can be considered a priori products
of genetic variation, as the investigation of the etiology
of torus mandibularis shows.

ACKNOWLEDGEMENTS

The author gratefully acknowledges all of the
assistance generously given by Dr. Simon Hillson and
Dr. Daniel Antoine and Dr. Robert Kruzynski of the
Department of Anthropology at the Natural History
Museum of London. Additional thanks are in order
to Dr. Louise Humphreys and to the entire staff of the
Department of Anthropology at the Natural History
Museum.

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