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Disproportionate sexual dimorphism in the human face.

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Disproportionate Sexual Dimorphism in the Human Face
W STUART HUNTER AND STANLEY M GARN
T h e Department of Pnedia trzc Dentistry. tlze University of W e s t e r n
Chitarto,London, Canada a n d Center for Htcrnan Groiutlz a n d
Deuelopment, the Uritueiszty of Michigan,
Ann Arbor, Michigan
ABSTRACT
Analysis of facial dimensions of 86 young adults and their 76
parents indicates that a disproportionate sexual dimorphism exists in the ramus
of the mandible, demonstrating a regional difference in growth response. The
male ramus is on the average 1 4 % longer than the female ramus, whereas
other facial dimensions approximate an 8% sex difference.
The findings have relevance to the analysis of skeletal remains and suggest
the desirability of age specific discriminant function analysis for the sexing of
adult mandibles.
The sexing of human skulls has been a
matter of interest for nearly 100 years as
Giles and Elliot (‘63) have described. Since
shape is obviously a n important factor in
sex discrimination, the use of various
cranial and facial dimensions for the sexing of crania has recently received much
attention as a result of the development
of discriminant function analysis. However, the general basis for discrimination
for sex lies in the fact that male skulls
are as a rule larger than female skulls.
The purposes of this study were (1) to
institute a pilot exploration of the nature
of sexual dimorphisms i n facial dimensions of young adults and their parents,
( 2 ) to employ such dimorphisms as were
found in the improvement of sex identification and (3) to study the effects of the
dimorphisms on the shape of the face and
on the vertical position of the dentition in
the face insofar as these might relate to
clinical orthodontic evaluations.
METHODS AND MATERIALS
The data consist of measurements taken
from tracings of lateral head roentogenograms (all obtained under standard conditions) of 162 subjects from 38 nuclear
families. I n these tracings, symmetry is
assumed and the mid-point between left
and right landmarks (Gonion, for example), was used. For a discussion of the
technical details see Enlow (‘68) chapter
12. The average ages for the samples used
are shown in table 1 .
AM.
J. PHYS. ANTHROP.,36: 13&138
The sample was gathered in Ann Arbor
during 1966-67 and consists, with a
few exceptions of established mid-western middle-class families of European
origin. With two exceptions, all 162 subjects could establish occlusal contact with
at least some posterior teeth of their own.
The initial study utilized six measures of
the periphery of the face and cranial base
shown in figure 1 . Evaluation of these
measures revealed disproportionate sexual
dimorphism in the ramus and posterior
cranial base. Therefore, a n additional
analysis of the components contributing
to the vertical dimensions of the posterior
region of the face was undertaken to determine whether the mandible itself and
the dentition are positioned differently
according to sex. For this purpose, the
PM vertical (Posterior Nasomaxilla) described by Enlow et al. (’69) was used as
a reference line for three additional measures made parallel to the PM vertical as
shown in figure 2. As a check on the apparently critical Articulare-Gonion measure, a n additional adjacent measure,
Condylion-Gonion (parallel to the posterior border of the ramus) was also taken.
All tracings and measures were repeated
and discrepancies i n excess of 0.5 m m
were corrected.
Following the initial comparison of differences between means and the computation of sex differences as percentages
of the female values, discriminate functions were obtained using a discriminant
133
134
W. STUART HUNTER AND STANLEY M. GARN
TABLE 1
C o m p a r a t i v e a g e s of the p a r e n t s and
t h e i r a d u l t oflspring
N
Parent
Offspring
38
48
Parent
Offspring
38
38
Mean
Male
55.5
23.7
Female
52.8
23.8
\
S.D.
Range
5.9
4.4
47-73
17-37
5.8
5.4
44-71
17-39
\
\
Neutral
Occlusal
Axis
Fig. 2 Additional
roentgenographic
measurements obtained to examine the components
of the posterior vertical dimensions of the face.
The numbered measurements are: 7. CondylionGonion, 8. Sphenoethmoid-Condylion, 9. Condylion-Neutral Occlusal axis, 10. Neutral Occlusal
Axis-Gonion. PTM is pterygomaxillary fissure.
For further description of PM vertical, SE and
NOA see text.
Findings
Fig. 1 The landmarks used for this roentgenographic study and the measures made between
them. The numbers refer to measures as follows:
1. Articulare-Gonion, 2. Gonion-Menton, 3. Nasion-Menton, 4. Sella-Nasion, 5. Sella-Articulare,
6. Articulare-Gnathion.
analysis program for two groups,' which
permits the use of two or more variables.
Although the computer analysis yields a
cut off value for discrimination, i n some
cases slight improvement could be obtained by empirically adjusting the cut
off, which was done.
Previous analysis of these data (Hunter
and Garn, '69) had disclosed significant
secular change in the face from parent to
offspring generations. Hence, for the
most part, the two generations were dealt
with separately.
As may be seen in table 2, analysis of
the six measures shown in figure 1 disclosed that for ramus length (ArticulareGonion) the sex difference expressed as a
percentage of the smaller (female) measure is in excess of 14% at both age levels.
This is slightly higher than values which
may be computed for Giles' ('64) data for
a similar measurement. For four of the
other five measures, the per cent differences range from 6.1% to 9.7% with a n
average of 8.1 % . The remaining posterior
vertical measure (Sella-Articulare) approximates a 13% difference. Although
these measures are in no way exhaustive,
they do sample the general structure of
the face and suggest a more detailed examination of the posterior vertical dirnensions of the face.
The analysis of some obvious cornponents of posterior vertical facial dimen1 Adapted by t h e University of Michigan Statistical
Research Laboratory from UCLA program BMD04M.
135
DISPROPORTIONATE SEXUAL DIMORPHISM
TABLE 2
Sex differences n n d percentnge sexztnl d i m o r p h i s m f o r S I X men n f n ci n l
d i m e n s i o n s f o r p a l e i i t s tind f o r offspring
Male
Female
Measurement
N
Parent
Offspring
38
48
59.2
58.8
38
38
51.7
51.4
7.5
7.4
14.5
14.4
Parent
Offspring
38
48
80.8
83.6
38
38
75.2
76.2
5.6
7.4
7.5
9.7
Parent
Offspring
38
48
133.7
134.2
38
38
125.1
122.8
8.6
11.4
6.9
9.3
Parent
Offspling
38
48
78.8
80.2
38
38
73.9
75.6
4.9
4.6
6.6
6.1
Parent
Offspring
38
48
37.6
39.9
38
38
33.3
35.4
4.3
4.5
12.9
12.7
Parent
Offspring
38
48
124.6
125.9
38
38
113.8
115.3
10.8
10.6
9.5
9.2
Mean
N
mm
1. Articulare-
Gonion
2. Gonion-
Menton
3. Nasion-
Menton
4. Sella-
Nasion
5. Sella-
Articulare
6. Articulare-
Gnathion
Per cent 1
sexual
dimorphism
Group
Mean
Difference
mm
1
Per cent sexual dimorphism
x m -2f
T X 100
xf
TABLE 3
S e x dgferences a n d per c e n t sexual d i m o r p h i s m f o r f o u r posterior vertical height
m e a n m e a s u r e m e n t s f o r parents a n d for offspring
Male
Female
Measurement
Group
N
Mean
N
mm
7. Condylion
to Gonion
8. Spheno-
ethmoid to
Condylion
9. Condylion
toNeutral
Occlusal
Axis
10. Neutral
Occlusal
Axis to
Gonion
1
Mean
Difference
mm
mm
Per cent 1
sexual
dimorphism
Parent
Offspring
38
48
70.8
71.3
38
38
62.0
62.9
8.8
8.4
14.2
13.4
Parent
Offspring
38
48
31.7
32.0
38
38
30.5
30.7
1.2
1.3
3.9
4.2
Parent
Offspring
38
48
44.2
43.5
38
38
40.3
38.5
3.9
5.0
9.7
13.0
Parent
Offspring
38
48
25.0
26.2
38
38
20.5
22.6
4.5
3.6
22.0
15.9
Per cent sexual dimorphism
xm-Ff
%f
x 100
sion (fig. 2) are shown in table 3 and
indicate that (1) the disproportionate dimorphism is in the ramus rather than in
its position relative to cranial base and
(2) that although the dentition of males
is positioned proportionately further from
the cranial base than in females, it is not
disproportionately so. The proportional
excess for measures eight and nine combined (Sphenoethmoid to Neutral Occlusal
Axis) is 7.2% for the parent sample and
9.1% for the adult offspring sample.
Having established the magnitude and
location of the sexual dimorphisms in
these samples, attention was then given
to the utilization of the dimorphisms in
136
W. STUART H U N T E R A N D STANLEY M. GARN
TABLE 4
Results of various discriminant analyses for sex showing t h e best three combinations
w h e n two, three, foiir,five, a i d szr variables are zised
Per cent misclassified
Variable
Variable
numbers
names
Parent
sample
38 males
38 females
Offspring
sample
48 males
38 females
Combined
sample
86 males
76 females
9.2
13.2
10.5
11.6
8.1
11.6
13.0
11.1
14.2
Na-Me,S-Na,Ar-Gn
S-Na,Ar-Gn,Co-Go
S-Na,S-Ar,Ar-Gn
7.9
9.2
6.6
11.6
10.5
10.5
12.3
10.5
15.4
Go-Me,S-Na,S-Ar,
Ar-Gn
Go-Me,Na-Me,S-Na,
CO-GO
S-Na,S-Ar,Ar-Gn,
CO-GO
6.6
8.1
15.4
7.9
7.3
9.9
6.6
8.1
11.7
Go-Me,Na-Me,S-Na,
S-Ar,Co-Go
Na-Me,S-Na,S-Ar
Ar-Gn,Co-Go
Go-Me,S-Na,S-Ar,
Ar-Gn,Co-Go.
7.9
5.8
8.6
7.9
8.1
9.3
6.6
8.1
12.3
7.9
5.8
10.5
S-Na,Ar-Gn 1
Ar-Gn,Co-Go
S-Ar,Ar-Gn
All six variables
1
See text for description of abbreviations.
the sexing of crania and mandibles, using
discriminant analysis. From the ten measures obtained i n this investigation, six
were selected for examination on the
basis of their potential utility either
for standardized roentgenographic or
for craniometric studies. Thus, NasionMenton, Gonion-Menton, and Condylion-Gonion may be obtained either
craniometric ally or roentgenographic ally.
Sella-Nasion, Articulare-Gnathion
(approximately equivalent to CondylionGnathion) and Sella-Articulare are essentially roentgenographic. All possible combinations of the above six measures were
tested and the combinations of greatest
discriminatory effectiveness when two,
three, four and five variables are used
were selected for inclusion in table 4.
When three variables are used instead of
two, the percentage misclassified as to
sex is reduced from 9.2% for the best
combination of two variables to 6.6% for
the best combination of three variables
for the parent sample. On the other hand,
the use of three variables rather than
two provides no improvement for the offspring sample. In general however, some
reduction in misclassification is accomplished by using more variables, so that
when all six are used, 7.9% of the parent
sample is misclassified and only 5.8% of
the offspring sample is misclassified.
The sex differences for the parent generation and for the offspring generation
were compared and found not statistically
different. However, pooling of the data
did not result in improved discriminatory
effectiveness as may be seen i n the last
column of table 4.
In general, the results of the discriminant analyses for a contemporary population using a radiographic procedure
compare favorably with those reported by
Giles ('64) for the mandible alone and by
Giles and Elliot ('63) and Kajanoja ('66)
for crania using direct measurements on
cadavers i n which misclassification accounts for from 13% to 20% of the
samples.
DISPROPORTIONATE SEXUAL DIMORPHISM
DISCUSSION
The amount of facial sexual dimorphism for the sample of adults studied
appears to be on the order of 8% as i t is
in fact for stature in this sample (8.3%
for parents and 8.6% for adult offspring),
except for measures associated with ramus height for which it is on the order
of 14% . Disproportionate sexual dimorphism in the adult face then, is localized
in the ramus of the mandible and seems
to be especially large in the gonial area.
The dimorphisms for two posterior face
height measures (Articulare-Gonion and
Sella-Articulare) are disproportionately
larger in both age groups than the other
perimeter measures, with percentage differences from 12.7% to 14.5%. The
breakdown of the components of the
posterior vertical dimensions (table 3) indicates that the disproportionate dimorphism lies in the ramus and not in the
positioning of the mandible relative to the
cranial base. With respect to sexual discrimination, either (or both) of the measures Articulare-Gnathion and CondylionGonion appears in all of the combinations
shown in table 4 in which the most effective discriminant combinations are summarized. The large ramal dimorphism
then proves to be both directly and indirectly useful in sex discrimination.
It should be noted that the greater sexual dimorphism for the dimension in
question exists both in the young adult
group and in the parental group. Further,
the fact that the sex differences for the
;
15.-
-
measures from Neutral Occlusal Axis to
Gonion (fig. 2, table 3) are both absolutely
and proportionately greater for the parent
sample than for the offspring sample
suggests that the localized dimorphism
increases with age disproportionately in
the gonial area. Hence, continuing mandibular bone growth during adulthood
would appear to be selective with the
gonial area the target site.
Pooling the generations results in lessening of the discriminatory power as
may be seen for the combined sample in
figure 3 in which each bar represents the
average percentage of subjects misclassified by the three best discriminant
analyses as shown in table 4. Separation
of the sample by generation reduces the
classification errors on the average by
approximately 3 % . An additional 3% reduction in misclassification is obtained
by using four or more variables. Thus,
separation of samples by age would a p
pear to be as useful as the taking of additional measures in the sexing of unknown mandibles.
It is of interest that the use of two mandibular measures, Articulare-Gnathion
and Condylion-Gonion (or Articulare-Gonion) as shown in the second line of table
4, provides a rather good level of discrimination, especially for the parent sample
of this study, thus providing a n effective
means for sexing isolated mandibles. It
should also be pointed out that quite good
levels of sexual discrimination may be
obtained for the human facial skeleton
-
+.-
Y,
-U
lrl
E
2
al
137
5
U
L
Number of variables used
Fig. 3 The reduction in percentage of subjects misclassified by a discriminant analysis
for sex when 2,3,4,5and 6 variables are used and when the sample is separated by age. For
details see table 4.
138
W. STUART HUNTER AND STANLEY M. GARN
generally, through concomitant use of tion of samples by age improves the ef(1) a measure of general facial size show- fectiveness of the discriminant function
ing the usual amount of sexual dimor- and should be undertaken as the first
phism such as Nasion-Menton or Articu- step in the sorting of commingled remains.
lare-Gnathion and (2) either of the two Subsequent analysis might well follow the
measures showing disproportionate sexual protocols outlined by Giles ('70b).
This study then, points to the extent
dimorphism - Sella-Articulare or Condylion-Gonion. Similarily , Giles ('70a) has of localized difference in the magnitude
observed that for crania alone, the mea- of sexual dimorphism; it indicates that
sure of maximum bi-zygomatic width pro- the relative magnitude of sexual dimorduces almost as good results (81.9% phism in the face continues to increase
correct) as utilization of eight measures with age; it points to the discriminatory
effectiveness of certain mandibular mea(83.3% to 86% correct).
Disproportionate sexual dimorphism in sures and finally, to the extent of improvethe face is postpubertal as pointed out by ment in sex discrimination even i n adulthood when age is taken into account.
Krogman ('62). A study by Boerman ('67)
using mixed longitudinal-cross-sectional
ACKNOWLEDGMENTS
data from 5 to 17 years of age did not
This study was supported in part by
show disproportionate sexual dimorphism
in the ramus by age 17. Since it clearly United States Public Health Service, grant
exists in the data of the present study by HD02272 and by Medical Research Counthe beginning of the third decade of life, cil of Canada, grant MA 3547. The auit appears to be distinctly a feature of thors are indebted to the families who so
adult growth, which in addition continues generously contributed their time to proto be enhanced at least into the sixth vide data for this study and to Richard L.
decade of life. This offers at least a par- Miller for data reduction.
tial explanation for Giles' ('7Oa) observaLITERATURE CITED
tion that i t is the younger males and
older females who tend to be inisclassified Birkby, W. H. 1966 A n evaluation of race and
sex identification from cranial measurements,
by discriminant analysis.
Am. J. Phys. Anthrop., 24: 21-28.
Consideration of shape differences Boerman,
I. E. 1967 Vertical facial developalone reveals that the dimorphism under
ment from ages five to seventeen. M.S. Thesis
University of Michigan.
discussion, while disproportionate for
ramus height, is not manifested in abso- Enlow, D. H. 1968 The Human Face. Hoeber,
New York.
lute or metric size differences i n that di- Enlow, D. H., R. E. Moyers. W. S. Hunter and
mension any more than it is in the manJ. A. McNamara 1969 A procedure for the
analysis of intrinsic facial form and growth,
dibular length dimension (see table 2).
Am. J. Orthodont., 56: 6 2 3 .
The use of percentages is therefore helpGiles, E. 1964 Sex determination by discrimful in clarifying relative amounts of
inant function analysis of the mandible. Am. J.
dimorphism.
Phys. Anthrop., 21: 129-136.
1970a Sexing crania by discriminant
Finally, the sexing of unidentified skelfunction analysis: effects of age and number of
etal material using the sexual dimorphism
variables. Proceedings, VIIIth International
discussed here, assumes that a reasonable
Congress of Anthropological and Ethnological
number of subjects is available for study
Sciences, Tokyo, 1 : 59-61.
1970b Discriminant function sexing of
and that the sample includes approxithe h u m a n skeleton. In: Personal Identification
mately equal numbers of males and
i n Mass Disasters. T. D. Stewart, ed. National
females. The data presented here are
Museum of Natural History. Smithsonian In.from a contemporary population. Secular
stitute, Washington. pp. 9 S 1 0 9 .
change (Hunter and Garn '69) as well as Giles, E., and 0. Elliot 1963 Sex determination
by discriminant function analysis of crania. Am.
racial differences, probably preclude the
J. Phys. Anthrop., 2 1 ; 53-68.
use of the figures presented in tables 2 Hunter,
W. S., and S. M. Garn 1969 Evidence
and 3 for the sexing of earlier and rafor a secular trend i n face size. Angle Orthocially different populations, although
dont., 39: 320-323.
Birkby ('66) concluded that sex discrim- Kajanoja, P. 1966 Sex determination of Finnish crania by discriminate function analysis.
ination in unknown populations can
Am. J. Phys. Anthrop., 24: 2 S 3 3 .
achieve levels of accuracy comparable to Krogman, W. M. 1962 The Human Skeleton in
those found here. However, the separaForensic Medicine. C. C Thomas, Springfield.
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