DEPARTMENT OF ANATOMICAL
SCIENCES
AN321 - FORENSIC OSTEOLOGY
SEX DETERMINATION FROM THE
SKELETON
W. B. Wood
Senior Lecturer
The University of Queensland
INTRODUCTION
Sexing of human skeletal remains
is a vital part of forensic identification. Once sex is determined the
possibility of correct identification is immediately increased by fifty
percent.
Sex distinguishing
characteristics of the skeleton are dependent on the existence of sexual
dimorphism which results from the adaptations of the female to the function of
childbearing, the influence of sex hormones and cultural differences between
the sexes.
Although present at all ages,
sexual dimorphism becomes most marked after puberty when the secondary sexual
characteristics have fully developed. Hence sex can be most readily and most
reliably ascertained in skeletons from mature individuals.
Senility may influence sexual
characteristics of a skeleton and therefore alter the reliability of sex
determination in the aged.
An expert is aware of the range
of variation of sexual traits within the skeleton and the degree of overlap
that normally exists between males and females. The determination of sex is
rarely based on any one skeletal feature alone. Wherever possible as many
criteria as are available are assessed before coming to a definitive
conclusion.
The degree of development of
sexual characteristics within the skeleton, may vary greatly from one racial
group to another. (Stewart 1948, Johnston et al 1989). Hence there is a need
for awareness of and familiarity with this inter-racial variation in sexual
traits. Because of this fact, racial affiliations of skeletal material should
be ascertained before the appropriate sexing and aging criteria are applied.
Visual (morphological)
examination remains the quickest and easiest method of determining sex in the
great majority of unknown skeletal remains and in experienced hands will result
in 95%-100% accuracy when the whole skeleton is available for assessment.
Quantitative metrical methods
where available should be used to back up the morphological assessment. These
metrical methods use sectioning points( female< >male), or discriminant
functions. Their reliability varies depending on the bones assessed but usually
lies somewhere between 70-95%.
The development of discriminant
functions based on skeletal measurements has helped greatly in removing the
subjective basis from sex determination. Their use tends to back up the
morphological assessments made by the expert osteologist and make reliable sex
assessment available to the less experienced. Such functions that are available
should be restricted in their application to those population groups from which
the functions were derived.
The possibility of
distinguishing male and female skeletal material by chemically determining the
citrate levels of the bones has been reported in the literature (Kiszeley 1974,
Dennison 1979). This method would have great potential in dealing with
fragmented or juvenile skeletal material, but has yet to be proven as a useful
and reliable tool for sexing bones.
The skeletal elements that are
of most use in sex determination are listed below in descending order of
reliability:
the
pelvis - hipbones and sacrum
the
cranium
the
long bones, especially the femur, humerus & tibia
other
bones, e.g. the sternum, clavicle, calcaneum
the
teeth
The accuracy of sex
determination in adults is stated by Krogman & Iscan (1986, p259) to be as
follows
entire
skeleton 100%
pelvis
alone 95%
skull
alone 90%
skull
+ pelvis 98%
long
bones 80-90%
long
bones + skull 90-95%
long
bones + pelvis >95%
In immature (prepubertal)
skeletons the chances of a correct sex allocation is 50/50 unless the pelvis is
present which improves the chances of correct sex allocation to 75-80%.
SEX DETERMINATION FROM THE ADULT PELVIS
The pelvic bones (hip bone and
sacrum) provide the best and most reliable means of sex determination in
adults. Both morphological and metrical characteristics are used in this
assessment and include:
Morphological Sexing Features of
the Hip Bone:
-
The shape of the greater sciatic notch: J shape in the male; L shape in the
female
-
The presence of a well-marked preauricular sulcus in the female; absent or slight
in the
male
-
The elevation of the auricular surface above the adjacent postauricular area in
the female; coplanar in the male
-
The width and shape of the body of the pubic bone: wide and rectangular in the
female;
narrow and triangular in the male
-
The presence of a ventral pubic arc in the female; absent in the male
-
The presence of a ventral pubic concavity (lateral recurve) in the female; the
ventral pubic
ramus is straight or convex in the male
-
The thickness of the ventral pubic ramus: thin in the female; thick in the male
-
The presence of "parturition" pitting on the dorsum of the pubis of
many females; absent
in males
-
The estimated size of the subpubic angle: wide in the female narrow in the male
Sex differences in the adult
pelvis are largely unrelated to race.
Suchey, Brooks & Katz (1988)
have developed a reference collection of casts of the pubic bone of both males
and females which assist greatly with correct sex assignment.
Metrical Sexing Features of the
Hip Bone
Metrical features of the hip
bone also assist with sex assessment especially the ischiopubic index. This is
the length (in mm) of the pubis divided by the length (mm) of the ischium
multiplied by 100. It was demonstrated by Washburn (1948) as correctly allocating
sex in over 90% of 300 cases. Because some difficulty may be experienced in
locating the exact measuring point within the acetabulum other variations on
this index have been developed St. Hoyme (1982, 1984), Schulter-Ellis &
Hayek (1984). All seem to work equally effectively.
The Sacrum:
I have not found the sacrum by
itself to be extremely reliable in sex determination. It is commonly variable
in the number of its sacral segments; sacralisation of L5 is not infrequent;
& arthritis may distort its auricular surfaces. All these factors can
influence correct assessment.
Morphological characteristics often quoted as assisting with
visual sex determination from the sacrum are:
-
The relative width of the ala of the sacrum compared with the width of the
first sacral
body (1:1 in the female; 1:2 in the male)
-
The shape of the ventral (pelvic) concavity: flattened anterior contour S1-3 in
the female;
regular curve anteriorly from S1-S5 in the
male
-
The extent of the auricular surface relative to S2 and S3 lateral masses:
limited to S2
lateral mass in the female; extends onto S3
lateral mass in the male
Discriminant functions for sex
determination in American whites, blacks and Japanese have been developed by
Kimura (1982b) utilising the transverse width of the base and the transverse
width of the wing (ala). The functions resulted in correct sexing in 75%
Japanese, 80% whites and 83% blacks.
SEX DETERMINATION FROM THE CRANIUM AND MANDIBLE
The crania and mandible may be
assessed by morphological (subjective) or metrical (objective) methods
It is only after puberty that
the sexual dimorphism of the human cranium and mandible becomes readily
apparent. This dimorphism varies greatly from one population group to another
eg the morphological traits of an aboriginal female cranium may approach or
exceed the same traits of the European male.
There is therefore the need for
the use of race specific standards of development of sexual dimorphic features,
and to apply those standards only after the race of the skeleton has been
determined. Unfortunately as far as I am aware standards of this type (in the
form of comparative casts) are available only for the assessment of Australian
Aboriginal skeletons (Larnach & Freedman 1964) and have not yet been
developed for use on any other population group. They certainly would assist
with making subjective sex assessment of the cranium and mandible more reliable
especially for the inexperienced.
Morphological Sexing Features of
the Male and Female Cranium and Mandible.
In experienced hands this can
result in 85-90% accuracy
Compared with the male, the
adult female cranium is usually:
-
smaller, more rounded, smoother and juvenile looking the forehead is more
vertical
-
the brow ridges are not as pronounced
-
the superior and lateral orbital margins are sharper
-
the mastoid processes are smaller
-
the muscle markings of the occipit, temporal region and on the zygomatic bone
are not as
pronounced
-
the palate and teeth tend to be smaller
-
the chin tends to be pointed or rounded rather than square
Metrical Sexing Method of the
Male and Female Cranium and Mandible
This method utilises metrical
differences in the male and female cranium and mandible in various combinations
for the development of discriminant functions (Giles & Elliot 1963).
The functions should be applied
only to crania originating from the population on which the functions were
derived.
Brown (1981) has adapted the
Giles and Elliot method to the sexing of Australian Aboriginal crania.
The method is reported to have a
reliability of 80-90%.
SEX DETERMINATION FROM THE LIMB LONG BONES
Morphological Sexing Features:
In general the limb bones of
females are more gracile and less well marked by muscle attachments than those
of males. The articular ends of the bones are smaller and the shafts less
robust.
Metrical Sexing Features
The above observations are
reflected in the metrical features of the bones especially in the lesser
diameter of the articular ends of the bones and the lesser circumference of the
long bone shafts.
Using the diameters of the
femoral and humeral heads, sex may be determined with a claimed accuracy of
70-85%.
SEX DIFFERENCES IN THE SCAPULA AND CLAVICLE
Not extensively studied for the
scapula because of its poor preservation rate. Refer to Jit & Singh 1966.
SEX DIFFERENCES IN THE CALCANEUS, TALUS & PATELLA
Linear dimensions and volumes of
the calcaneus, talus and patella also are sex dependent and may be used in sex
assessment.
SEX DIFFERENCES IN THE ATLAS
Clavelin & Derobert (1946)
claim that the width of the atlas vertebra reveals sex differences with males
rangeing from 74-90mm with a mean of 83mm while females range from 65-76mm with
a mean of 72mm.
SEX DIFFERENCES IN THE STERNUM
Sex differences in the sternum
are claimed by Dwight 1881, 1890, Fawcett 1938, Jit et al. 1980 but St.Hoyme
& Iscan in Iscan & Kennedy (Ed) 1989 claim that such differences have
not yet been convincingly demonstrated.
SEX DETERMINATION FROM THE RIBS AND COSTAL CARTILAGES
The patterns of calcification
and ossification of the costal cartilages in ageing adults has been reported to
be recognisably different in the two sexes such that radiology of the anterior
chest wall will reliably sex 97-99% of unknown skeletons. (McCormick et al
1985). Iscan (1985) and Iscan & Loth (1986) have reported metrical
differences in the dimensions of the rib shaft which may be useful in
distinguishing between the sexes.
SEX DETERMINATION IN FOETAL AND JUVENILE BONES
Lack of studies (and
availability) of large foetal & juvenile skeletal samples of known age sex
and race make confident sexing of the isolated foetal or juvenile skeleton
difficult and of questionable reliability. A number of studies of limited sample
size have been reported (Thomson 1989, Boucher 1957, Hunt & Gleiser 1955,
Imrie 1958, Weaver 1980).
A discriminant function method
reported by Choi & Trotter (1970) claims an accuracy of 72%. The method
requires an almost complete skeleton.
SEX DETERMINATION USING MOLECULAR GENETIC TECHNIQUES
With the recent development of
modern molecular biology, it has now become possible to accurately sex bones
whose sex is traditionally problematic (eg in children) or where bone fragments of otherwise unrecognizable origin
require identification. (Mannucci et al. 1994)
The most reliable methods make
use of X-specific DNA segments, and the most common of these techniques
involves the analysis of a segment of the amelogenin gene. Using the polymerase
chain reaction (PCR), a portion of the amelogenin gene - which has counterparts
in both the X and Y chromosomes - is amplified in a single reaction. A small
deletion in the X chromosome makes it possible to distinguish the X and Y
chromosomes from one another. The results of the test are unambiguous and easy
to interpret.
The test has a high degree of
sensitivity, it can be applied to even the most highly degraded tissues, and
only a very small amount of tissue is required
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