DEPARTMENT OF ANATOMICAL SCIENCES
AN321 - FORENSIC OSTEOLOGY
SEX DETERMINATION FROM THE SKELETON
W. B. Wood
The University of Queensland
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 long bones, especially the femur, humerus & tibia
other bones, e.g. the sternum, clavicle, calcaneum
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
- 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
- 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.
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
- 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
Boucher BJ 1957 "Sex Differences in the Foetal Pelvis." AJPA 15:581-600.
Black TK 1978 "A New Method for Assessing the Sex of Fragmentary Skeletal Remains: Femoral Shaft Circumference." AJPA 48:227-232
Brown P 1981 "Sex Determination of Aboriginal Crania from the Murray River Valley: A Reassessment of the Larnach and Freedman Technique". Archaeology in Oceania, 16(1):53-63.
Choi SC & M Trotter 1970 "A statistical study of multivariate structure and race-sex differences of American white and negro fetal skeletons". AJPA 33:307-313
Dwight T 1881 "The sternum as an index of sex and age." J Anat Physiol 15:327-330
Dwight T 1889/90 "The sternum as an index of sex height and age." J Anat 24:527-535
Fawcett E 1938 "The sexing of the human sternum." J Anat 72:633
Flander LB 1978 "Univariate and Multivariate Methods for Sexing the Sacrum." AJPA 49:103-110
Giles E & O Elliot 1963 "Sex determination by discriminant function analysis of crania." AJPA 21:53-68
Giles E 1964 "Sex determination by discriminant function analysis of the mandible." AJPA 22:129-135
Hunt EE & I Gleiser 1955 "The Estimation of Age and Sex of Preadolescent Children from Bones and Teeth." AJPA 13:479-487
Imrie JA 1958 "Assessment of Age Sex and Height from Immature Human Bones." Brit Med J No1 28-131
Iscan MY (1985) "Osteometric Analysis of Sexual Dimorphism in the Sternal End of the Rib." J Forensic Sci 30:1090-1099
Iscan MY & KAR Kennedy 1989 Reconstruction of life from the skeleton. Alan R Liss Inc
Iscan MY & SR Loth 1986 "Estimation of age and determination of sex from the sternal rib." In Forensic Osteology. KJ Reichs (Ed), C.C. Thomas, Springfield, 68-69
Jit I & S Singh 1966 "The sexing of adult clavicles." Indian J Med Res 54:551-571
Jit I, Jhingan V & M Kulkarni 1980 "Sexing the human sternum." AJPA 54:217-224
Johnson DR, O'Higgins P, Moore WJ & TJ McAndrew 1989 "Determination of race and sex of the human skull by discriminant function analysis of linear and angular dimensions." For Sci Int., 41:41-53
Johnson DR, O'Higgins P, Moore WJ & TJ McAndrew 1990 "Determination of race and sex of the human skull by Discriminant Function Analysis of Linear and Angular Dimensions - An Appendix." For Sci Int., 45:1-3
Kimura K 1982 "A base-wing index for sexing the sacrum." J Anthropol Soc Nippon 90(Suppl):153-162
Kiszeley I 1974 "On the possibilities and Methods of the Chemical Determination of Sex from Bones." Ossa 1:51-62
Krogman WM & MY Iscan 1986 The Human Skeleton in Forensic Medicine. C.C. Thomas, Springfield
Mannucci A, KM Sullivan et al 1994 Forensic application of a rapid and quantitative DNA sex test by amplification of the X-Y homologous gene amelogenin. International J of Legal Medicine 106:190-193
McCormick WF, Stewart JH, and LA Langford 1985 "Sex Determination from Chest Plate Roentgenograms." AJPA 68:173-195
Schulter-Ellis FP & LC Hayek 1984 "Predicting race and sex with an acetabulum-pubis index". Collegium Antropol 8;155-162
St. Hoyme LE 1982 "A simple statistical method for estimating sex distribution and dimensions in dissociated long bone series". Ossa 7:119-127
St. Hoyme LE 1984 "Sex differences in the posterior pelvis." Collegium Anthopol 8:139-154
St.Hoyme LE & MY Iscan 1989 "Determination of sex and race: accuracy and assumptions." In Reconstruction of life from the skeleton. MY Iscan & KAR Kennedy (Eds). Alan R Liss Inc.
St. Hoyme (1982, 1984)
Stewart TD 1948 "Medico-legal Aspects of the Skeleton. I. Sex, Age, Race and Stature." Am J Phys Anthropol 6: 315-322
Stewart TD & WF McCormick 1983 "The Gender Predictive Value of Sternal Length." Amer. J. For. Med. Path. 4:217-220
Stradalova V 1975 "Sex Differences and Sex Determination on the Sacrum." Anthropologie 13:237-244
Suchey JM, Brooks ST & D Katz 1988 "Instructional materials accompanying female pubic symphyseal models of the Suchey-Brooks system." Distributed by FRANCE CASTING (Diane France, 2190 West Drake Road, Suite 259, Fort Collins, Colorado 80526
Thomson A 1898 "The Sexual Differences of the Foetal Pelvis." J Anat & Physiol 33 (3):359-380
Washburn SL 1948 "Sex differences in the pubic bone." AJPA 6:199-207
Weaver DS 1980 "Sex Differences in the Ilia of a known Sex and Age Sample of Fetal and Infant Skeletons." AJPA 52:191-195