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Significance of the Classical Morphological Criteria for Identifying Gender Using Recent Skulls, by Graw (Forensic Science Communications, January 2001)

Significance of the Classical Morphological Criteria for Identifying Gender Using Recent Skulls, by Graw (Forensic Science Communications, January 2001)

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January 2001 - Volume 3 - Number 1

Research and Technology

Significance of the Classical Morphological Criteria for
Identifying Gender Using Recent Skulls

Paper presented at the 9th Biennial Meeting of the International Association
for Craniofacial Identification, FBI, Washington, DC, July 24, 2000

Matthias Graw
Doctor of Medicine
Institut fhr Gerichtliche Medizin
Thbingen, Germany

Abstract | Introduction |Material and Metho ds
Results
| Discussion | References


Abstract

The diagnosis of gender using the skull is based mainly on morphological characteristics. The differential expression of these characteristics is described in the relevant literature almost identically. These descriptions date back to the eighteenth century and arise from observations of a small series of investigations.

To clarify the significance that can be attributed to the 17 primarily morphological characteristics used in gender differentiation from a current point of view, a collection of 137 forensically recent adult skulls from southwestern Germany was investigated. A test group (n = 91; 58 males, 33 females) and a control group (n = 46; 34 males, 12 females) were chosen by random sampling.

The examination of the morphological characteristics of the test group revealed that, in principle, gender could be differentiated from such characteristics. The variability of the individual morphological characteristics, however, caused them to be of varying degrees of importance for the diagnosis. Only five traits could be attributed with a probability of 70–80 percent and were regarded as sufficiently reliable: the glabella, arcus superciliaris, processus mastoideus, crista supramastoidea, and mandibula (overall impression). By combining these five traits in the analysis, the gender of approximately 91 percent of the skulls could be correctly determined.


Introduction

In forensic science and anthropology, the skull is more frequently and thoroughly investigated than any other section of the human skeleton. However, opinions are divided on how to differentiate between male and female skulls in the most reliable way. According to Prokop and Göhler (1976), it is more difficult to determine the gender of an individual if the skull alone is available. Holland (1986) and Bass (1987), however, regard the skull after the pelvis as the best section of the skeleton for reliable gender differentiation.


Material and Methods

To evaluate the validity and reliability of determining an individual's gender, the relevant morphological features of the skull were assessed on the basis of criteria listed by Acsádi and Nemeskéri (1970) and Ferembach et al. (1979) using 137 recent adult forensic skulls from southwestern Germany (92 G, 44.6 ± 4.7 years; 45 E, 47.7 ± 19.2 years).

The individuals were divided into a test group (n = 91; 58 GG, 43.9 ± 13.0 years; 33 EE, 46.5 ± 20.4 years) and a control group (n = 46; 34 GG, 45.7 ± 17.3 years; 12 EE, 50.9 ± 16.1 years) by random sampling. Statistical analysis was carried out using the SPSS software (SPSS Incorporated, Chicago, Illinois).

The following features and their appearance were assessed:

  • Angulus mandibulae: Visual and palpable assessment of the surface of the lower jaw angles: smooth (1), very slight ridges (2), small ridges (3), prominent ridges (4), and very prominent ridges (5). In the case of (4) and (5), the angulus can appear to be turned outwards. 
  • Arcus superciliaris: Visual and palpable assessment of prominence: very weak (1), slightly marked (2), average prominence (3), rather prominent (4), and very prominent (5). In the case of (4) and (5), a crestlike prominence is observed. In the case of (1) and (2), the weak contour is round. See Figure 1.

  • Crista supramastoidea: Visual and palpable assessment: very slightly raised (1), weakly raised (2), moderately raised (3), very pronounced (4), and extremely pronounced (5). See Figure 2.

  • Forma orbitae: The contour of the orbital cavities was assessed from the front perspective: round (1), fairly round (2), moderate (3), more or less rectangular (4), and rectangular (5).

  • Glabella: Inspected from the frontal and lateral perspective, the course of the curve: missing or only faintly developed (1), just visible (2), moderately shaped (3), pronounced (4), and extremely pronounced (5). See Figure 3.

  • Inclinatio frontale: The inclination of the forehead up to its transition into the parietal bone, in particular the lower forehead up to the height of the tubera frontalia, viewed from the front: concave or vertical (1), nearly vertical (2), slightly inclined (3), slightly receding (4), and strongly receding (5).

  • Mandibula: The overall visual assessment combines the appearance of corpus and rami as well as size and strength: very delicate (1), fairly delicate (2), moderate (3), strong (4), strong, and massive appearance (5). See Figure 4.

  • Margo supraorbitalis: The contour is palpated with the fingertip from the inside to the outside: very thin with sharp edges (1), fairly thin (2), moderate or indifferent (3), thicker, more rounded edges (4), and clearly rounded edges, thick, occasionally concave (5).

  • Mentum: Viewed from the cranium and the front: round, small, with a conical tip (1), small (2), moderate (3), strong (4), and strong with prominent mental tubercles (5).

  • Planum nuchale: Visual and palpable assessment of the surface structure of the basal occipital bone and accentuation of the lineae nuchae: virtually smooth (1), hardly developed (2), moderate (3), prominent (4), and very prominent (5).

  • Processus mastoideus: Lateral (length and surface) and basal aspects (basis, surface structure): very small, delicate, narrow, smooth (1), small (2), moderate (3), large (4), and very large, massive, broad, rough (5). See Figure 5.

  • Processus zygomaticus ossis temporalis: Palpable and visual inspection: very frail, low, thin (1), frail (2), moderate (3), high, strong (4), and very high, massive (5). The processus zygomaticus of the os frontale is occasionally included in anthropological assessments, but this was not the case in the present evaluation.

  • Protuberantia occipitalis externa: Palpable and visual inspection: hardly detectable (1), faint appearance (2), moderate (3), strong (4), and with a projecting cone (5).

  • Tubera frontalia: Palpable and visual assessment: prominent (1), less prominent (2), moderately developed (3), hardly developed (4), and not present at all (5). 

Photo of male arcus superciliaris Photo of female arcus superciliaris.


Figure 1. Male (left) and female (right) arcus superciliaris.


Photo of male crista supramastoidea.

Photo of female crista supramastoidea.


Figure 2. Male (top) and female (bottom) crista supramastoidea.


grawfig1as.jpg
 grawfig1bs.jpg


Figure 3. Male (left) and female (right) glabella stage 6 (G) or 2 (E), according to Broca (1875).


Photo of frontal view of skeletal mandibles (overall impression: female, left; male, right).


Figure 4. Mandibles (overall impression, E left, G right), frontal view.


grawfig3a.jpg

grawfig3b.jpg


Figure 5. Male (top) and female (bottom) processus mastoideus.
  • Tubera parietalia: The tubercles of the parietal bone were assessed according to the criteria used also for the frontal tubercles.

  • Os zygomaticum, height: The height of the cheekbone in relation to its width and the entire face: very small (1), small (2), medium (3), large (4), and very large (5).

  • Os zygomaticum, surface: Palpable differentiation: very smooth (1), fairly smooth (2), moderately smooth (3), uneven profile (4), and very profiled (5). 

Results

The findings for the test group are summarized in Table 1. According to these findings, the appearance of the investigated features can be applied to the differentiation of gender. The error rate and, thus, the extent to which correct determination of gender is possible vary considerably. Only those features in Table 1 that are printed in bold guarantee a reliable number of correct diagnoses (> 70 percent) and an error rate of less than 10 percent. These features include the arcus superciliaris, crista supramastoidea, glabella, and the overall impression of mandibles and processus mastoideus. See Figure 5.

Some features are clearly unidirectional (have a distinctly female or male appearance). These include: a less inclined "steep" forehead is observed primarily in females (present in 45.5 percent EE and 5.1 percent GG); a more inclined forehead is an unreliable male feature (G: 27.3 percent; E: 72.4 percent); the tubera frontalia (E: 63.6 percent; G: 15.5 percent) and parietalia (E: 30.6 percent; G: 6.9 percent) are more pronounced in females; a missing tubera frontalia (E: 21.2 percent; G: 67.3 percent), and the parietalia (E: 36.4 percent; G: 84.5 percent) cannot be regarded as a typical male feature. A conelike protrusion of the protuberantia occipitalis externa is, however, only found in male skulls of the test group (E: 0 percent; G: 22.4 percent). A slight protrusion can be observed on both male and female skulls. (G = 48.5 percent; E = 10.3 percent).

The assessment of the correlation between morphognostic features and age (# 49 vs. $ 50 years) led to few notable differences between males and females. The overall impression of the mandibles correlates with age, but it is more prominent in females. With increasing age, and thus with a reduced number of teeth, the lower jaw appears more frail and atrophic. The shape of the orbital cavity is less round in older women (p = 0.045), and this tendency towards virilization of older female skulls can also be observed in the margo supraorbitalis (p = 0.056) and in the reduced size of the tubera parietalia (p = 0.069). The threshold of significance is not reached, however.

The five characteristics necessary for reliable gender identification were used to assess 46 skulls of a control group. The gender of 42 (91.3 percent) skulls was correctly determined. The gender of one male and one female skull was incorrectly classified. The results were inconclusive on two male skulls.


Discussion

The determination of gender using the skull is based on morphological features. In the relevant publications, the gender-specific appearance is described almost identically. These descriptions date back to the eighteenth century and arise from observations of a small series of investigations. Broca (1875), for example, examined only 7 EE and 11 GG skulls. Individual descriptions were collected by textbook authors (Stewart 1952; Krogman 1962; Prokop and Göhler 1976; Hunger and Leopold 1978; Bass 1987; Hoyme and Iscan 1989; Herrmann et al. 1990). The morphological descriptions put forward by these authors have remained basically unchanged over the years. No fundamental new findings or features confirmed by more recent investigations have since been communicated. In their investigation on the diagnosis of age and gender using the skull, Ferembach and colleagues (1979) described 17 morphognostic features and oriented themselves according to reports published previously by Acs<di and NemeskJri (1970).

The assessment of typognostic features naturally underlies a certain subjective influence. The experience of each investigator with his "internal standard" always leaves its mark, and in addition, the individual features are usually not investigated independently of each other. The skull is assessed in its entirety, subconsciously at least. In cases of doubt, the classification of a single, gender-specific feature can thus be decisively influenced one way or the other. The published data must, therefore, first be interpreted against this background (the significance of the individual features for a certain investigator and the respective sample).

Taking these facts into consideration, the features are molded in a gender-specific way. The appearance of certain features differs significantly between female and male skulls. Because of their variability, as demonstrated by the number of false determinations and indifference types, gender-specific features can occasionally lead to different values. The relatively high number (33 percent) of skulls that were either incorrectly determined or which could not be specified according to gender is unsatisfactory. Therefore, the value of features such as inclinatio frontalis, tubera frontalia, and parietalia, processus zygomaticus, height of the os zygomaticum, planum nuchale, upper ridge and shape of the orbitae, and angulus mandibulae for the reliable determination of gender must be critically assessed.

In view of the ordinal scaling which consists of five values, a characteristic feature can be guaranteed to determine gender if there is a median difference between males and females of more than one, or even two, combined with a relatively small standard deviation and error rate. These conditions are best met by glabella, arcus superciliaris, processus mastoideus, crista supramastoidea, and the overall impression of the mandibles.

In particular, the marks of the muscle insertions at the back of the head (planum nuchae) and the angles of the lower jaw and the os zygomaticum revealed only minor differences in the median values. The high value that is usually attributed to these features cannot be reconfirmed by the findings of the present investigation. It is possible that a tendency towards finer structures in the course of acceleration (Knussmann 1980) is obvious. This can be observed when populations of the migration age are compared (Wahl 1988). In contrast to this are the results gained from mastoid and crista supramastoidea, which should still be regarded as essential features in a reliable determination of gender.

The shape of the orbital cavity, which is distinctly different in males and females according to the computer-assisted two-dimensional evaluation (Schleyer et al. 1971; Schiwy-Bochat 1996), proved to be less significant when inspected visually. Although an investor-associated shift of the threshold value can be questioned in view of a median value within the "male range," the difference in the median value is still too small to lead to a satisfactory diagnosis. Apparently, the visual assessment is inferior to the two-dimensional computer-assisted analysis because of the three-dimensional orbital contour.

A "sharpening" of the margo supraorbitalis was defined by Broca (1875) as a female characteristic and has been used as such since then. Palpable findings and thus the strength (thickness) of this ridge were relied upon. Therefore, the value of the margo supraorbitalis for the diagnosis of gender is only minor. If one relies only on the presence or absence of a ridge on the upper, inner edge of the orbital cavity, this feature can be reliably assessed. Presence or absence correlates with gender and is not side-dependent. The probability of correctly determining gender rises to more than 70 percent and, therefore, lies in the order of magnitude of the features mentioned in this investigation (Graw et al. 1997; 1999)

The discrepancy between these data and data listed in publications by Krogman (1962), Hunger and Leopold (1978), Novotny et al. (1993), and Ferembach et al. (1979), in particular as regards the value of some features, can be attributed primarily to three reasons:

1. The tables represent summaries of international publications. They are not derived from personal investigations or a specified series of specimens. Shifts in population-dependent validations are generally acknowledged but were neither confirmed nor described.

2. The individual findings were derived from inhomogeneous material of different regional origin and age. Some publications date back to the beginnings of systematic anthropology (Ackermann 1788; Welcker 1866; Ecker 1866; Broca 1875; Rebentisch 1893). In contrast, the German skulls used in this investigation are modern in the forensic sense. Influences due to acceleration are taken into account when the respective findings are compared.

3. Short adjectives such as "small" and "thin" are used to describe certain appearances, but these can cannot be claimed as objective and are based to a great extent on the experience of the investigating researcher. Because of this subjective influence, a shift in the validity is possible.

When the individual features (glabella, arcus superciliaris, processus mastoideus, crista supramastoidea, overall impression of the mandibles) are weighted identically, 42 of the 46 skulls of the control group can be determined correctly (91.3 percent). The gender of one male and one female skull was incorrectly determined, and two more skulls could not be assessed (inconclusive results). Compared to the diagnostic reliability of 77 percent (Stewart 1948) and 92 percent (Krogman 1962), respectively, the results are definitely positive and indicate that it is possible to restrict the reliable determination of gender to these five characteristics in forensic anthropological practice.


References

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