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Method Background and Overview

Overview
Craniofacial identification methods are used to assist the identification of skeletal remains via the analysis of skulls and faces. This may involve the overlay of a face on a skull in an attempt to determine if a match exists between the two (craniofacial superimposition), or the prediction of a face from the skull (facial approximation; see Fig. 1). Both methods are underpinned by knowledge of human anatomy, despite their use of different technical protocols. Craniofacial superimposition and facial approximation are subsumed within the discipline of physical anthropology (see Krogman and Iscan, 1986; Montagu, 1941; Reichs, 1998; Stewart, 1979). Artists have commonly been called upon to assist with sculpting or drawing tasks when undertaking the latter method.

Although they contribute to the identification process, craniofacial superimposition and facial approximation are not generally used to provide identifications in a stand-alone manner. Instead, they are used to help generate information that can be investigated using higher powered methods. Consequently, the term "craniofacial identification" does not provide a literal description of the methods but rather a convenient short-hand summary of their basis and their realm of application. This is similar to use of "skeletal identification" to describe basic forensic anthropology techniques (e.g., Kerley, 1978), including biological profile assessments, since these methods rarely hold the potency to produce identifications when open and/or large samples of individuals are considered.

Superimposition offers the most reliable information (particularly in closed samples where the skeletal remains are a priori known to belong to one of only a few individuals), but in many cases superimposition methods are used as exclusion tools. Facial approximation methods are, at present, much poorer indicators of a person's identity and their results are suggestive at best. Presently, both methods largely depend on hard-to-soft tissue relationships which are more accurately described as propositions rather than comprehensively researched anatomical rules (see below). Despite this, superimposition methods hold greater credibility than facial approximation methods because real faces are used in the process and these faces direct and constrain the results of the analysis. In contrast, known faces cannot be used in facial approximation and so provide no constraints over the methods. Despite a relatively long history (>100 years), both methods possess numerous variations and a lack of consensus about which variation is best indicates that craniofacial identification techniques are still in want of scientific validation.

Both facial approximation and superimposition methods developed from early attempts to verify the identities of skeletons thought to represent well-known historical figures (examples include Bach & Schiller (see Welcker, 1883; Welcker, 1888)). Such analyses were originally achieved by comparing skulls or constructed faces to artistic portraits rather than images of living people. As a consequence, results of these studies cannot be considered to be precise. Nowadays, methods have progressed to comparisons based on photographic representations of the living target individual (person to whom the skull belongs), thus making assessments more reliable.

Craniofacial Superimposition
Superimposition refers to the process of overlaying upon one image, a semi-transparent version of another image. This method can be used with regards to skulls and faces or it can be used in relation to other infracranial body regions and may only involve the comparison of skeletal elements (see e.g., Adams and Maves, 2002). In the former instance, however, it is known as craniofacial superimposition (Taylor and Brown, 1998).

Craniofacial superimpositions were initially conducted using tracings of skulls and faces made from photographs (see e.g., Fig. 2). The first forensic application of the method was in this context (see Glaister and Brash, 1937). In more recent times, methods have moved solely to the comparison of photographs (Sen, 1962; Webster, 1955) and then to video images (Brown et al., 1978; Helmer and Gruner, 1977; Snow, 1976). Video-superimposition is now the most frequently employed method, and involves mixing a video image of a skull with a photograph of the person in question (usually also captured as a video image) to determine the degree of anatomical correspondence (see Fig. 1).

By varying the opacity levels of one video image and superimposing it on the other using video effects (e.g., wipes and fades) the degree of match between the skull and face can be assessed. Clearly, for a match to be entertained as authentic the soft tissue profiles of the face must encompass the entirety of the skull (Taylor and Brown, 1998), and major features must lie at the appropriate corresponding regions (lips over teeth, eyes within orbits, nose over nasal aperture etc.). Soft tissue depths may be used to evaluate the fit of the face with a skull (Helmer, 1984), as well as other more specific relationships, including: the position of the temporal line of the skull with that of the soft tissue, the position of the endo- and ecto-canthions with their relative ligamentous attachments to the bone, the position of the anterior nasal spine relative to the medial crus of the nose, and the shape of the chin in relation to the underlying mandible (for more details see: Austin-Smith and Maples, 1994).

In superimposition, it is extremely important that the orientation of the skull matches, as close as possible, the orientation of the face in the photograph (Taylor and Brown, 1998). The photography conditions between the two image sets (i.e., original photographs and video of the skull) should also be as similar as possible. A failure to closely approximate any of these conditions (including lighting, distance from the camera and focal length of the lens) will result in inconsistencies between the skull and the face even if the two are, in fact, a correct anatomical match (Taylor and Brown, 1998). Negative images of the skull with positive images of the face have also been used to enhance the visualization of the superimposition and facilitate skull-face comparison (Glaister and Brash, 1937). 

Presently, it is not possible to draw firm statements concerning the overarching performance of superimposition methods because formal published studies on the accuracy and reliability of the methods have been infrequent, have used small samples, and have often not been replicated. This is in part due to the time intensive nature of such studies. So far, research indicates that superimposition is useful as an exclusion tool (i.e., determining to whom the skeletal remains do not belong; see Ubelaker, 2002) and that errors are reduced with the use of two or more views (i.e., frontal and profile). In perhaps the most comprehensive study so far conducted, Austin-Smith and Maples (1994) found that, when comparing three skulls to frontal and profile views of 97 persons, false positive or incorrect identification rates were 0.6%. Only one false positive superimposition result has been reported in the scientific literature in relation to forensic casework (see Dorion, 1983), however, this does not guarantee the lack of any other occurrences due to possible publishing bias.

Facial Approximation
As discussed above, facial approximation methods are regarded to be less reliable than craniofacial superimposition techniques since they deal with face prediction.  Although these methods first developed in Europe (see Eggeling, 1913; His, 1895; Kollmann and Buchly, 1898; Merkel, 1900), the first written account of facial approximation methods employed for forensic casework comes from the United States (see details provided by Wilder and Wenworth, 1918).

From their initial conception, facial approximation methods have been controversial (Eggeling, 1913; Gerasimov, 1968; Montagu, 1947; Suk, 1935). They have also heavily relied on mean soft tissue depth data of the face (see His, 1895; Kollmann and Buchly, 1898; Gerasimov, 1955; Krogman and Iscan, 1986; Prag and Neave, 1997; Wilkinson, 2004 and Fig. 3), with all major facial approximation methods depending on soft tissue depths and face anatomy to some degree (Stephan, 2006; see also Gerasimov, 1955; Ullrich, 1958). The frequently propounded view that some methods rely on the face anatomy, but others on the soft tissue depths alone, is inaccurate. Gerasimov (who is often cited in favor of the anatomical methods) used mean soft tissue depth values for his approximations (see figures provided in Gerasimov, 1955; Ullrich, 1958) and valued them so highly that he undertook his own needle puncture studies on fresh cadavers to measure them for his own work (Gerasimov, 1955).

There is wide agreement that the ultimate aim of facial approximation is to construct a face that can be purposefully and readily recognized as the person to whom the skull belonged. However, attainment of casework "success" should not be confused with the purposeful realization of this goal. Advertisements of facial approximations are typically accompanied by case descriptions in addition to the estimated face, which are likely to influence performance. Consequently, the success of the method in forensic contexts cannot be used as a measure of the ability of facial approximation methods to generate recognizable faces. Attainment of the method aims can only be verified under controlled conditions where confounding variables can be eliminated or minimized and non-targets must be used in the assessments (see Stephan, 2002a; Stephan and Henneberg, 2001). The demonstrated usefulness of facial approximation in forensic casework unquestionably illustrates the value of the method to act as a vehicle for recognition, but purposeful and reliable recognition of facial approximations in the absence of other influencing factors has yet to be demonstrated (Haglund and Reay, 1991; Stephan, 2003). It is also important to note here that for facial approximations to be successful in forensic casework potentially only one person needs to think they recognize the face. Thus, an argument exists that facial approximation methods need not be accurate or reliable (especially since identification universes of individuals with similar biological profiles who are also missing are often small). Of course, if facial approximation methods achieved purposeful recognition their value would be dramatically increased.

Often, the poor method performance observed in many studies is explained away as a result of limited practitioner experience. While this might be a contributing factor in some cases, the inaccuracies associated with the soft tissue prediction guidelines that are used, are more fundamental and hold much greater concern. Indeed, recent studies have confirmed that many soft tissue prediction guidelines used in facial approximation (and superimposition) perform poorly.  For example, traditional methods of setting the position of the eyes have been found to contain large cumulative errors (Stephan et al. 2009; Stephan and Davidson, 2008; Stephan, 2002b; Wilkinson and Mautner, 2003; see Fig. 4). Similar findings go for prediction of the ear (Farkas et al., 2000), nose (Hoffman et al. 1991; Rynn and Wilkinson, 2006; Stephan et al., 2003) and mouth (Stephan, 2003b; Stephan and Murphy, 2008). Moreover, many untested subjective guidelines have been used in facial approximation and craniofacial superimposition despite the fact that quantified and well studied relationships exist in other disciplines and these latter guidelines often invalidate the former ones (Stephan and Devine, 2009; Stephan, 2010).Taken together, these findings undermine claims that overarching facial approximation methods are accurate, scientific, and based on solid anatomical foundations (see e.g., Wilkinson, 2010).

In light of the errors associated with recent methods, it is not surprising that facial approximations constructed using these methods are rarely recognized, or recognized at low rates (for accuracy studies see Snow et al., 1970; Stephan and Arthur, 2006; Stephan and Henneberg, 2001; Stephan and Henneberg, 2006; Stephan et al., 2005; Wilkinson et al., 2006). As inaccurate prediction methods have been identified improvements are being made (see above cited studies), but the challenge remains to improve methods so much that correct and purposeful recognition of the predicted faces can be easily and reliably achieved (Stephan, 2009).

It is also worth noting here that facial approximations of hominids, while interesting, cannot be justified on scientific grounds since the methods employed and/or the faces constructed cannot be verified/tested using antemortem facial appearances. Since little research has been directed towards establishing the most appropriate methods to predict these faces, even larger components of the method are left to the subjective interpretation than in forensic casework. Consequently, paleoanthropological facial approximations must be viewed using a strong degree of skepticism (Montagu, 1947; Stephan, 2003a).

Facial approximation methods have also been known by many other names previously. The most popular of these is “facial reconstruction”. There are two main problems with this term: i) the face is not reassembled from existing parts as reconstruction implies; and ii) reconstruction overemphasizes the exactness, reliability and scientific validity of the methods. Most practitioners now acknowledge that the term "facial approximation" is the most appropriate name for the prediction of facial appearances from the skull (George, 1987; Taylor, 2001; Haglund and Reay, 1991), although some continue to avoid its use due to its less sensational nature. The suitability of "facial approximation" is perhaps best demonstrated by Gerasimov who lucidly summarized the aim of the method as "approximation so close to the appearance of a living person that even an unknown individual could be identified" (Gerasimov, 1955, 1968, 1971).

Ridicule is sometimes directed at the term "facial approximation" since it is said that methods, if in the realms of scientific study, should be precise and exact. Besides being mistaken (see e.g., Popper, 1959; Kuhn, 1970; Feyerabend, 1999), this view expects too much of an elementary technique that, despite a lengthy history, is still emerging.

To reference the above content please cite:
Stephan CN (2011) Craniofacial identification: method background and overview. www.CRANIOFACIALidentification.com

References

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