1. Introduction to Craniofacial Superimposition

The main focus of forensic anthropology lies in the determination of the identity of human remains when skeletal information becomes the last resort for forensic assessment (Burns 2007; Yoshino and Seta 2000; Alemán et al. 1997). In the last few decades, anthropologists have focused their attention on improving the techniques that help us in make the most accurate identification.

Before making an identification decision, anthropologists follow different processes to assign ancestry, sex, age, and height to human remains from the study of ante-mortem information (which can be retraced from visual material and interviews with relatives or witnesses) and post-mortem data (the skeletal remains found, i.e., bones). Different methodologies have been proposed for this purpose, with each implemented according to the features of the population group from which the method was derived (Alemán et al. 1997; İşcan 2005; Gonzalez-Colmenares et al. 2007; Urquiza et al. 2005; Landa et al. 2009). As these methods are population specific, they require the pre-determination of ancestry. The aforementioned anthropological studies are usually taken as the first step in the identification process, prior to the application of any other technique, as the determination of the main biological parameters (sex, age, stature, build, teeth, possible pathologies, etc.) reduces the number of individuals for comparison. Nevertheless, there are several other identification procedures that are more reliable than skeleton-based identification and can be applied either with the forensic anthropology evaluation or without it, such as (Stratmann 1998):

The application of these methods can be problematic, as on some occasions there is not enough information (ante- or post-mortem) available in order to conduct them. With regard to post-mortem data, the state of preservation of a corpse can vary considerably as a result of several chemical and mechanical factors. While the skeleton usually survives both natural and non-natural decomposition processes (fire, salt, water, etc.), the soft tissues (skin, muscles, hair, etc.) progressively degrade and are lost. The main disadvantage of DNA testing is the relatively large amount of high-quality tissue material required, which is not common in remains that were buried a long time ago. On the other hand, with regard to ante-mortem information, the first method requires the existence of a print database, the second is dependent on the availability of dental records (although in comparison to bones and skin, the teeth are more resistant to the effects of fire and salt water), the third requires previous X-ray images (among other information), and the last method depends upon a second DNA sample, either from the same person or from a relative.

Hence, anthropological identification based only on skeletal information can be considered as a final option for forensic identification when none of the previous methods can be applied. If the soft tissue or DNA methods cannot be applied but the skeletal studies provide positive results, then more specific skeleton-based identification techniques can alternatively be applied. These can include craniofacial superimposition (CFS) (İşcan 1993; Rathbun 1984; Taylor and Brown 1998).

Craniofacial superimposition is a forensic process in which photographs or still frame images taken from video recordings of a missing person are compared with the skull that is found. By projecting both photographs on top of each other (or, even better, matching a scanned three-dimensional skull model against the face photo/series of video shots), the forensic anthropologist can try to establish whether they belong to the same person (Krogman and İşcan 1986) based on the similarity of morphological features common to both.

Therefore, in every system, for skull identification by craniofacial superimposition, two objects are involved: a skull and an image of a face. The latter is typically a photograph although it can be sometimes replaced by a series of video shots or, more rarely, a portrait of the missing person. The final goal, common to every system, is to assess the anatomical consistency between the skull and the face.

Before reviewing the literature of this forensic identification technique, one should note that different terms have been used to refer to CFS over more than a century of its development. This has been mainly due to the use of close synonyms and also because of the coining of new, more specific terms depending on the supporting technical devices used throughout the developmental history of the overall technique. The following points justify our choice of “craniofacial superimposition” as the most general and currently appropriate name for this forensic identification method.

As a result of the rapid developments in video technology, the term “video superimposition” was then used when this tool became common in forensic identification (Seta and Yoshino 1993; Pesce Delfino et al. 1993; Shahrom et al. 1996; Yoshino et al. 1997). Finally, the use of computers to assist anthropologists in the identification process has evolved the next generation of CFS systems.¹ The latter approach is usually referred to as “computer-aided” or “computer-assisted CFS”² (Pesce Delfino et al. 1986; Ubelaker et al. 1992; Aulsebrook et al. 1995; Yoshino et al. 1997).

Hence, when using the generic term CFS, we are assuming neither a particular acquisition device nor a given data format for the input of our problem. We simply consider that any CFS method will deal with a 2D image of the disappeared person (typically a photograph) and corresponding images derived from the 3D skull recovered (possibly alongside other skeletal remains).

We are aware there are some authors who prefer to use the term “photographic supra-projection” (Bronkhorst 2006; Stratmann 1998). We avoid its use because it does not explicitly indicate the matching of a skull with a face.

Finally, CFS should not be confused with craniofacial identification either. It is important to recognize that the latter is used as an umbrella term, which includes both CFS and the very different technique of facial approximation³ (Clement and Ranson 1998; Wilkinson 2009; Stephan 2009). Both methods are underpinned by the knowledge of human craniofacial anatomy. It is this principle that ties these two techniques together despite the use of different technical protocols for each of them.