Dorsal hand comparison
The hand provides one of the most widely used types of forensic evidence – fingerprints, created by the ridge detail of the fingers and palmar surface. However, the evidentiary value of the hand doesn’t end with the palmar surface. Combining the principles of image comparison with biometric knowledge, researchers and investigators have found that the backs of the hands – the dorsum – may also be forensically useful for excluding or potentially identifying individuals based on the anatomical features present in photo or video evidence.
Common uses
With the threat of being caught on camera, perpetrators of crimes increasingly cover their faces to avoid being easily identified (Slot & Geradts, 2014). Dorsal hand comparison can be employed in cases where image evidence depicts hand detail but does not feature clear biometric information of the head and face. Similar techniques utilised in facial image comparison can be applied to compare questioned images of an unknown or questioned individual’s dorsal hand to reference photos of a suspected individual’s hands. Images of masked individuals with uncovered hands, camera footage where heads of victims or perpetrators are out-of-frame, or close-up POV photo or video captured by an unknown person of interest with their hands in view, are a few instances that may be encountered by investigators where dorsal hand examination may be a viable tool.
Dorsal hand comparison is being increasingly applied in cases involving child sexual abuse materials (CSAM), particularly by agencies in the United Kingdom (Ferguson & Raitt, 2013; Hackman & Black, 2023). While contact offenders in CSAM cases may typically try to avoid being identified on video, videos depicting CSAM offences frequently feature perpetrators hands or parts of their bodies, which can open avenues for employing forensic image analyses and anatomical comparisons (Ferguson & Raitt, 2013). Hand examination evidence has been used in UK courts by both the prosecution and defence for several years, with perpetrators of sexual abuse being convicted with the help dorsal hand analysis and c (Hackman & Black, 2023).
Anatomical features of the dorsum
Like the face, the dorsal hand can be divided into different morphological components for comparison, each containing anatomical structures that can be analysed and then compared between images.
Examiners with sufficient anatomical knowledge and technical competency compare the morphology, presence, patterns and locations of structural features and fine details to discern similarities and dissimilarities between the questioned and reference hand images. The observed similarities and dissimilarities are then evaluated to ascertain the likelihood of the images depicting hands from the same or a different individual. The discriminating power of features such as dorsal vein patterns, knuckle creasing, and birthmarks is based on their embryonic development, leading individual hands to have distinctive appearances and variations of these features (Dan, et al., 2023).
There are a diverse range of anatomical features on the dorsal surface of the hand that can be utilised for analysis and comparison.
The dorsal surface above the metacarpal bones is where vein networks can be observed underlying the skin, while creases of the knuckle joints and the lunulae of the fingernails (the lighter-coloured half-moon shapes at the bottom of the nail beds) can be assessed for their patterns, appearance, or presence of abnormalities. There is also the potential for observing more distinctive features such as moles, freckles, sunspots, scars, hyperpigmentation, or tattoos, all of which can be used as points of comparison between sets of images.
Like fingerprints, the superficial dorsal veins can be observed for their minutiae – this includes branching or bifurcation points, disappearing points (i.e. where visibility of a vein branch appears to ‘end’ on the back of the hand), and bridges or connections (Wang, et al., 2008). Knuckle crease patterns at the finger joints have proven useful as they are often visible in image evidence, and are not typically as affected by usage and injury as prints on the palmar surface, remaining reasonably stable over time and with finger flexion (Bhattacharya, et al., 2017; Dan, et al., 2023). Observations on knuckle creases may be recorded in terms of their orientation, number, and depth (e.g. superficial versus deeper creases, the latter remaining more stable with greater degrees of finger flexion) (Dan, et al., 2023).
The particular morphology, location, and amount of marks or scars on the back of the hand can offer a significant amount of discriminating power in determining whether a set of hand images depict the same or different individuals (Black, et al., 2014a; Busch & Lynnerup, 2015; Jackson & Black, 2014; Malone, 2015).
Our hands undergo wear through use and exposure to the elements throughout our lives, increasing the acquisition of distinctive features over time. The frequency of scarring, for example, can be correlated with age, certain locations on the hands, and the left versus the right hand (Black, et al., 2014a; Macdonald-McMillan, 2011). Melanocytic nevi, or moles, can also occur on the hands at birth or with age over time, though some research has suggested that nevi are less common on the hands, which may further increase their discriminating power when observable (Black, et al., 2014b). There are also solar lentigines, dark spots that can be acquired on the backs of the hands from sun exposure, and freckles, all of which can be useful as points of comparison between hand images.
Method Development
Dorsal hand comparison has yielded opinion-based evidence used primarily for exclusionary purposes, but it is important to keep in mind that all analytical techniques employed in forensic investigation must be validated to demonstrate their reliability and efficacy. There is a growing body of literature detailing research and case applications of dorsal hand comparison. Studies have detailed that is possible to achieve accurate and consistent results in “matching” hand images (Black, et al., 2014a; Black, et al., 2014b; Busch & Lynnerup, 2015; Slot & Geradts, 2014). Examiners using dorsal hand comparison in forensic casework have published about procedural approaches and case outcomes (Ferguson & Raitt, 2013; Hackman & Black, 2023; Jackson & Black, 2014).
Generally, researchers and analysts apply a morphological approach for assessing the dorsal hand, much the same as is most commonly used for facial image comparison (Slot & Geradts, 2014). This method involves analysing the morphology, characteristics, and orientations of the anatomical features depicted in each image one-by-one, then individually comparing each feature between the questioned and reference images to assess their similarities and dissimilarities. An examiner will then weigh the similarities and dissimilarities against same source versus different source propositions, to determine which is more likely given the visual evidence. They will then provide an evaluative opinion on whether or the questioned and reference images depict to the same or different individuals.
While dorsal hand comparison has seen growth in forensic application over the past decade, there is still room – and need – for more validation testing. Many studies relating to dorsal hand examination focus on the testing of methods to classify and analyse particular anatomical features in isolation, such as knuckle creases or skin marks, rather than an end-to-end validation of the entire hand comparison process. There is also a lack of a standardised approach or system for dorsal hand analysis and image comparison, which limits the extent to which assessment results can be reliably reproduced and repeated (Malone, et al., 2015). While different organisations and practitioners may vary in their specific procedure, there lacks a general methodology or set of best practices that have been tested and validated to establish or define efficacy. Practitioners and researchers may design or adapt their own methodological approach and test it on small organisational image databases, but this has not necessarily led to widespread acceptance and adoption by a majority of examiners so far.
Limitations
The possibility for carrying out a dorsal hand image comparison will depend on the suitability of the images provided to examiners. It may be challenging or outright unfeasible to carry out a reliable comparison of a set of images, if image quality factors – such as low resolution, lighting, or blur – and subject factors – such as particular hand poses – create too many limitations for analysis. When extracting still frames from videos of persons of interest, this can especially pose a challenge; if a subject’s hand or fingers are visible in frame for only a few seconds, it can be difficult to obtain images of suitable quality and/or with an appropriately usable amount of information (Busch & Lynnerup, 2015).
While image comparison of dorsal hands may use a similar methodological approach as facial image comparison, experience in conducting facial examination is not enough for analysts to take on cases involving hand images. Prospective dorsal hand examiners should receive pertinent specialised training and education in hand anatomy and tailored competency assessments, to ensure that they can produce accurate and repeatable results and demonstrate the competency necessary for providing reliable expert opinions (Malone, et al., 2015; Malone, 2015). This will require effort, resources, and cooperation on the part of examiners and agencies – and of course, a validated methodology.
Future direction
In order to better ascertain the likelihood of occurrence of particular dorsal hand features within given populations, some researchers have begun compiling and/or consulting hand image databases (Black, et al., 2014a; Jackson & Black, 2014; Slot & Geradts, 2014). Like having databases for fingerprints and faces, the aim is to serve an investigative purpose, test algorithmic assessments of hand data, and better allow practitioners to quantify and report likelihood ratios for their observations on dorsal hand anatomy (Jackson & Black, 2014; Slot & Geradts, 2014). Presently, hand image databases are limited by use to the organisations that are curating them (i.e. not necessarily publicly available), and may not represent a wealth of demographic groups (Black, et al., 2014b; Wang, et al., 2008; Zhao, et al., 2007). However, this only highlights the need and opportunity for increasing expansion.
While hand image comparison can be a valuable form of evidence, it is still recommended that it should support other analyses and evidentiary information in building a case, as opposed to being used alone or as a form of positive identification (Busch & Lynnerup, 2015). Investigators and organisations who see dorsal hand comparison as a useful potential tool to aid in cases involving photo and video evidence are best to get in touch with practitioners already working and performing research in dorsal hand analysis. They may also consider investing in method development and validation studies, as well as training, for the creation of accurate, reliable methods and growth of practitioner expertise in this emerging field.
References
Bhattacharya, N., Dewangan, D. & Dewangan, K., 2017. An Efficacious Matching of Finger Knuckle Print Images Using Gabor Feature. ICT Based Innovations, Advances in Intelligent Systems and Computing, Volume 653, pp. 153-162.
Black, S., MacDonald-McMillan, B. & Mallett, X., 2014a. The incidence of scarring on the dorsum of the hand. International Journal of Legal Medicine, 128(3), pp. 545-553.
Black, S. et al., 2014b. The incidence and position of melanocytic nevi for the purposes of forensic image comparison. International Journal of Legal Medicine, 128(3), pp. 535-543.
Busch, J. & Lynnerup, N., 2015. Identifying suspects by matching hand photographs with video evidence. Forensic Science, Medicine, and Pathology, Volume 11, pp. 504-508.
Dan, L., Hackman, L., Rinaldi, V. & Trucco, E., 2023. The persistence of knuckle creases during finger flexion for the identification of perpetrators from digital images of their hands. Forensic Science International, Volume 348, pp. 1-10.
Ferguson, P. & Raitt, F., 2013. 'If a picture paints a thousand words...': the development of human identification techniques in forensic anthropology and their implications for human rights in the criminal process. The International Journal of Evidence & Proof, Volume 17, pp. 127-156.
Hackman, L. & Black, S., 2023. Forensic examination of the hand. Journal of the Royal Anthropological Institute (N.S.), 29(S2), pp. 116-131.
Jackson, G. & Black, S., 2014. Use of data to inform expert evaluative opinion in the comparison of hand images—the importance of scars. International Journal of Legal Medicine, Volume 128, p. 555–563.
Macdonald-McMillan, B., 2011. The quantification of dorsal hand features of interest to assist forensic human identification. MSc Thesis: University of Dundee.
Malone, C., 2015. Photographic Analyses Using Skin Detail of the Hand: A Methodology and Evaluation. Journal of Forensic Sciences, 60(2), pp. 326-330.
Malone, C., Salyards, M. & Hein, M., 2015. Inter-/Intra-observer Reliability of Hand Assessment Using Skin Detail: A Count-based Method. Journal of Forensic Sciences, 60(6), pp. 1605-1612.
Slot, A. & Geradts, Z., 2014. The Possibilities and Limitations of Forensic Hand Comparison. Journal of Forensic Sciences, 59(6), pp. 1559-1567.
Wang, L., Leedham, G. & Cho, D.-Y., 2008. Minutiae feature analysis for infrared hand vein pattern biometrics. Pattern Recognition, Volume 41, pp. 920-929.
Zhao, A., Wang, Y. & Wang, Y., 2007. Extracting Hand Vein Patterns from Low-Quality Images: A New Biometric Technique Using Low-Cost Devices. s.l., s.n., pp. 667-671.