Bitemark Analysis: A NIST Scientific Foundation Review
Forensic Bitemark Analysis Not Supported by Sufficient Data
National Institute of Standards and Technology
Kelly Sauerwein, John M. Butler, Christina Reed and Karen K. Reczek
Abstract
This report summarizes a review of the scientific foundations of bitemark analysis conducted by the National Institute of Standards and Technology (NIST). Bitemark analysis typically involves examining patterned injuries left on a victim or object at a crime scene, recognizing those injuries as bitemarks, and comparing those marks with dental impressions from a person of interest. This review specifically focuses on pattern injuries found on human skin. Over 400 sources were considered via literature searches and input from previous efforts by the National Institute of Justice Forensic Technology Center of Excellence. Our NIST review also utilized input from an October 2019 Bitemark Thinkshop organized by the Center for Statistics and Applications in Forensic Evidence (CSAFE) where experts and stakeholders associated with bitemark analysis were convened to discuss key issues. Based on this input, our study found a lack of support for three key premises of the field: 1) human dentition is unique at the individual level, 2) this uniqueness can be accurately transferred to human skin, and 3) identifying characteristics can be accurately captured and interpreted by analysis techniques. Furthermore, our review noted a lack of consensus among practitioners on the interpretation of bitemark data as well as thoughts on how to move the field forward. If the field seeks to advance, the key takeaways provided in this review are starting points for areas needing improvement, not an exhaustive list of specific shortcomings.
Executive Summary
All scientific methods have limits and one must understand these limits to use a method appropriately. This is especially important in forensic science as critical decisions impacting life and liberty are often based on the results of forensic analyses.
The American Board of Forensic Odontology (ABFO) defines a bitemark as a “physical alteration or representative pattern recorded in a medium caused by the contact of teeth of a human or animal.” For human bitemarks, this pattern would demonstrate features, traits, or characteristics that distinguish the patterned injury as a bitemark (ABFO 2018). Bitemark analysis typically involves the examination of patterned injuries left on a victim or object at a crime scene, identification of those injuries as bitemarks, and comparison of those marks with dental impressions from a person of interest (POI).
The assumption that an individual can be identified from bitemarks left on human skin has, for several decades, seen a steady increase in scientific scrutiny. In 1960 following an experiment where multiple people left bitemarks in food items, a British dentist concluded “evidence which involves the identification of a person by tooth-marks left as bruises in flesh should never be admitted [in court], and evidence involving bitemarks in, for example, foodstuffs should be examined extremely critically” (Fearnhead 1960). Unlike the use of dental information to identify human remains, bitemarks are primarily made from only the anterior teeth and are prone to distortions due to bite force, location of the bite, and movement of the biter or victim during the biting event – all of which can lead to an innocent person not being excluded as the source of a bitemark.
This scientific foundation review examined the existing bitemark literature to answer two questions: 1) Can bitemarks be accurately associated with teeth that left them? and 2) What data exist to support or refute this claim in bitemark analysis? The aim of this foundation study is to promote a better appreciation of the capabilities and limitations of the practice within the forensic community as well as among other stakeholders, including investigators and legal professionals. Given the questions already arising from practitioners within this field about the legitimacy of the fundamental assumptions required to establish a verifiable source of a bitemark (Avon et al. 2010) and the frequency at which such claims are disproven with DNA testing (Bowers 2006), this review also focused on the limitations inherent to this practice and under what conditions they are being observed.
Obtaining input from experts outside of NIST is an integral component of a NIST scientific foundation review. As described in Chapter 3, the NIST team followed the process outlined in NISTIR 8225 for conducting this review. This involved:
- collecting and evaluating the peer-reviewed literature,
- assessing publicly available data from interlaboratory studies, proficiency tests, and laboratory validation studies,
- exploring other available information, including position statements and non-peer reviewed literature, and obtaining input from members of the relevant community through interviews, workshops, working groups, and other formats for the open exchange of ideas and information.
In addition, this NIST review also sought community input from the 2019 CSAFE Thinkshop involving practitioners, stakeholders, and researchers. A conclusion from this workshop was that there is a critical need for research to explore the scientific foundations of bitemark analysis, including assessing the reliability and validity of determinations made as to bitemark type (human vs nonhuman vs not a bitemark) and in linking dentition to bitemarks.
It is noted that bitemark analysis represents only a portion of forensic dentistry (odontology) activities. Antemortem dental records, for example, involving the full human dentition, routinely enable postmortem identification of human remains. This review does not explore the whole discipline of forensics odontology; the focus is on bitemarks left on human skin.
Three primary postulates are important for successful bitemark analysis: (1) that dental characteristics, especially the arrangement of the anterior teeth, differ substantially among individuals (i.e., uniqueness), (2) skin or other marked surfaces can reliably capture those differences (i.e., transference), and (3) a bitemark examiner can reliably compare anterior dentition information with the bitemark image (i.e., interpretation) (Hale 1978, Pretty & Sweet 2001, Saks et al. 2016). This review considers each of these three postulates and finds limited data to support them. Therefore, the ability of bitemark analysis to accurately exclude or not exclude individuals as a source of the mark is not supported.
Key takeaways identified as part of this foundation study include the following (numbering is based on their sequence within the chapter where they are derived):
KEY TAKEAWAY #1.1: Forensic bitemark analysis lacks a sufficient scientific foundation because the three key premises of the field are not supported by the data. First, human anterior dental patterns have not been shown to be unique at the individual level. Second, those patterns are not accurately transferred to human skin consistently. Third, it has not been shown that defining characteristics of that pattern can be accurately analyzed to exclude or not exclude individuals as the source of a bitemark.
KEY TAKEAWAY #2.1: The entire human dentition is not represented in a bitemark. Bitemark patterns typically only represent the anterior teeth and thus not the full possible dentition of an individual, limiting the amount of information available for an analysis.
KEY TAKEAWAY #4.1: There is a lack of research into population frequencies, specific identifying characteristics, and measurements that support the notion that human anterior dental patterns as reflected in bitemarks are unique to individuals.
KEY TAKEAWAY #4.2: Accurate transference of an anterior dentition pattern in the form of a bitemark on human skin can be limited by distortions caused by skin elasticity, unevenness of the biting surface, location of the bite, and movement of the biter and/or victim during the biting event.
KEY TAKEAWAY #4.3: Comparisons between bitemark patterns made on skin, for example multiple bitemarks from the same individual on the same victim, have shown that there exists intra-individual variation in bitemark morphology on the human body such that bitemarks from the same biter may not appear consistent.
KEY TAKEAWAY #4.4: Bitemarks in cadaver-based research studies are representative of highly controlled experimental conditions and these results may overestimate the accuracy of analysis methods. Bitemarks in actual cases, where controlled conditions are not present, are prone to higher levels of inaccuracy.
KEY TAKEAWAY #4.5: As reflected in research studies to date, bitemark examiners may not agree on the interpretation of a specific bitemark, including whether the injury is a bitemark, the features present, and the exclusion or non-exclusion of potential biters.
KEY TAKEAWAY #5.1: Repeated calls for additional data by critics and practitioners (since at least 1960) suggest insufficient support for the accurate use of bitemark analysis and a lack of consensus from the community on a way forward.
Calls have been made for empirical studies to assess the limitations of bitemark analysis for decades. Since 1960, those in the bitemark community have been highlighting the lack of empirical research and the need to address reliability concerns in bitemark methods. These calls have largely gone unheeded.
This report describes an examination of publicly available literature and information pertaining to bitemark analysis. If the field seeks to advance, the key takeaways provided in this report are starting points for areas needing improvement, not an exhaustive list of specific shortcomings.
Read the report
About the Authors
Kelly Sauerwein
John M. Butler
Christina Reed
Division 602 - Special Programs Office
Laboratory Programs
National Institute of Standards and Technology
Karen K. Reczek
Division 601 – Standards Coordination Office
Laboratory Programs
National Institute of Standards and Technology