Bite marks and their limits

What it is

A human bite usually leaves two opposing curved rows of bruises or abrasions, one from each jaw. It can appear in sexual assault, child abuse and fights, and its pattern, site and age are worth documenting. The value of comparing a bite mark with a suspect's teeth is much weaker than was once claimed. A 2023 review by the US National Institute of Standards and Technology (NIST) concluded that bite-mark analysis lacks a sufficient scientific foundation: it has not been shown that the front teeth are unique to each person, that skin records their pattern accurately (skin stretches, the victim moves, swelling and healing distort it), or that examiners can reliably exclude or include a person. Examiners often disagree even on whether an injury is a bite. The saliva left in the mark, sampled for DNA, is far more useful than the shape.

How to tell it apart

• Other patterned injuries (from objects, rings, medical devices) and animal bites can look like human bites.
• Bruise colour cannot date the injury reliably.
• A pattern consistent with a human bite does not identify the biter.
• Marks from the same biter on the same victim can look different from one another.

How to recognise it

• Oval or round mark made of two U-shaped arcs facing each other, one from each jaw.
• Individual tooth marks may appear as bruises, abrasions or small lacerations.
• Examiners disagree surprisingly often on whether a mark is human or animal, from an adult or a child.

How it forms

• Pressure of the teeth bruises and scrapes the skin; deep bites can tear it.
• Skin is elastic and curved, and the bitten person usually moves, so the mark is distorted when it forms.
• Swelling, healing and decomposition change its shape afterwards.

Next step: what to do and document

• Swab the mark for saliva DNA before the area is washed, and keep the swabs under chain of custody.
• Photograph the mark with a scale and the case number, before and after cleaning.
• Describe site, shape, size, arcs and each tooth mark in neutral terms, without naming a biter.
• In a child, start the protection and reporting process for suspected abuse.
• In the report, state that comparison with a suspect's teeth has weak scientific support and that DNA is the identifying evidence.

Common mistakes

• Washing the mark before taking the saliva swab.
• Stating that the mark matches a suspect's teeth as if it were an identification.
• Calling any curved bruise a bite without considering other objects.
• Photographing without a scale or at an angle, which distorts the shape.

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Educational content on forensic and legal medicine; not legal advice and no substitute for the technical guides of Colombia's National Institute of Legal Medicine and Forensic Sciences. Laws change: each page links the official text and says when it was last checked. For a real case, follow the guide in force and ask the competent authority.

Sources: Bitemark Analysis: A NIST Scientific Foundation Review (NISTIR 8352) (National Institute of Standards and Technology, 2023); The Minnesota Protocol on the Investigation of Potentially Unlawful Death (2016) (Office of the United Nations High Commissioner for Human Rights, 2017).

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