Fingerprint forensics for beginner crime-fiction writers

If you’re including fingerprint science in your fiction writing, these tips will help you get the basics right.

Fingerprints are an established part of forensic procedure. If they’re in your novel, you’ll need to think about the science, the degree to which you’ll stay true to reality, and how much detail to include without turning your fiction into a textbook.

Here are 9 tips to get you started.

1. Know how a fingerprint is formed
In ‘Introduction to Forensic Science’, Penny Haddrill describes fingerprints as follows:

‘On our fingers and toes there are many fine ridges. These ridges form recognisable patterns and are formed during the gestation period of the foetus during pregnancy. Fingerprints are produced from the contact between the ridges present on our fingers and a surface. The transfer is achieved by the deposition of the secretions from our glands associated with the ridges on our skin or by contaminants present on the finger surface.’

2. Know your terminology
If you’re seeking authenticity, the terminology you use will be determined by where your novel’s investigators are based. If your fiction is set in the UK at least, ‘fingerprint’ and ‘fingermark’ mean different things:

• PRINT: A fingerprint refers to what is taken from a known/controlled source. For example, if your suspect is arrested, their fingerprints will be taken by the police using specialist equipment. The prints will be of high quality and include all the ridge detail.
• MARK: A fingermark refers to what is taken from an unknown/uncontrolled source. Your investigator might find fingermarks at a crime scene or elsewhere on an object that’s suspected of being related to the crime scene. Fingermarks are the ridge patterns (either full or partial, and from one digit or several) left behind on surfaces. The marks will usually be of lower quality, and might be smudged or otherwise contaminated.

3. Categorize your fingermarks correctly
In Explore Forensics, Jack Claridge offers three categories of mark (though, again, in your novel’s jurisdiction, these might be referred to as prints):

• Impressed marks (visible to the naked eye): These are found when a person’s finger has been pressed into a malleable substance such as wax, clay, or wet paint. They’re sometimes referred to as plastic marks.
• Patent marks (visible to the naked eye): These are found on a surface that’s come into contact with a finger on which there is a residue of, say, blood, oil, dirt, or some other liquid or powder material.
• Latent marks (invisible to the naked eye): The skin secretes sweat from pores. Sweat mixes with external particles in the air, and the body’s natural oils, and sits on the ridges of the fingers. When those fingers come into contact with a surface (particularly something hard or shiny, such as glass) the ridges leave a fingermark that can be exposed under high-intensity light sources or displayed by dusting with a fine powder or chemicals.

4. Understand where the science wobbles  
It is believed that no two fingerprints are alike but there is no empirical evidence to prove this.

While it’s true that, to date, identical fingerprint matches have not been found, there are enough similar ‘matching points’ between two people’s prints such that false positives and negatives have occurred.

Says Laura Spinney in Nature:

‘Fingerprint analysis is fundamentally subjective. Examiners often have to work with incomplete or distorted prints — where a finger slid across a surface, for example — and they have to select the relevant features from what is available. What is judged relevant therefore changes from case to case and examiner to examiner.'

You might want to bear this in mind when your characters are doing or talking about fingerprint uniqueness.

5. Get the twin stuff right
Identical twins have almost identical DNA because they develop from one zygote (created by one egg and one sperm) that splits into two embryos.
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Fingerprints, however, are formed during foetal development. Here's Spinney again:

‘The ridges and furrows on any given fingertip develop in the womb, [and are] shaped by such a complex combination of genetic and environmental factors that not even identical twins share prints.’

With that in mind, don’t make the mistake of hanging your plot on murderous monozygotic mayhem created by a couple of trickster twins. Their DNA might be an almost perfect match, but their paw prints won’t be.

6. Acknowledge real-world procedure and bias
The Centre for Forensic Science identifies four separate stages in the methodology of fingermark collection: ACE-V.

• Analysis – all the features of the mark, including any distortions, are analysed by the examiner. Remember, the mark may well be of lower quality than an exemplar fingerprint because it’s smudged or partial or has been contaminated in some way. Marks are analysed in terms of their overall pattern (e.g. loop, regular arch, tented arch, whorl), their ridge characteristics (more detailed ‘minutiae’), and the size, shape and frequency of pores and ridge edges.
• Comparison – the features are compared with a fingerprint in the database.
• Evaluation – the examiner decides whether there’s a match. It might be impossible to conclude either way because a comparison can’t be carried out between a partial mark and a full print.
• Verification – more than one specialist examiner should verify the match.

​In Forensics, Val McDermid discusses a miscarriage of justice that occurred because of flawed procedure (pp. 134–7). A partial fingermark left at the scene of a horrific bombing in Madrid in 2004 was analysed in comparison with a suspect’s fingerprints.

​In other words, the A and C were not separate procedures. The examiner went looking for points of similarity rather than taking into account the differences, even though only a partial fingermark was available. This introduced unacceptable bias and led to erroneous findings. She summarizes the FBI’s later conclusions:

'First of all the expert should analyse the mark in detail, describing as many minutiae as she can. Only afterwards should she examine possible matches and carry out a comparison. When analysis and comparison happen simultaneously, experts run the risk of finding matching minutiae [ridge characteristics) because they are looking for them.’

Does this mean you can’t have flawed procedure in your novel? Not at all – it could be a great plot point.

Just bear in mind that the issue of bias is on the agenda for the forensics community internationally. Read ‘A Review of the FBI’s Handling of the Brandon Mayfield Case’ in its entirety if you want more insights into the challenges of fingerprint forensics.

Furthermore, many police forces are having to endure budget cuts. Stretched resources can lead to corner-cutting. How might that affect your story?

7. Familiarize yourself with processing and enhancement
Latent fingermarks are currently processed using the following techniques. To find out more, read Dr Chris Lennard’s paper, ‘The detection and enhancement of latent fingerprints’, presented at the 13th INTERPOL Forensic Science Symposium.

• Powder suspension – enhances latent marks on non-porous surfaces. A suspension of coloured metal and detergent is painted onto the surface, then washed off, leaving the fingermark exposed. The mark is photographed.
• Powders – enhances latent marks on non-porous surfaces. Coloured metal powder – aluminium for glass; black powder for UPVC – is brushed onto the surface, exposing the mark. The mark is photographed or lifted using tape.
• Cyanoacrylate – enhances latent marks on non-porous surfaces. This compound is found in superglue. The vapour reacts with some marks, producing white crystals on the mark’s ridges. The crystalline can be dyed or dusted with dark powder to further enhance the mark. The mark is then photographed.
• Ninhydrin – enhances latent marks on porous surfaces. The surface is covered with the chemical then baked. It reacts with amino acids in the finger mark and turns purple. The mark is photographed.

Exemplar fingerprints – those taken directly from an individual in controlled circumstances – are captured via two methods:

• Ink-rolling (called Tenprints in the UK) – the fingertips and palms are rolled in ink and re-rolled onto card, then photographed.
• Scanning (called Livescan in the UK and US) – the fingertips and palms are scanned electronically.

Read about how two law-enforcement organizations – one in the UK and one in the US – use ink and scanning technology here:
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• FBI: Recording legible fingerprints
• Northumbria Police: Tenprints

​8. Use the right fingerprint databases
Images of fingermarks extracted from a crime scene can be uploaded to databases containing both fingerprints (and other biometrics) of known individuals and fingermarks that have yet to be identified.

The main fingerprint databases are:

• UK: IDENT1
• US: IAFIS – Integrated Automated Fingerprint Identification System
• Australia: National Automated Fingerprint Identification System
• Eurodac – specifically for identifying asylum seekers and illegal immigrants
• INTERPOL fingerprints database

9. Decide how far to bend the rules
Do you need to worry about any of this? After all, it’s fiction!

​It depends on the subgenre of your writing, and how your readers are likely to respond to deviations from reality. If your novel is set in an alternative world or in the future, you can play it however you want.

If, however, you’re writing a realistic procedural, you could alienate sticklers if you get it wrong, especially those who are police officers, forensic scientists, scene-of-crime officers, and fingerprint examiners.

TV shows like Silent Witness and CSI offer audiences a single viewpoint. Most viewers accept that the real world involves more complex investigations with many more players. Books, like TV shows, entertain us, so readers too will indulge writers who bend the rules to a degree.

Even if you don’t want to go for maximum authenticity, consider sprinkling your narrative with factual information that grounds your investigation just enough to head off those whose fingers are hovering over the one-star button in Amazon’s review pane!

Further reading and related resources

• 3 reasons to use free indirect speech in crime fiction​
• A Review of the FBI’s Handling of the Brandon Mayfield Case, US Department of Justice, 2006
• Crime writing resources for indie authors
• Fingerprint Examination – Terminology, Definitions and Acronyms, Forensic Science Regulator, 2013
• Forensics: The Anatomy of Crime, by Val McDermid. Wellcome Collection, 2015
• Introduction to Forensic Science (online course), with Penny Haddrill, Centre for Forensic Science, University of Strathclyde, 2018
• Latent Prints, by Jack Claridge. Explore Forensics, 2017
• Playing with sentence length in crime fiction. Is it time to trim the fat?
• ​Playing with the rhythm of fiction: commas and conjunctions
• Science in court: The fine print, by Laura Spinney. Nature 464, 344–6, 2010
• The Detection and Enhancement of Latent Fingerprints, presented at the 13th INTERPOL Forensic Science Symposium, by Dr Chris Lennard, 2001
• Why ‘suddenly’ can spoil your crime fiction: Advice for new writers

Louise Harnby is a line editor, copyeditor and proofreader who specializes in working with crime, mystery, suspense and thriller writers.

She is an Advanced Professional Member of the Chartered Institute of Editing and Proofreading (CIEP), a member of ACES, a Partner Member of The Alliance of Independent Authors (ALLi), and co-hosts The Editing Podcast.
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