How Are Fingerprints Found? Developing What Nobody Can See

Ridges on the fingertips carry pores that secrete sweat, and the skin also picks up oils, so touching a surface leaves a deposit reproducing the ridge pattern. The material is mostly water, which evaporates within hours, leaving salts, amino acids, fatty acids and whatever contaminants the finger had collected. That composition determines which development method will work, since different techniques target different components, and it changes with time, temperature and humidity, which is why old prints and prints on surfaces exposed to weather are harder to recover. The deposit is fragile, so handling a surface destroys what is on it, which is why examination order at a scene matters and why prints and other evidence types compete for the same surfaces.

Each technique targets a particular component of the residue:

• •Powders, which adhere physically to the moisture and oils and are used on hard non-porous surfaces
• •Cyanoacrylate fuming, where superglue vapour polymerises on the residue producing a white deposit
• •Ninhydrin, which reacts with amino acids and is the standard method on paper
• •Physical developer, which targets fatty components and works on wetted paper
• •Vacuum metal deposition, which coats a surface unevenly according to the residue
• •Fluorescent dyes and alternative light sources, which improve contrast on difficult backgrounds

An examiner compares a recovered mark with a known print by locating features where ridges end, split or form other characteristic details, and assessing whether the arrangement corresponds. The method used across much of the field proceeds through analysis of the mark, comparison with the candidate, evaluation of the correspondence and verification by a second examiner. There is no fixed number of matching features required in most jurisdictions, which was once specified and was abandoned because it has no statistical basis. The decision is a judgement by a trained examiner rather than a measurement, which is the crux of the criticism, since a judgement is susceptible to expectation and the examiner frequently knows what result the investigation wants.

The response to the criticism has been methodological rather than a retreat from the technique, and the changes are specific. Blind verification means the second examiner does not know the first examiner's conclusion, which removes a source of confirmation. Linear sequential unmasking requires the examiner to document their analysis of the mark before seeing any candidate print, which prevents the candidate shaping what they see in the mark. Case information unrelated to the comparison is withheld. Proficiency testing has been strengthened. Statistical models estimating the strength of the evidence from the features present are under development and would replace a categorical identification with a likelihood, which is how DNA evidence is presented and is where several researchers argue the field should go. Testimony now avoids claims of absolute certainty in many jurisdictions.

Confidence in this evidence was substantially reduced by scrutiny beginning in the 2000s. A high-profile misidentification in 2004, in which several examiners linked a mark from a bombing in Madrid to a lawyer in the United States who had no connection to it, demonstrated that error was possible at the highest level of practice. A review by the United States National Research Council in 2009 found that claims of absolute certainty and zero error rate were not supported by research, and that the underlying assumption of uniqueness had not been tested in a way that supports individual identification claims. Subsequent work measured error rates directly, finding them low but not zero, and demonstrated that examiners are influenced by contextual information. Practice has changed, with blind verification and restricted case information now recommended, and claims of certainty moderated in testimony.

Prints are deposits of sweat, oils and amino acids, and development methods each target a different component, which is why the surface determines the technique. Comparison is an examiner's judgement about corresponding ridge features rather than a measurement. A 2004 misidentification and a 2009 review established that claims of zero error rate were unsupported, and practice changed in response.