Forensic scientists have used fingerprints in criminal investigations as a means of identification for many years. Fingerprint identification is one of the most important criminal investigation tools due to two aspects of fingerprints: their persistence and their uniqueness. The word persistence refers to the fact that fingerprints do not change over time. The friction ridges which create fingerprints are formed while inside the womb and grow proportionally as the baby grows. Permanent scarring is the only way a fingerprint can change. The word uniqueness acknowledges that fingerprints are unique to an individual. Even identical twins have been shown to have different fingerprints.
Types of Prints
In general, the purpose of collecting fingerprints is to identify the fingerprint-leaver. This person may be the suspect, a victim, or even someone else who happened to be at the place where the print was found. There are three types of fingerprints that can be found: latent, patent, and plastic. Latent fingerprints are made of the sweat and oil on the skin’s surface. This type of fingerprint is invisible to the naked eye and requires additional processing in order to be seen. This processing can include basic powder techniques or the use of chemicals. Patent fingerprints can be made by blood, grease, ink, or dirt. This type of fingerprint is easily visible to the human eye. Plastic fingerprints are three-dimensional impressions and can be made by pressing your fingers in fresh paint, wax, soap, or tar. Like patent fingerprints, plastic fingerprints are easily seen by the human eye and do not require additional processing for visibility purposes.
Surface Characteristics and Collection Methods
Characteristics of the surface in which the print is found are important in deciding which collection methods should be employed on scene. The general characteristics of the surface are: porous, non-porous smooth and non-porous rough. The distinction between porous and non-porous surfaces is their ability to absorb liquids. Liquids sink in when dropped onto a porous surface, while they sit on top of a non-porous surface. Porous surfaces include paper, cardboard, and untreated wood. Non-porous smooth surfaces include varnished or painted surfaces, plastics, and glass. Non-porous rough surfaces include vinyl, leather, and other textured surfaces. For porous surfaces, scientists sprinkle chemicals like ninhydrin over the prints and then take photographs of the developing fingerprints. For non-porous smooth surfaces, experts use powder-and-brush techniques, followed by lifting tape. For rough surfaces, the same powdering process should be used, but instead of using regular lifting tape for these prints, scientists use something that will get into the grooves of the surface. A gel-lifter or Mikrosil (a silicone casting material) can be used effectively to lift a print on these surfaces.
Analysis of Collected Prints
Once someone collects the print, the analysis can begin. During analysis, examiners determine whether there is enough information present in the print to be used for identification. This includes determining class and individual characteristics for the unknown print. Class characteristics are the characteristics that narrow the print down to a group but not an individual. The three fingerprint class types are arches, loops, and whorls. Arches are the least common type of fingerprint, occurring only about 5% of the time. This pattern is characterized by ridges that enter on one side of the print, go up, and exit on the opposite side. Loops are the most common, occurring 60-65% of the time. This pattern is characterized by ridges that enter on one side of the print, loop around, and then exit on the same side. Whorls present a circular type of ridge flow and occur 30-35% of the time. Individual characteristics are those characteristics that are unique to an individual. They are tiny irregularities that appear within the friction ridges and are referred to as Galton’s details. The most common types of Galton’s details are bifurcation, ridge endings, and dots or islands.
Comparison of Prints
After an examiner completes the analysis, they compare the unknown print side by side with a known print. The unknown print is the print found at the crime scene, and the known print is the print of a possible suspect. First, the class characteristics are compared. If the class characteristics of the two prints are not in agreement, then the first print is automatically eliminated. If this is the case, another known print may be compared to the unknown print. If the class characteristics appear to match, the examiner then focuses on the individual characteristics. They look at each individual characteristic point by point until they have found a possible match.
Evaluation of Comparison
After the examiner completes the comparison, they can make a proper evaluation. If there are any unexplainable differences between the unknown and known fingerprints, then they can exclude the known fingerprint as the source. This means that if the class characteristics are in disagreement, then the conclusion would be exclusion. However, if the class characteristics as well as the individual characteristics are in agreement and if there are no unexplainable differences between the prints, the conclusion would be identification. In some cases, neither of these conclusions is possible. There may not be a sufficient quality or quantity of ridge detail to effectively make a comparison, making it impossible to determine whether or not the two prints came from the same source. In these instances, no conclusion can be made and the report will read “inconclusive.” The three possible results that can be made from a fingerprint examination are therefore exclusion, identification, or inconclusive.
Verification of the Evaluation
After the first examiner reaches one of the three conclusions, another examiner must verify the results. During this verification process, the entire exam is repeated. The second examiner does the repeated exam independently from the first exam, and for an identification conclusion, both examiners must agree. If they agree, the fingerprint evidence becomes a much stronger piece of evidence if and when it goes to court.
Databases such as AFIS (Automated Fingerprint Identification System) have been created as ways of assisting the fingerprint examiners during these examinations. These databases help provide a quicker way to sort through unlikely matches. This leads to quicker identification of unknown prints and allows fingerprints to be as widely used as they are in criminal investigations.