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THE VALUE OF NEGATIVE
TESTIMONY Bureau of Alcohol, Tobacco & Firearms, Forensic Science Laboratory, Atlanta, GA
Bureau of Alcohol, Tobacco & Firearms, Forensic Science Laboratory, San Francisco, CA
In many instances, latent prints that are recovered from evidence either do not belong to the suspects or are not of value for identification. Testimony regarding negative results can be extremely useful during a trial. This paper will discuss factors affecting latent print recovery, the value of this type of testimony, and how to effectively present testimony for negative results.
Perspiration is approximately 99% water hence it is
transparent on the surface. The term latent print is used because latent means
hidden or difficult to see. To make a latent print visible, different techniques
are used to include chemical, powders and specialized high intensity light. Once
the latent print is made visible, it is photographed and in the case of a
powdered print, lifted from the surface and placed on a latent lift card. Porous
surfaces, which readily absorb water, include paper products and cardboard.
Latent prints that are
Non-porous surfaces do not readily absorb water. These items include various types of metals, plastics and glass. When a latent print is left on a non-porous object, it sits on the top of the surface, thus making the print extremely fragile as the slightest brush can destroy it's value for identification. "Failure to recover latent fingerprints does not in itself necessarily mean that the prints were wiped off. The absence of latent fingerprints on an evidentiary item may be due to numerous factors, each depending on conditions existing before, during, and after the finger touched or handled the surface examined." 2
An identifiable latent print can be defined as a set of friction ridges with sufficient detail and clarity to be positively identified to an individual. In order to successfully develop an identifiable latent print, four basic factors must occur: sweat, surface, contact and environment.3 There must be sufficient sweat on an individual's fingers to leave an adequate latent print. If there is too much sweat, the latent print will more likely be smudged. If there is not enough sweat, the examiner may not be able to develop the print.
Different individuals perspire at different volumes at
various times of the day. If a person is engaged in strenuous work or is
experiencing stress due to a risky task being performed such as the commission
of a criminal act, perspiration usually emanates at a greater rate than when the
person is at rest.4 The weather can also play a role in the sweat
factor because when the temperature becomes colder, "the sweat pores that exude
perspiration tend to close." 5 The surface is also a main factor in latent print development. The best surface for a latent print to be deposited on is a smooth, clean, flat surface. A surface that is textured, grooved, or dirty makes identifiable latent print recovery most difficult. Friction ridge continuity can also be affected by the surface to the object being handled. If a person is handling a crumpled piece of paper or plastic, the friction ridges that come in contact with the item will be continuous only if the particular item stays in the crumpled state. In most cases though, once pressure is relieved on the item, the paper or plastic will begin to spread out, resulting in a loss of friction ridge continuity.6
Once there is an adequate surface and a sufficient amount of sweat on the fingers, there must be good contact between the finger and the surface. If the finger grazes the surface or is not in contact with the surface long enough to deposit the perspiration, an identifiable latent print probably will not be deposited. Another problem caused by improper contact is pressure distortion. This problem is not as critical with porous surfaces as it is with non-porous surfaces. In the case of non-porous surfaces, if too much pressure is applied to the surface, the ridges may be smudged or distorted to the point where the examiner will not be able to use them for identification.
Probably the most important factor is the environment. Assuming that the first three factors have taken place, the environmental factor encompasses everything that happens to the latent print from the time it is deposited to the time that it is developed and preserved. "Temperature, humidity, and handling are the three primary factors within this group which may adversely affect a latent fingerprint." 7 Since latent fingerprints are 99% water, as soon as the fingerprint is deposited evaporation occurs. The rate of evaporation is directly proportional to the temperature and humidity conditions surrounding the latent print.
Regardless of whether a latent print identification was made or if no latent prints of value were recovered, a pretrial conference with the prosecuting attorney is essential. Without one, the prosecutor will proceed with the trial based solely on what is written in the laboratory report of examination. Usually in the case of a negative report, all the attorney will have to work with is a phrase slating that no latent prints of value were developed. Obviously, this is not sufficient to explain how the items were processed and more importantly, why no latent prints were found. The prosecutor must understand that the expert's testimony presents a more complete picture of the overall examination.
Often a defense attorney will try to use the defense that the defendant is innocent because his or her fingerprints were not developed on the evidence. The defense relies on the jury's preconceived notions regarding fingerprints that they have formulated by viewing entertainment media. The majority of jurors are usually under the impression that a person will leave an identifiable fingerprint on anything that he or she has touched. It is the responsibility of the prosecutor and the examiner to educate the jury on what is fact and what is fiction regarding the science of fingerprints. There are times when the defense will call the examiner to testify to his or her negative results. The testimony given should be the same as if the prosecutor had requested the testimony. Since the examiner is usually employed by a governmental entity, should a pretrial conference be scheduled with the defense attorney, the prosecutor should be notified of the request. In most instances, the prosecutor will wish to attend the pretrial conference.
By giving the jury an accurate understanding of latent print recovery, whether or not the testimony is requested from the prosecution or the defense, the examiner will be able to maintain an unbiased position. It is important, to remember that the examiner is on the witness stand to discuss how the examination was accomplished and what the results of the examination were. The guilt or innocence of the defendant is to be determined by the judge or jury. The results of the fingerprint examination are only to assist them in understanding the facts of the case.
In the Oklahoma City Bombing trial, "a prosecution expert testified that the fingerprints couldn't be found on several pieces of evidence to tie McVeigh to the blast that destroyed the Alfred P. Murrah Federal Building." 8 The FBI went so far as to rip the counter out of Elliott's Body Shop where a witness had seen McVeigh picking up the truck two days before the bombing. "The counter was transported to the FBI laboratory for analysis but the examiner was unable to find McVeigh's fingerprints. They also processed "co-defendant Terry Nichols' pickup truck, which prosecutors said Nichols drove to get McVeigh in Oklahoma City after McVeigh allegedly stashed a getaway car there three days before the blast," without success.
Under redirect testimony by the prosecution, the fingerprint expert insisted that someone could touch something and still not leave prints. "It's very common," he added. The fingerprint expert's job is to educate the jury as to the facts pertaining to the development of identifiable latent prints. Latent fingerprints are chance impressions. The examiner should not specifically tell the jury that the defendant is not innocent solely because his or her fingerprints were not developed on a particular evidentiary item. If the expert sufficiently testifies to the four factors listed above with a clear definition of what a latent print is, the jury should be able to make the connection themselves.
1) Midkiff, Charles R., "Lifetime of a Latent Print How Long? Can You Tell?", Journal of Forensic Identification. Vol.43No.4 July/August 1994, p.390. 2) Olsen, Robert D., Scott's Fingerprint Mechanics. Charles C. Thomas, Springfield, IL., 1978, pll7. 3) Wertheim, Pat A., International Association For Identification Advanced Ridgeologv Comparison Technique, unpublished, 1996. 4) Moenssens, Andre A., Fingerprint Techniques. Chilton Book Co., Radnor, PA, 1971, pl30-131. 5) Alien, Sharon, "Separating Fact From Fiction", Law Enforcement Technology. January 1992, p24-25. 6) Bib. 7) Olsen, Robert D., Scott's Fingerprint Mechanics. Charles C. Thomas, Springfield, IL., 1978, pl21. 8) Johnson, Kevin, "McVeigh's prints not on evidence, expert testifies", USA Today. May 16, 1997, p4A. 9) Ibid.
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Last modified: 04/18/08 |
