Forensic Science

A. Criminalistics

The majority of the forensic services provided in a robust crime laboratory are of a discipline called criminalistics. Put simply, this area of forensic science seeks to process physical evidence collected from a crime scene and produce a final report based on analysts’ findings. It is also the broadest category of forensic science, with many subspecialty units and much expertise. The easiest way to classify these services is to divide them by the units typically found in a robust crime lab: controlled substances, serology/biological screening, DNA, trace analysis, firearms/explosives, tool-marks, questioned documents, latent prints, and toxicology. While a vast amount of different analyses fall within these areas, these highlighted areas are nonexhaustive. It is also important to note that the widest amount of contemporary controversies revolve around many of the more subjective analyses performed by these analysts.

Over the last decade, the Bureau of Justice Statistics (BJS) commissioned a census of publicly funded crime labs to gain a better understanding of the collective trends in forensic services in the United States. By order of usage of service, controlled substances examination has persisted in being the most requested over the years of the census (Durose, 2008). Simply put, these requests concern seized substances thought to be illicit or unidentified controlled drugs. To perform the examination, analysts use a two-pronged process to first screen substances and then use this preliminary data to run a confirmation analysis if the initial one screens positive for a controlled substance. This secondary analysis has the power to examine the unknown substance both qualitatively and quantitatively with high levels of statistical certainty. Thus, at the end of a controlled substance analysis, investigators will learn the consistency of the substances submitted for testing, down to their molecular makeup. For example, if an unknown white powder is examined, controlled substance analysis will show the different components that constitute that powder and to what extent these components make up the whole sample—perhaps 85% cocaine, 5% lidocaine, and 10% baking powder (sodium bicarbonate).

This differs slightly from toxicological services, as the analyses in this area serve to qualify and quantify controlled substances and their metabolites in biological matrices (e.g., blood, saliva, hair, urine, and vitreous fluid of the eye), as well as toxic substances (e.g., mercury, arsenic, and cyanide), alcohol, over-the-counter products, and many other foreign compounds to the body. Depending on the circumstances, investigators typically request only a certain subset of toxicological examinations to be performed, as a full “tox” screen is costly and wasteful. In particular, two situations call for a more comprehensive toxicological examination: in cases of offender/probationer/parolee drug screening and in post-mortem toxicology. In the BJS census, toxicological services were requested a far second (298,704 requests in 2002; 251,585 requests in 2005) behind controlled substances (844,183 requests in 2002; 855,817 requests in 2005). There is limited subjectivity in these areas of forensic science— thus, controversies are limited to individual cases.

Latent print analysis seems ubiquitous in forensic science and police investigations. Examining visible (patent) or invisible (latent) fingerprints and comparing them to known samples or a computer database called AFIS (Automated Fingerprint Identification System) is a long-standing tradition in the field, and is the third most common type of request for forensic services (Durose, 2008). Based on the premise that no two fingerprints are alike—even identical twins have fingerprints that differ—the criminal justice system as well as private security firms have invested heavily in a fingerprint-driven identification system. Using proficiency tests, or controlled examinations designed to gauge the accuracy and reliability of forensic analyses, researchers have proven to be very accurate in their identification, given typical casework circumstances. Thornton and Peterson (2002) find that the existence of misidentifications is a rare event in typical casework (fewer than 0.5%of comparisons); correct identification lies within the 98%–99% range under normal circumstances. Fingerprints are among only a few other analytical tests, such as DNA typing and blood typing, that share such high success rates.

Firearm and toolmark analysis, the next most requested service, is an example of a subset of forensic analysis that contains elevated amounts of subjectivity, which increases the likelihood of error. Shotgun shells and shot pellets, discharged bullets, bullet casings, and any sort of firearm and its ammunition can be examined to understand the origin of a spent bullet, the trajectory of shots fired, and much more. The physical construction of firearms and their mechanisms make relatively unique impressions on fired bullets suitable for these analyses. Forensic science examinations dealing with toolmarks work in a remarkably similar manner. Impressions left by screwdrivers, crowbars, knives, saws— any tool imaginable in a garage—can give investigators an idea of what tools were used in the commission of a crime. If these tools can be identified, additional evidence left on these objects may be collected, if found.

It is true that as time passes, unique wear and tear on these items may produce remarkably unique impressions on objects (e.g., bullets, walls, bone, etc)—especially when these items are frequently used. When this occurs, the ability of forensic firearm and toolmark examiners to make a determination of whether the suspect impression embedded in an object shares a “common origin” with a sample impression made by the firearm/tool in a laboratory increases in confidence. Regardless whether if these conditions are met or not, these forensic examiners have good success in making these determinations; however, their success can wane in comparison to objective analyses such as DNA testing and blood typing. It is important not to overweigh the probative value of these examinations, especially when environmental conditions such as decay or damage make these analyses exceedingly more difficult.

DNA analysis, what has become the gold standard in forensic identification, is the next most requested service in the United States (Durose, 2008).According to the BJS, this service has remained the most backlogged during the census of publicly funded crime labs in the country. This should not be surprising, as this type of forensic analysis is demanding on both human and operational resources. While the field has come a long way from the origins of the use of DNA in the criminal justice system over three decades ago, the average time to complete these requests is typically much longer than for any other forensic service. For example, a typical forensic toxicological analysis may take anywhere from a week to a month, but comparing DNA samples from a suspect or several suspects to biological samples gathered from a crime scene may take anywhere from a few months to a year. Many times, if backlogs become a surmounting problem and local or state funding permits, outsourcing to private labs may be an option. In fact, about 28% of the crime labs included in the BJS census have outsourced their DNA casework to private labs (Durose, 2008).

The value of DNA analysis is twofold: (1) Several kinds of DNA analyses are embraced by robust methodologies that include error rates that can be measured, calculated, and interpreted to yield results that are concrete and objective. These results can be interpreted to estimate the likelihood of both a false positive (e.g., the likelihood of finding a “match” when, in fact, the samples from a crime scene and a suspect do not “match”) and a false negative (e.g., the likelihood of not finding a “match” when, in fact, the samples from a crime scene and a suspect should “match”). (2) These types of requests also have the power to provide exculpatory and inculpatory evidence with the same amount of certainty, accuracy, and reliability. Both types of evidence are equally important in criminal justice, particularly when a person’s freedom is on the line: exculpatory evidence includes any proof of an individual’s innocence, while inculpatory evidence provides proof of guilt.

Even years after a crime occurs, DNA analysis has proven itself to be the chief piece of analysis in many criminal cases. The past few decades have seen wrongful convictions overturned by DNA analyses at the cost of proving other forensic science evidence (or at least the interpretation of this evidence) wrong. Saks and Koehler (2005) point out that forensic science testing errors and false or misleading testimony by forensic expert witnesses are the second and fifth most common issues (respectively) in the wrongful conviction cases overturned by Project Innocence. This organization consists of a group of attorneys and advisors working pro bono that have been highly critical of many components of the criminal justice system, including a variety of areas in forensic science. Since the late 1980s, over 225 convicted felons typically serving life sentences have been exonerated by the efforts of Project Innocence using DNA analysis as the cornerstone of their litigation. On their Web site and in their promotional literature, Project Innocence echoes Saks and Koehler’s calls for reform in forensic science, particularly within areas that only give limited probative value. This includes much of the remaining facets of criminalistics not previously discussed: serology and biological screening, trace evidence analysis (e.g., hairs, fibers, glass, paint, etc.), impressions (e.g., bite marks, shoeprints, tire marks, etc.), fire and explosive examination, and questioned documents.

Each of these areas of analysis has its strengths and weaknesses, but all of them have been shown to assist investigators in their casework. Serology and biological screening is an example of a subset of forensic services that allows any investigator to narrow down the possibilities of suspects or helps the investigator understand the circumstances and nature of the event(s) in question, yet it has limited probative value. While a variety of these forensic services are able to produce results with reliable statistics and defined error rates, critics remain steadfast that these results can be misleading to jurors. Blood grouping methods are a good example: These methods allow analysts to examine a sample of blood and produce a report that identifies the blood type of the “donor.” In stark contrast to the cost and effort of DNA analysis, these reports can be produced rapidly and at a low price. The issue, however, becomes the lack of power these analyses have in narrowing suspects with a good degree of certainty, as many people share the same blood type. “Presumptive tests” for suspected semen and saliva samples are examples of less powerful biological analyses that can yield useful results, giving investigators reasonable evidence that these samples do, in fact, consist of seminal fluid or saliva. If there is sufficient biological material and these samples are viable enough to run DNA analysis (e.g., the material has not been contaminated or degraded below qualifying levels), further analysis can be run to refine these preliminary results. Forensic analysts may also choose to use other methods, such as microscopy and species typing, to refine these results if DNA analysis is not an option.

Other kinds of forensic tools, such as particular types of trace analysis and questioned document analysis, do not have as good a track record of producing reliable, accurate, and powerful results. Observers, however, should not cast them off as not being useful. For example, if an analyst were to find a hair in the trunk of a car bound to a piece of duct tape that was consistent with a victim’s head hair, the car owner would have a lot of explaining to do. This is not to say that this hair couldn’t have come from another source—in fact, the analyst would be hard-pressed to come up with a statistic of the likelihood that the hair came from the victim’s head. If, in fact, the analyst offered this statistic, it would be a disservice to a jury, the defendant, and even the victim since this information is uncertain and not based on sound statistical principles. If DNA material—whether nuclear DNA material or a kind called mitochondrial DNA material—were available for examination, then analysts would gain the power to include specific statistics in the present case to aid in the interpretation of the findings. Otherwise, a certain degree of caution should be used in interpreting the results and weighed accordingly when making a decision based on the information found in a final report.

In particular, questioned document analysis has received a significant amount of criticism, particularly in its ability to determine “matched” writing samples (or more accurately stated, consistent writing samples). Proficiency testing has proven to yield weak results in this area (see Peterson & Markham, 1995b). Yet, handwriting comparisons are the most commonly requested service in the area of questioned documents. Based on the reasonable assumption that people’s handwriting evolves over time, and that writing habits contain idiosyncrasies, both conscious and subconscious, analysts look for consistencies in writing samples for particular classes and characteristics of writing behavior. This holds true even when a person tries to disguise his or her writing to conceal authorship. For the most part, these services are more critical in civil trials where the burden of proof does not have to meet the “beyond a reasonable doubt” standard. Other types of questioned document analysis can fortify these results to offer more resolute findings. These include the analyses and comparisons of paper, inks, and printer and typewriter output. It must be stated, however, that few of these analyses come with the ability to include standard statistics and error rates, leaving them open to the aforementioned criticism.

While the above is not an exhaustive list of forensic services performed by many crime labs, it should offer a sampling of analyses that make up a spectrum from objective to subjective. While those from the subjective end of the spectrum may not be able to conclusively yield the proverbial finger pointed at a wrongdoer or give black-and-white answers, they can further clarify what occurred or did not occur with a series of events under investigation. Obviously, very few pieces of evidence can offer a smoking gun, so to speak, on their own. It is not the sole responsibility of the forensic analyst to make this clear; it is the responsibility of all of the key players in the courtroom work group—judges, prosecutors, attorneys, jury foreman, and the jury—to use their role to get the most out of each analysis, report, and expert testimony to be able to reach a just verdict. While many of the critiques of the more subjective aspects of forensic science merit close attention, the importance should be stressed on the proper weighing of this evidence when offered at trial. As mentioned above, these analyses do hold scientific value but only to a limited extent. The results must be weighed carefully with all of the other evidence, testimony, and circumstances about a particular trial in question.

While the forensic services at a crime lab play an important role in contemporary criminal justice and civil courts, other key services are offered outside of the crime lab that are important to mention. Two areas in particular stand out—forensic pathology, since these services are utilized so regularly, and forensic anthropology, for its topical importance in solving identification mysteries worldwide. The following two sections describe these aspects of forensic science, often considered off in their own realms and separate due to where they are organizationally located, in the government (pathology, and a minor part of anthropology) and in academia (anthropology).

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