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No chapter abstract available.

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Forensic science is the application of scientific theories and methodology to practical problems, particularly legal issues or disputes. The forum for resolution of a legal issue is a trial, in which a judge presides as the source of law and a jury of citizens decides the facts. An “expert” witness can give opinion testimony to assist the jury. Testimony from scientific experts is the classic form of expert testimony. Admissibility simply means that the judge will decide whether the jury should hear and consider some particular evidence. Generally, evidence that tends to prove or disprove guilt or innocence should be heard. But sometimes, while some evidence may tend to prove guilt or innocence, it still may be too prejudicial for a jury to consider. The judge is the “gatekeeper” and must determine what evidence is admissible. As to expert witnesses, the judge must decide if the jury needs an expert to help them understand the issue, if the expert is offering relevant and reliable testimony that will provide proof of guilt or innocence, and whether the expert is qualified to give the testimony.

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The rule announced in Frye v. United States, 54 App. D.C. 46, 293 F. 1013 (1923) became the standard for judges to determine the admissibility of forensic science evidence. The court said that the science behind the testimony “must have gained general acceptance in the particular field in which it belongs.” While the general acceptance test relies on the opinion of scientists, it has been criticized because it ignores the speed at which science develops and because it delegates a judicial responsibility to non-lawyers. The general acceptance test lasted in most courts until 1993, when the U.S. Supreme Court decided the case of Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 U.S. 579 (1993) The Supreme Court held that the Federal Rules of Evidence superseded any ruling in Frye and announced a nonexclusive set of five criteria federal judges must use to decide the admissibility of forensic science evidence: whether the theory or technique can be and has been tested; the known or potential rate of error; the existence and maintenance of standards controlling the technique’s operation; publication (or lack thereof) in a peer-reviewed journal; and general acceptance in the relevant scientific community. The subsequent case of Kumho Tire Co., Ltd. v. Carmichael, 526 U.S. 137 (1999), held that the Daubert rule applies to all experts. In our system of federalism, States may apply their own rules of evidence, and while most States have adopted the Daubert standards, some States still use Frye or some variant of it.

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DNA has become the ultimate tool to identify the perpetrator of a crime. It is equally good at identifying the person who did not leave that evidence and is a valuable method to prove innocence as well as guilt. DNA has been able to exonerate many people who were wrongfully convicted. In many of those cases, innocent persons had been wrongfully convicted based on several traditional forensic science techniques. In almost a third of the exonerations, some form of forensic science was used to wrongfully convict them in the first place. This led Congress to commission the NAS to conduct a thorough study of the state of the forensic science in the United States. The 2009 NAS report concluded that, apart from nuclear DNA analysis, no other forensic method had been shown to consistently and reliably link evidence to a specific individual or source. The 350-page report found that many traditional forensic sciences lacked the empirical evidence to support their courtroom claims of individualization or identification.

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The criminal justice system has not thus far been successful in preventing unscientific testimony. When forensic science evidence is challenged, judges rarely exclude it and usually rely on the fact that it has been admitted in older cases. The reason for this failure relates to the basic difference between scientific training and legal training. The scientific method involves observation, questioning, forming a hypothesis, gathering data through experimentation, analyzing that data, and drawing conclusions. On the other hand, lawyers, and lawyers who become judges, are trained to use the concept of precedent and stare decisis – that the answer to present questions can always be found by looking to the past. The law is always biased in favor of the status quo, not because it is the correct result but because it should not be disturbed. When new science tries to enter into the legal system, it clashes with the judicial system’s reliance on precedent. When it comes to admitting evidence, the burden of proof is always on the party that wants to admit the evidence, and for forensic science, this means that the prosecutor must prove that the expert testimony meets the requirements of Daubert or Frye, depending on the jurisdiction. Nevertheless, some courts shift the burden to anyone opposing forensic science evidence that has been traditionally admitted. Recent amendments to the Federal Rules of Evidence may remedy that problem.

Pages: 83

No chapter abstract available.

Pages: 5

For decades, forensic science witnesses have testified that two samples, such as fingerprints, ballistics, or handwriting, are a “match” and that they are “100% certain” that no other person could have left the crime scene sample. The scientific reality is that no comparison of samples can result in 100% certainty. Using the term “match” implies, and is interpreted by jurors to mean, that there is no other person in the world who would have the same characteristics. Similarly, “certainty” claims are not scientifically based. Scientific conclusions, if possible, should be expressed in terms of probability or likelihood based on available data. Forensic science is becoming more reliant on quantitative analysis to validate evidence. Statistics are based on the collection and analysis of data, while probability is the likelihood that an event will occur based on data. The likelihood ratio is the ratio of two probabilities of the same event under different hypotheses. The best example is what has become the standard for testimony about DNA profiling, where the expert describes not a “match” but the likelihood of finding a person other than the defendant in the population with the same DNA characteristics as the crime scene DNA.

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DNA profiling is universally recognized as the standard against which many other forensic individualization techniques are judged because the probabilities of a false positive are quantifiable and often miniscule. DNA is in the nucleus of virtually every cell in the human body. The history of the discovery of deoxyribonucleic acid (DNA) was not related to the criminal justice system but grew out of pure biological research. James Watson and Francis Crick discovered that the structure of the DNA was a double helix, connected by four chemical bases called adenine (A), guanine (G), cytosine (C), and thymine (T). In 1985, geneticist Alec Jeffreys discovered a way to develop a DNA profile that would lead to its use in criminal justice. Various techniques have evolved for analyzing DNA, with the most common being short tandem repeat (STR) technology that intensifies the differentiation between one profile and another. Next-generation sequencing (NGS) is the latest advancement in forensic DNA profiling and allows analysis of multiple genetic markers, resulting in a more comprehensive profile. The FBI’s Combined DNA Index System (“CODIS”) is a computer software database of DNA profiles from offenders, unsolved crime scene evidence, and missing persons. CODIS uses STR analysis at 13 different locations on the human chromosome. If the crime scene sample and the CODIS profile have matching alleles at all 13 STRs, the DNA expert makes a statistical calculation of the frequency at which that occurs in the same ethnic group in the population. That frequency provides the basis for probabilistic testimony about the likelihood that another person with the same characteristics exists in the relevant population. DNA evidence is now universally admitted by courts in the United States; however, the possibility of human error in the collection or analysis of the sample can affect its admissibility.

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Microscopic hair examination and analysis have been taking place for a long time, but it has been discovered that they have a very limited capability to accurately identify individuals. Traditionally, comparing human hair is based on a microscopic comparison of the evidentiary sample with a known sample from the defendant. Based on a comparison of major and secondary characteristics, the examiner formulates an opinion as to exclusion or consistency. Although previously used by investigative agencies, it is now uniformly recognized that microscopic hair comparisons do not constitute a basis for a definitive identification and in fact have led to wrongful convictions. The National Academy of Sciences found no scientific support for the use of hair comparisons for individualization in the absence of nuclear DNA. The advent of DNA, and particularly the mtDNA method, has changed the field of forensic hair analysis from a microscopic examination into a DNA analysis of the cells in the hair. More recent developments have taken the science a step further. Scientists have now discovered that there is indeed identifiable nuclear DNA in the strands of hair even without a root.

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Fingerprint comparison is a form of pattern identification. The history of fingerprinting in general goes back hundreds of years. Its use in the criminal justice context is more recent, but fingerprint comparison testimony has been admitted as evidence in criminal prosecutions in the United States for over one hundred years. Courts have accepted the proposition that each person’s fingerprint is unique and that fingerprint comparison is almost infallible as a means of forensic identification. Fingerprint experts often declare a “match,” a positive identification, even stating that it is “100% certain” that the defendant is the person who left a crime scene fingerprint. Indeed, fingerprint comparison has been referred to as the “gold standard” of pattern identification. In recent years, that assumption of infallibility has been called into question. There are documented errors stemming from unclear or partial prints, examiner bias, and system issues within fingerprint databases. In its present state, fingerprint comparisons may not be entitled to the great weight that criminal courts and jurors have long ascribed to it. Strides have now been made to improve fingerprint databases and move to a more probabilistic assessment of the likelihood that a crime scene fingerprint was left by a particular suspect or defendant.

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Toolmarks result when a hard tool contacts a softer object, leaving marks or striations. Expert toolmark testimony is based on the idea that each set of markings is unique. Testimony about a variety of tools, such as screwdrivers and crowbars, has been admitted. Ballistics relates to firearms and is a form of toolmark comparison. Weapons are a prime source of evidence, and experts try to identify a particular gun as the source of bullets or cartridges from the crime scene or other ammunition components. Most firearms have a groove in the barrel, called rifling, to cause the bullet to spin for accuracy. The rifling leaves distinctive marks on the bullet. Marks are also left by the firing pin on the cartridge and by the ejector mechanism as it removes the cartridge. Using a dual microscope, examiners compare the marks on bullets, cartridge cases, and expended shells to a test firing of the suspected gun. Firearms comparison testimony has been routinely admitted in court. But the problem with ballistics is similar to the issues concerning other pattern identifications. The assumption of uniqueness is questionable, the standards for comparison are not well established, and the conclusions are subjective. Recent advancements in ballistics in the development of computerized databases of firearms and ammunition and in the development of three-dimensional imaging processes may provide a more scientific basis for ballistics testimony.

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Comparison of handwriting samples is called questioned document examination and involves the comparison of documents and writing to identify persons who are the source of the writing or to reveal forgeries. It is based on the claimed ability to determine the authorship of a piece of handwriting by examining the way in which the letters are inscribed, shaped, and joined and by comparing it to known examples of writing by a suspect. Handwriting examiners claim that no two people write alike and that no one person writes the same way twice. It was first widely accepted after it was part of the cornerstone of the prosecution case in State v. Hauptmann, the Lindbergh kidnapping case. Since then, it has been almost widely accepted as admissible. With electronic communication, the use of cursive writing has declined, and the use of handwriting comparison by law enforcement has likewise decreased, although cursive is still used to sign a number of legal and other documents that may be the subject of criminal investigation. As with several other such routinely accepted types of pattern evidence, scientists are now skeptical about the reliability of handwriting comparison expert testimony. The FBI and the U.S. Postal Service still have active handwriting analysis units and are pursuing new, more scientific methods of analysis.

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Digital forensics is the analysis of electronic information that is stored or transmitted in binary form. Electronic devices are everywhere in our society, and they all generate data, which may be important in criminal investigations. In the criminal justice context, the data may be evidence of a crime that was committed electronically, such as credit card or other fraud, but more often, it is substantive evidence that is relevant to other crimes, such as the suspect’s location, communications, or image or audio recordings. The data from electronic devices is often recoverable. The seizure of digital evidence by the police presents privacy concerns, and the courts have ruled that in many instances the police must obtain a particular search warrant before they can search digital data. The admissibility of digital evidence under Daubert standards must be established. However, there are now established standards for such evidence, and the National Institute of Standards and Technology tests many products and provides the basis for meeting the Daubert requirement for testing and proof of its error rate.

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A crime scene may often contain impressions from shoes or other footwear. Forensic footwear examiners attempt to identify the type of footwear that left the impressions and then compare those impressions to footwear associated with a suspect. Like fingerprints, footwear impressions may be patent and visible to the naked eye or latent, requiring some device or material to make them visible. Whether patent or latent, the impression must be preserved, either physically or photographically, for comparison by the examiner. The extremely large quantity and quality of available footwear make manual identification difficult, and software and computerized databases are being developed to assist in the process. Like other forms of pattern evidence, the opinions of forensic footwear examiners are subjective and can raise admissibility issues.

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No chapter abstract available.

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The primary use of forensic science evidence relating to fires is to attempt to identify the source of the fire and to determine whether and what type of accelerant was used to start or spread a fire. Traditionally, most experts who offer fire origin testimony, known as “cause and origin investigators,” are not scientists and have little or no scientific training. They are often local fire marshals who testify primarily based on their technical knowledge and what they learned “on the job” from other similar investigators. Origins of fires are difficult to determine since practitioners in the field recognize that no two fires will burn alike and the evidence has usually been destroyed by the fire itself. Cause determinations, especially for accelerants, proceed on a much stronger scientific footing. The presence of an accelerant is most often detected by its odor, and agencies use dogs or commercial electronic sniffers to detect areas for possible investigation. Gasoline or other petroleum-based products are the dominant form of accelerants used in intentionally set fires. Once an accelerant is detected, fire debris is examined in the laboratory to identify common petroleum-based products using gas chromatography. While the National Academy of Sciences Report was critical of the lack of scientific bases for fire testimony, in the past several years much has been done to attempt to improve and standardize forensic fire investigation, primarily through the development and use of the National Fire Protection Association Guide for Fire and Explosion Investigations.

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Bloodstains and bloodstain patterns are often the subject of forensic science testimony in assault or homicide cases. Initially, the investigative question can simply be whether a particular stain is blood or not. Preliminary and laboratory tests are available to determine if a stain is blood. More complex is the question of what conclusions can be drawn from a pattern of bloodstains or spattering. Patterns from the spattering of blood have been used to form the basis for testimony about such issues as the sequence of events, the distance from which a victim was shot, claims of self-defense, and any number of other homicide or assault dynamics. It may include the determination of whether a stain resulted from arterial spurting, blood dripping, aspirated (coughed) blood, back spatter, angular deposits, flight paths, or many other characteristics of the blood path that led to the recorded bloodstain pattern. Such an analysis involves complicated and technical issues of fluid dynamics. Unfortunately, untrained police witnesses without any training in fluid dynamics often purport to offer conclusions about the patterns that do not meet admissibility requirements.

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When a gun is fired, gases from the exploding materials contain particles from the explosion, especially the primer material, that can then condense on the shooter’s hands or clothing. Most of these particles are not visible to the naked eye, so police use adhesive lifters to remove the particles for testing. Special spectroscopic instruments known as scanning electron microscopy coupled with energy dispersive X-Ray spectrometry (SEM-EDS) can identify chemicals that are typically used in gunshot primers, such as lead, barium, and antimony. Testimony that gunshot residue was found on the defendant’s hands or clothing is often admissible, but there are limitations and concerns. The chemical particles are not unique to primers and may come from other sources, and there are no standards for determining how many particles are required to allow a gunshot residue conclusion. A gunshot residue analyst also cannot describe how or when the chemical particles were deposited.

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There are two primary issues regarding the admissibility of the testimony of a medical examiner or “coroner” regarding the cause and manner of death. The first is the qualifications of the expert. The position of coroner has historically been an elected or appointed government position with little or no requirements for medical expertise. A medical examiner is usually a physician and optimally a forensic pathologist. The second issue concerns the admissibility of medical examiner testimony as to the “manner” of death. Cause of death is a medical conclusion about the injury or illness that resulted in death. The manner of death is opinion testimony about how the injury resulting in death was inflicted. Administratively, that manner will be listed on the death certificate as natural, accident, suicide, homicide, or undetermined. It is not based on medical evidence but rather on ordinary facts supplied by the police or other nonmedical sources. While many courts routinely admit such opinions, they may not qualify under FRE 702 or Daubert analysis

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No chapter abstract available.

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While the ultimate use of forensic science is in the criminal courtroom, there are many important advantages of forensic science without regard to admissibility in a criminal trial. Forensic odontology is especially useful in the identification of human remains. Death certificates are important administratively for private matters and public health purposes. Although generally not permitted for private employers, the use of polygraphs may be valuable for obtaining or keeping government employment or for civil purposes. Drug testing may be used for employment or safety purposes without regard to any criminal use.

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While there are limitations on the admissibility of several forms of forensic science in a criminal trial, police can and do use some inadmissible forensic science techniques for investigative purposes. Many pattern identification procedures, such as fingerprints, footwear, microscopic hair analysis, or even bite marks, can effectively exclude possible suspects and narrow the field for further investigation. Polygraphs are also used to implicate or eliminate suspects. Other techniques can provide enough information to pursue a warrant for admissible evidence. Facial recognition software has inherent difficulties but can be an effective initial investigation tool. Genetic genealogy, combining DNA and genealogical information, has become a very effective investigation technique. While its results are not admissible, it can lead police to a suspect from whom they can usually obtain discarded confirming DNA samples.

Pages: 13

No chapter abstract available.

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Artificial intelligence (AI) is the use of computer systems to perform tasks that normally require human intelligence. Generative AI creates new content, like text or images, based on patterns in data. Generative AI is revolutionizing communication, work, and innovation. The use of generative AI in forensic science poses both opportunities and challenges for the criminal justice system. The Supreme Court has adopted a new rule of evidence, FRE 707, that requires AI-generated evidence to meet the same Daubert and FRE 702 requirements for all expert testimony. The current state of forensic science AI may not be able to meet those criteria. The court system may be reluctant to admit AI-generated evidence.

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It is apparent that there are significant issues with the admission of some forms of forensic science evidence in criminal cases. One basic problem in our adversarial system is the lack of defense objections or requests for Daubert or Frye hearings. When objections are made, many judges do not understand or are resistant to performing their gatekeeping responsibility. One remedy may lie in more meaningful forensic science education of lawyers in law school and more focused continuing education requirements for both lawyers and judges. A second pathway to improvement of forensic science practices, both in the laboratory and the court, may lie in increased legislative and regulatory supervision of the forensic science evidence system. While legislative efforts have generally not been forthcoming, there is some evidence that regulatory commissions can improve the forensic science system.