Because forensic science aims to glean information from a wide variety of clues and evidence associated with a crime, it deals with a broad range of tools and with evidence of highly variable quality. In general, the forensic science disciplines are pragmatic, with practitioners adopting, adapting, or developing whatever tools and technological aids they can to distill useful information from crime scene evidence. Many forensic science methods have been developed in response to such evidence—combining experience-based knowledge with whatever relevant science base exists in order to create a procedure that returns useful information. Although some of the techniques used by the forensic science disciplines—such as DNA analysis, serology, forensic pathology, toxicology, chemical analysis, and digital and multimedia forensics—are built on solid bases of theory and research, many other techniques have been developed heuristically. That is, they are based on observation, experience, and reasoning without an underlying scientific theory, experiments designed to test the uncertainties and reliability of the method, or sufficient data that are collected and analyzed scientifically.

In the course of its deliberations, the committee received testimony from experts in many forensic science disciplines concerning current practices, validity, reliability and errors, standards, and research.3 From this testimony and from many written submissions, as well as from the personal experiences of the committee members, the committee developed the consensus views presented in this chapter.

BIOLOGICAL EVIDENCE

Biological evidence is provided by specimens of a biological origin that are available in a forensic investigation. Such specimens may be found at the scene of a crime or on a person, clothing, or weapon. Some—for example, pet hairs, insects, seeds, or other botanical remnants—come from the crime scene or from an environment through which a victim or suspect has recently traversed. Other biological evidence comes from specimens obtained directly from the victim or suspect, such as blood, semen, saliva, vaginal secretions, sweat, epithelial cells, vomitus, feces, urine, hair, tissue, bones, and microbiological and viral agents. The most common types of biological evidence collected for examination are blood, semen, and saliva. Human biological evidence that contains nuclear DNA can be particularly valuable because the possibility exists to associate that evidence with one individual with a degree of reliability that is acceptable for criminal justice.

in National Research Council, Committee to Review the Scientific Evidence on the Polygraph. 2003. The Polygraph and Lie Detection. Washington, DC: The National Academies Press. It does not cover forensic pathology, because that field is addressed in Chapter 9.

3

A complete list of those who provided testimony to the committee is included in Appendix B.



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