Unlike many forensic techniques that were developed empirically within the forensic science community, with limited foundation in scientific theory or analysis, DNA analysis is a fortuitous by-product of cutting-edge science. Eminent scientists contributed their expertise to ensuring that DNA evidence offered in a courtroom would be valid and reliable (e.g., in the 1989 New York case, People v. Castro), and by 1996 the National Academy of Sciences had convened two committees that issued influential recommendations on handling DNA forensic science.12 As a result, principles of statistics and population genetics that pertain to DNA evidence were clarified, the methods for conducting DNA analyses and declaring a match became less subjective, and quality assurance and quality control protocols were designed to improve laboratory performance.
DNA analysis is scientifically sound for several reasons: (1) there are biological explanations for individual-specific findings; (2) the 13 STR loci used to compare DNA samples were selected so that the chance of two different people matching on all of them would be extremely small; (3) the probabilities of false positives have been explored and quantified in some settings (even if only approximately); (4) the laboratory procedures are well specified and subject to validation and proficiency testing; and (5) there are clear and repeatable standards for analysis, interpretation, and reporting. DNA analysis also has been subjected to more scrutiny than any other forensic science discipline, with rigorous experimentation and validation performed prior to its use in forensic investigations. As a result of these characteristics, the probative power of DNA is high. Of course, DNA evidence is not available in every criminal investigation, and it is still subject to errors in handling that can invalidate the analysis. In such cases, other forensic techniques must be applied. The probative power of these other methods can be high, alone or in combination with other evidence. This power likely can be improved by strengthening the methods’ scientific foundations and practice, as has occurred with forensic DNA analysis.
The term “illicit drugs” is widely used to describe abused substances. Other terms that are used include “abused drugs,” “illegal drugs,” “street drugs,” and, in the United States, “controlled substances.” The latter term refers specifically to drugs that are controlled by federal and state laws.13