Date: May 2, 1996
Contacts: Dan Quinn, Media Relations Associate
Darice Griggs, Media Relations Assistant
(202) 334-2138; Internet <>



WASHINGTON -- While confirming that the science behind DNA forensics is valid, a new report* from a committee of the National Research Council recommends new ways of interpreting DNA evidence to help answer a key question for jurors -- how likely it is that two matching samples came from different people.

The report advances a new combination of formulas to calculate the likelihood that a DNA match could be in error, and recommends ways to protect suspects from false incrimination because of improper handling of evidence.

"When a DNA sample from a suspect matches that found at a crime scene, we know one of two things has happened: either the samples came from the same person, or there has been a coincidence," said committee chair James F. Crow, professor emeritus of genetics, University of Wisconsin, Madison. "Courts need to use the principles of population genetics to tell jurors just how unlikely it is that two matching DNA samples have appeared by coincidence."

Calculating the chances of a random match has been complicated by the knowledge that the frequency of individual genetic profiles varies among different racial and ethnic groups. The Research Council's 1992 report on DNA technology in forensic science concluded that scientists did not know enough about these variations to apply them with confidence when calculating the likelihood of a match. The report advanced a conservative, interim formula to be applied to any suspect, regardless of that person's racial or ethnic background. This formula was designed to be favorable to the defendant, to lessen the chances of a false conviction.

The new report says that this so-called "interim ceiling principle" is no longer necessary and recommends a set of statistical calculations that takes population structure into account. People of the same race are more likely to have a similar genetic makeup than people of different races, and data bases of DNA profiles are providing an ever clearer picture of the genetic makeup of different racial and ethnic groups. If the race of the person who left the evidence sample is known, the committee recommends using the data base for that person's race in the calculations. The report also offers formulas for cases where the racial source of the DNA crime scene sample is not known, or where the suspect is from a segment of the population for which the data base is not yet sufficient.

The new approach to analyzing DNA evidence reflects the increasing volume and sophistication of DNA profile data bases. "The technology for DNA profiling and the methods for estimating frequencies and related statistics have progressed to the point where the reliability and validity of properly collected and analyzed DNA data should not be in doubt," the report said.

DNA -- or deoxyribonucleic acid -- is the inherited, coded genetic material that determines much of one's appearance and physiology. Forensic scientists extract DNA from samples of blood, semen, saliva, skin, or hair follicles found at the scene of the crime and develop a profile by analyzing several locations in the DNA where individual variations are common. This profile is compared with DNA taken directly from the suspect. If the profiles do not match, the suspect is cleared -- as happens in about one-third of all rape cases, for example. If the samples match, the weight of the evidence depends greatly on an understanding of the likelihood of such a match. This question has been debated since DNA was first introduced into courts in the mid-1980s. The committee hopes its report will greatly narrow the range of uncertainty of the calculated likelihood.


The report underscores the importance of proper handling of evidence from the point it is collected at the crime scene through processing at the laboratory. Because no amount of effort can completely eliminate the risk of laboratory error, the best protection an innocent suspect has is the opportunity for an independent retest, the report says. Whenever feasible, forensic samples should be divided into two or more parts as soon as possible after collection and the unused parts should be retained to permit additional tests. Each portion should be stored and handled separately. If a suspect desires a re-analysis of the DNA sample, the additional tests should be performed independently, preferably in a different laboratory with different personnel.

Judging that it is unfeasible to estimate the likelihood of laboratory error, the committee made recommendations to improve laboratory performance and accountability. Laboratories should adhere to high-quality standards like those developed by professional organizations such as the DNA Advisory Board and the Technical Working Group on DNA Analysis and Methods, and they should make every effort to be accredited for DNA work by the American Society of Crime Laboratory Directors. Labs also should participate regularly in proficiency tests, and the results should be available for court proceedings.


"In the not distant future, distinguishing even close relatives like brothers will be routine, and DNA profiles will be treated much as fingerprints are today," Crow said. To reach that point the committee recommended research into the identification and validation of more and better ways of examining DNA profiles, with a view to making each profile unquestionably unique.

The committee also recommended that behavioral research be carried out to identify reasons why a potential juror might misinterpret evidence on DNA profiling and to assess how well various ways of presenting expert testimony on DNA can reduce any such misunderstandings.

The study was funded by the National Institute of Justice, the State Justice Institute, the National Science Foundation, the National Institutes of Health, and the Department of Energy. The National Research Council is the operating arm of the National Academy of Sciences and National Academy of Engineering. It is a private, non-profit institution that provides independent advice on science and technology issues under a congressional charter. A committee roster follows.

*Pre-publication copies of The Evaluation of Forensic DNA Evidence are available from the National Academy Press at the mailing address in the letterhead; tel. (202) 334-3313 or 1-800-624-6242. The cost of the report is $40.00 (prepaid) plus shipping charges of $4.00 for the first copy and $.50 for each additional copy. Reporters may obtain copies from the Office of News and Public Information at the letterhead address (contacts listed above).

NOTE FOR TELEVISION: A video news release will be available via satellite and on Beta SP cassette. The satellite feeds will be on Thursday, May 2, from 12:45 p.m. to 1 p.m. EDT on Telstar 401 Channel 3, audio 6.2 and 6.8; and on Friday, May 3, from 12:45 p.m. to 1 p.m. EDT, also on Telstar 401 Channel 3, audio 6.2 and 6.8. Videocassettes may be obtained by calling GVI at (202) 293-4488.

Commission on Life Sciences

Committee on DNA Forensic Science

James F. Crow, Ph.D.(1)(2) (chair)
Professor Emeritus of Genetics
University of Wisconsin

Margaret A. Berger, J.D.
Professor of Law
Brooklyn Law School
Brooklyn, N.Y.

Shari S. Diamond, J.D., Ph.D.
Professor of Psychology and Criminal Justice
University of Illinois

David H. Kaye, J.D.
Regents' Professor of Law
College of Law
Arizona State University

Haig H. Kazazian Jr., M.D.(2)
Chairman of the Department of Genetics and Seymour Gray Professor of Molecular Medicine in Genetics
University of Pennsylvania School of Medicine

Arno G. Motulsky, M.D. (1) (2)
Professor of Medicine and Genetics
University of Washington

Thomas A. Nagylaki, Ph.D.
Professor of Ecology and Evolution and of Genetics
University of Chicago

Masatoshi Nei, Ph.D.
Evan Pugh Professor of Biology and Director
Institute of Molecular Evolutionary Genetics
Pennsylvania State University
University Park

George F. Sensabaugh Jr., D.Crim.
Professor of Forensic Science and Biomedical Sciences
School of Public Health
University of California

David O. Siegmund, Ph.D.
Professor of Statistics and Associate Dean for the Natural Sciences
Stanford University
Stanford, Calif.

Stephen M. Stigler, Ph.D.
Ernest DeWitt Burton Distinguished Service Professor of Statistics
University of Chicago


Eric A. Fischer, Ph.D.
Study Director

(1) Member, National Academy of Sciences
(2) Member, Institute of Medicine