Frequently Asked Questions

Answered by Jane Siegel, Executive Director, and Jonathan H. Oberman, Associate Clinical Professor of Law, Innocence Project,
Benjamin N. Cardozo School of Law Clinic

Q: What is the chief misconception about DNA evidence?

There are several misconceptions. First, that DNA profiling is as specific as traditional fingerprints analysis. People tend not to grasp that when a suspect is included, and the inclusion is described as a "match," that refers only to a profile or small portion of the respective DNA taken from a suspect and recovered from a crime scene or evidence. Depending upon the given suspect's race or ethnicity the "matching" profile will occur with some degree of frequency--even if somewhat remote.

The strength of the inclusion thus depends upon the statistical probability of a random coincidental match. On the other hand, an exclusion is unqualified if the isolated DNA profiles do not correspond it means, absent references to any statistical analysis, that the suspect could not be the source of the crime scene sample.

Another misconception: DNA testing can only be done on some kind of biologic material (e.g., blood, saliva, semen, skin tissue, hair) that contains an individual's DNA. It cannot be done, for example, on a suspect's fingerprints.

Finally, many people don't realize that DNA testing can be done on old and even partially degraded material, although the value of the results may be compromised if the samples were contaminated as a result of having been improperly collected or stored.

Q: How widely is DNA accepted today by the public and in the criminal justice system?

DNA appears to be widely accepted--though not necessarily understood--by the public. It is almost uniformally accepted by state courts and is accepted in all federal courts.

Q: What generally are jurors' attitudes and reactions to DNA?

Testimony about the DNA testing can be confusing. Testimony about the underlying science--if too extensive, technical or academic--can befuddle jurors who may then disregard the significance of the results themselves. In general, jurors have given great deference to DNA results, except where reasonable doubts can be raised about the process by which the samples were recovered and/or tested.

Q: Is there opposition to DNA evidence in criminal trials?

There is basic agreement that the underlying science of the DNA testing is not open to serious challenge. Opposition, such as exists, may be directed to the specifics of the particular tests done or, reflect a reluctance on the part of the defendant to accept an inclusion or a prosecutor to accept an exclusion, especially where it might require reversal and dismissal of a case which had been previously tried to a conviction.

Q: What is the best way of retrieving a DNA sample (i.e., blood, saliva, hair, semen). Does it all offer the same information?

Hair poses particular challenges. The strands themselves will not generate a profile through application of standard DNA testing, which requires the presence of the hair follicle. New advances in mitochondrial DNA testing have produced accepted tests results in recent cases. This testing is prohibitively expensive and is, at this time, only being done by the FBI laboratory. Otherwise, equally valid DNA profiles may be isolated from each of the other bodily fluids mentioned (for semen to generate a DNA profile, the sample must contain sperm). Each can offer the same information with the same degree of precision. Clearly, taking a saliva sample from a suspect is the least physically intrusive.

Q: How often is DNA evidence incorrect and what are the reasons?

We are unable to estimate the error rate in part because far too many forensic laboratories that do DNA testing still are not subject to independent on-site inspection, double blind testing, and peer review. Results may be compromised by contamination of the sample that occurred during the collection process, or by a lab's failure to comply with its own testing protocols, or by some atmospheric or manual contamination in the lab.

Q: How good are DNA testing laboratories? Who monitors or regulates them?

This is an issue of ongoing concern. Barry Scheck and Peter Neufeld with the Innocence Project, for example, have for the last several years advocated for a standardized accreditation for forensic laboratories interested in doing DNA testing. They have thus far been unsuccessful in their efforts. Unfortunately, as a result, far too many labs are not being monitored or regulated. That is, forensic laboratories have escaped the kind of rigorous internal and external scrutiny that is applied to scientific laboratories in general.

Q: What are the costs of testing DNA?

Costs vary from lab to lab and depend upon the size and condition of the sample to be tested. The most simple--though still comprehensive and comparative--tests for a fairly well-sized, well-preserved sample will not cost less than $1,500.

Q: What are the civil liberties questions pertaining to DNA testing?

Civil liberties concerns exist with respect to the state's ability to obtain samples from a criminal suspect. Specifically, under what circumstances and after making what kind of factual showing should the state be able to compel an individual to provide biologic material that can be submitted for DNA testing (i.e., probable cause to believe the suspect committed the crime; reasonable suspicion to believe the suspect committed the crime; reasonable suspicion that the sample may produce material evidence in a particular case for which probable cause exists; reasonable suspicion that the sample may produce material evidence in some other case(s) in which the individual may be a suspect not withstanding the absence of additional proof). In addition, privacy concerns exist with respect to the uses the state may make of profiles stored in DNA data banks.

Q: What is the general status of DNA testing today?

DNA testing is done with great frequency in paternity cases. In addition, testing is being done with increased frequency in rape and homicide cases. Finally, numerous states have established DNA data banks. States vary in their requirement of who must be tested, ranging from all inmated in the prison system to only those who have been convicted of particular categories of offenses.

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