Use of DNA Evidence

THE USE OF DNA EVIDENCE

The Frye rule ( Frye v. United States , 1923) concerns the admissibility of scientific evidence. The principle states that new scientific tests must be generally accepted in appropriate scientific circles before evidence from them is admissible in courts. In addition, a “helpfulness” standard is applied in some states, which involves the use of expert witnesses to assist the court in interpreting facts from scientific evidence. Recent court cases have almost all allowed DNA testing to be admitted into evidence, although there have been occasional exceptions. By 1996, DNA evidence had been admitted in more than 2500 criminal cases in the United States. There are relatively few U.S. government labs doing DNA testing. Accredited private labs perform most forensic work, and these services are available to both the prosecution and defense.

The main impact of DNA technology has been the far greater certainty with which individuals can be associated with or excluded from a particular crime than was possible with traditional blood tests. Experience has shown that if DNA testing is given as evidence, there is a higher probability of conviction than without DNA testing. DNA evidence is commonly used in cases of rape. However, in most cases of rape, the accused admits knowing the alleged victim and identity is not an issue. DNA testing can also be used by law enforcement to narrow the number of possible suspects, given that simple DNA testing protocols can determine sex and racial characteristics.

The British judicial system has led the way in DNA testing. In one early case, DNA testing was used to exclude the person first suspected of the sexual assault and murder of a young girl. But to find the real perpetrator, the police screened more than 5000 men in the village by blood testing (ABO and phosphoglucomutase), only to find no match with anyone. Ironically, the murderer was discovered because it was revealed that he had paid another man to give blood for testing. DNA testing subsequently confirmed, with high probability, that his DNA matched that of the semen sample taken from the victim. A conviction was finally obtained. Overall, there is considerable public support in Britain for maintaining DNA profiles on the entire population.

In the United States, a national DNA data bank is presently maintained by the FBI and is often screened by computer searches to find suspects. Although some worry about invasion of privacy, those who feel that liberty includes the freedom to walk down the street without being assaulted regard these developments favorably. Can DNA information be misused? The answer, of course, is that any information can be abused. People with different racial or genetic characteristics have been persecuted in the past. Then again, detailed DNA sequence information is hardly necessary for identifying people by race. In practice, the vastly increased accuracy of DNA testing compared to, say, blood group analysis means that unique individual identification is usually possible and hence racial bias is largely excluded in DNA testing.