WASHINGTON, D.C.--FBI experts on DNA fingerprinting will now be permitted to testify that DNA from blood, semen, or other evidence at a crime scene came from a specific person. The new policy, announced at a press conference at FBI headquarters here yesterday, suggests that DNA analysis can now rightly be called "DNA fingerprinting," says Dwight Adams, chief of the scientific analysis section at the FBI laboratory in Washington, D.C. The term, he says, "invokes in the mind of the jury that we are identifying one individual to the exclusion of all others."
DNA experts had not been allowed to tell juries that DNA evidence nailed a suspect. Even in the O.J. Simpson murder trial, FBI forensic experts could only tell the jury that the chance was one in many billions that DNA evidence came from another person--implying that a handful of other people in the world could theoretically have committed the crime.
The FBI's new policy, which went into effect 1 October, states that if the likelihood of a random match is considerably less than 1 in 260 billion, the examiner can testify that the samples are an exact match. That's because advances in DNA analysis techniques and more complete data on the frequency of DNA patterns in different ethnic populations have sharpened estimates of a sample's uniqueness, FBI officials said.
The uncertainty in the calculations is vanishingly small, agrees James Crow of the University of Wisconsin, Madison, who chaired a 1996 National Research Council panel that assessed the forensic use of DNA. The danger in misidentifying the perpetrator, he notes, lies in "human frailty at the laboratory level"--so the best protection for a wrongly accused person is to insist that samples are retested. "The definition [of an exact match] is necessarily arbitrary but to me it seems reasonable," adds Crow. "I'll be interested to see how the courts react."