When DNA crime evidence is strong, prosecutors can attest that only one person out of as many as a billion is likely to have DNA matching that found at the crime scene.  This can make a persuasive case when combined with other evidence.  But are those odds correct?  Scientists said in the 1980s that the odds of two people having the same DNA profile were about 9 billion-to-1.  Today, the odds vary, depending on the expert.

Ronald Goodbread, a Washington, D.C. defense lawyer, says he’s heard experts cite probabilities ranging from 184 billion-to-1 to 3,000-to-1.  Because the figures are still disputed, Goodbread says the test is a questionable means of proving guilt beyond a doubt.  “Show me a system with that kind of margin of error, and I’ll show you a system I don’t want my life hanging on,” says Goodbread.

“This is an area in flux, and changes are made year to year or month to month,” says Mark Bloom, assistant director of the DNA learning center at New York’s Cold Spring Harbor Lab.  “People will flip-flop back again before DNA is used consistently in U.S. courts,” he says.  “If you inject DNA into a jury, the lay person says ‘Aha.  Super science.  Guilty.’  It’s light-years from that,” he says.

There have been major breakthroughs in the application of DNA testing in the conviction of offenders.  DNA testing was used for the first time in 1987 to secure a rape conviction in an Orlando, Florida court case.  Since then, DNA testing has been used in more than 24,000 American criminal cases.  In April of 1994, convicted murderer Timothy Spencer died in Virginia’s electric chair.  Spencer was the first person to be executed in a case based on DNA evidence. It is increasingly common to have DNA evidence presented in cases dealing with assault charges, particularly those of a sexual nature.

Prosecutors rarely hinge a case solely on DNA testing, says Rockne Harmon, deputy district attorney for Alameda County in Oakland, CA.  “It’s not like a fingerprint, but it has powers of its own when you need it.”  Both prosecutors and defense attorneys agree the technique can be flawless in excluding criminal suspects.  You don’t have to deal with statistical probabilities.  You either have a match or you don’t.  Out of 8,000 cases studied at the FBI lab, 35% of conclusive results eliminated the suspect.  Critics charge that prosecutors and the FBI claim more accuracy for the testing techniques than they warrant and they inflate probability figures to downplay the chances of a mismatch.

DNA evidence is especially useful in rape cases.  The FBI and Scotland Yard report that one-third of all suspects in rape cases are released before booking because DNA evidence exonerates them.  Until recently, few defense attorneys could refute the prosecutions’ complex calculations.  That left juries in awe of the apparent accuracy of DNA tests.  Most trials where genetic evidence is admitted end in conviction, and in only a handful of cases has the jury chosen to ignore DNA crime data.

In recent years, defense lawyers have grown more successful in attacking DNA profiling.  The debate has left appellate courts confused.  In most states, scientific evidence is admissible only when it has “gained general acceptance” in the scientific community.  That standard was recently overturned by the U.S. Supreme Court, giving judges more latitude in accepting such evidence.  California, however, still abides by the old restrictive standard, and at least one state appeals court has ruled DNA evidence inadmissible.

Within the decade, the faster PCR genetic test, coupled with more discriminating techniques, will require even less tissue and produce far more certain odds of a match.  Though DNA testing has been used in only a small fraction of rape and murder cases, juries are beginning to expect genetic evidence.

Pennsylvania’s Superior Court ruled in December of 1992 that every accused rapist in the state is entitled to DNA testing in an effort to prove innocence.  The ruling could force the state’s prosecutors to test DNA in all rape cases.  Today, most states allow DNA testing in court, even if its admissibility is not consistent from state to state.

The costs involved – both time and money – to prove a suspect’s guilt in court is a strong disadvantage of DNA fingerprinting’s use as conviction evidence.  Countless dollars have been spent on expert witnesses, not to mention lawyers, in a series of pretrial admissibility hearings around the country.  After weighing the competing claims of scientific experts, one court will find DNA crime evidence admissible, while the next court, often hearing from the same experts, will conclude just the opposite.