SAN FRANCISCO — Ruling in a "cold hit" murder case, the California Supreme Court decided Monday that prosecutors may tell juries in all cases of the rarity of finding a defendant's DNA "match" in the general population even when a database search has increased the likelihood.
The decision, written by Justice Ming W. Chin, was largely a victory for prosecutors, who favor using the rarity statistic because it often suggests a staggeringly small chance that crime scene evidence would fit a defendant's genetic profile at random.
But the state high court also opened the door for the defense to win admission of a second, more conservative calculation in cold hit cases, when a suspect is first identified through a database search.
The odds of a DNA match in those cases are higher because the chance rises with each comparison to a database profile.
"The database match probability may also be admissible," Chin wrote in a footnote.
Trial judges and prosecutors have often cited an earlier California appellate case in saying that the database statistic was not admissible, a practice Monday's ruling may end, some defense attorneys said.
The court also held that a 26-year delay in bringing Dennis Louis Nelson to trial for the murder of Ollie George, 19, did not violate his rights. Nelson was arrested after DNA from the crime scene was tied to his DNA profile in the state offender database.
The prosecution in that case told jurors that the odds of finding the same DNA profile in the general population were 1 in 930 sextillion. The defense, citing two leading panels of scientific experts, argued that the rarity statistic was misleading in database cases.
The state Supreme Court said the prosecution properly told the jury that "it was virtually impossible that anyone other than defendant could have left the evidence at the crime scene."
Many scientists believe that juries should hear both the rarity and database statistics, but judges frequently do not admit the more conservative database figure.
Deputy Los Angeles Public Defender Jennifer Friedman, who wrote a brief in the case, said the ruling was better than the case law that preceded it because now the prosecution "will have a difficult time preventing the defense from using" the database probability.
Retired Alameda County Prosecutor Rockne Harmon, now a DNA consultant, said he doubted the defense would sway any jurors with the database statistic, which he called "irrelevant."
If defense lawyers want to present that, "more power to them," Harmon said. Jurors don't believe their experts, he said, and "that's why people get convicted in these cases."
The Times reported in May that a San Francisco judge presiding over a murder trial allowed jurors to hear the rarity statistic of 1 in 1.1 million. But the judge refused to permit the defense to reveal that there would be a 1 in 3 chance of finding a DNA match in the database, even if the actual perpetrator was not among the profiles.
Chin acknowledged the database effect, but said adjusting for the database would not have mattered in Nelson's trial.
The odds of finding a random match would still have been astronomical, Chin wrote. But that "does not mean that [rarity statistics] are the only statistics that are relevant and admissible."
"In a different case, if the database were large enough and the odds shorter than those here, the database match probability statistic might also be probative," he wrote.