Sex conviction is overturned

A federal appeals court upheld a judge’s decision to overturn a sexual assault conviction on the grounds that a government scientist misrepresented the significance of DNA evidence.

In a 2-1 ruling, the 9th Circuit Court of Appeals said a Nevada sheriff’s DNA expert engaged in what is known as “the prosecutor’s fallacy” by equating the chance of a random DNA match in the general population to the likelihood that the DNA evidence came from the defendant.

The court said Troy Brown was convicted of sexually assaulting a 9-year-old girl in 1994 “most probably” because of “false but highly persuasive” testimony by the prosecution’s expert.

The analyst testified that DNA from the victim’s underpants matched Brown’s and that only 1 in 3 million people randomly selected from the population also would match. Pressed by the prosecutor, the analyst then testified there was a 99.99967% chance that Brown’s DNA was the same as the DNA on the underwear -- a probability of guilt that was not based on science, the court said.

-- Maura Dolan