The Role of Race in DNA Statistics: What Experts Say, What California Courts Allow

Abstract

When a defendant's DNA matches a sample found at a crime scene, how compelling is the match? To answer this question, DNA analysts typically use numbers - "relative frequencies," "random match probabilities" or "likelihood ratios." They compute and present these quantities for the major racial or ethnic groups in the United States, supplying prosecutors with such mind-boggling figures as "one in nine hundred and fifty sextillion African Americans, one in one hundred and thirty septillion Caucasians, and one in nine hundred and thirty sextillion Hispanics."

A line of California cases rejects this established practice on relevance grounds. The theory of these cases is that only the perpetrator's race is relevant to the crime; hence, it is impermissible to introduce statistics about other races. This article critiques this reasoning. Relying on the statistical concept of likelihood, it presents a logical justification for referring to a range of races and identifies some problems with the one-race-only rule. Nevertheless, even if the traditional practice is consistent with the doctrine of relevance, other ways to express the probative value of a DNA match exist. The paper therefore notes some ways to express the probative value of a DNA match quantitatively without referring to variations in DNA profile frequencies among races.

Keywords: DNA evidence, race, conditional relevance, likelihood ratio