DNA Probabilities in People v. Prince: When are racial and ethnic statistics relevant?

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 relative frequencies, random-match probabilities or likelihood ratios. They compute 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." In People v. Prince, a California Court of Appeals rejected this practice on the theory that only the perpetrator’s race is relevant to the crime; hence, it is impermissible to introduce statistics about other races. This paper critiques this reasoning. Relying on the 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. The paper also notes some ways to express the probative value of a DNA match quantitatively without referring to variations in DNA profile frequencies among races or ethnic groups.

💡 Research Summary

The paper examines the statistical treatment of DNA matches in criminal trials, focusing on the controversial practice of presenting genotype frequencies for major racial or ethnic groups. Using the California appellate decision in People v. Prince as a case study, the author critiques the court’s “one‑race‑only” rule, which held that DNA frequency statistics are admissible only when independent evidence establishes that the perpetrator belongs to the same racial or ethnic group as the defendant.

The author begins by framing DNA evidence in terms of a likelihood ratio (LR): LR = P(E | D) / P(E | I), where E denotes the observed DNA match, D the hypothesis that the defendant is the source, and I the hypothesis that some other individual is the source. An LR greater than one indicates that the evidence makes the defendant’s guilt more probable. The numerator, P(E | D), is essentially 1 when a match is observed and laboratory error is excluded. The denominator, P(E | I), is the probability that a random person from the relevant population would coincidentally possess the same genotype; this is precisely the genotype frequency in the population.

Because the denominator depends on how rare the genotype is in the overall population, the author argues that frequency information from any major subpopulation (e.g., Caucasian, Hispanic, African‑American) is relevant regardless of whether the defendant’s race is known. The court’s requirement that a “preliminary fact”—independent proof of the perpetrator’s race—must first be established is shown to be logically untenable. Even if other evidence strongly implicates the defendant, it does not affect P(E | I); only evidence that makes the genotype less likely in the alternative population would reduce the denominator. Consequently, the “conditional relevance” doctrine employed by the court fails both theoretically and practically.

A hypothetical dormitory murder scenario illustrates the point. If the defendant is the first person tested and matches, while all others do not, the LR simplifies to 1 / f, where f is the genotype’s population frequency. Providing the jury with f (or a range derived from several major ethnic groups) allows them to appreciate how surprising the match is. In Prince, the expert testified that the LR was 1.9 trillion for Caucasians, 2.6 trillion for Hispanics, and 9.1 trillion for African‑Americans—numbers that directly reflect the rarity of the genotype in each group. The appellate court rejected the Hispanic and African‑American figures as irrelevant because the perpetrator’s race was not proven, yet the author shows that these figures still convey essential information about the improbability of a coincidental match.

The paper also addresses concerns that presenting multiple ethnic frequencies might mislead jurors into assuming the perpetrator must belong to the group with the lowest frequency. While acknowledging that such a misperception is possible, the author contends that the risk is outweighed by the benefit of giving jurors a quantitative sense of rarity. Moreover, the use of broad population databases is justified because they provide upper and lower bounds on genotype frequencies, even if racial categories are socially constructed.

In sum, the author concludes that the People v. Prince decision misapplies statistical reasoning and that DNA evidence should be evaluated through a unified likelihood‑ratio framework that incorporates genotype frequencies from all relevant subpopulations. This approach preserves the scientific integrity of forensic evidence, avoids the pitfalls of the “one‑race‑only” rule, and offers a clearer, more consistent method for courts to assess the probative value of DNA matches.