Thursday, November 10, 2016

The Defense Attorney's Fallacy in United States v. Natson

Searching for cases that illustrate the range of statistical statements that courts encounter, I stumbled (in two senses) on United States v. Natson. On December 16, 2003, a hunter discovered the remains of Ardena Carter and a fetus in a remote area of the Fort Benning Military Reservation in Columbus, Georgia. A student at Georgia Southern University, Carter had been shot in the back of the head with a pistol.

A grand jury indicted her boyfriend, Michael Antonio Natson, for homicide, feticide, and carrying and using a firearm during the murder. Calling for the death penalty, the government contended that Natson, a military police officer, killed Carter to keep her from seeking child support. To establish paternity, the government turned to DNA testing. However, the fetal bones yielded only a partial (five-locus) DNA profile, and the government’s expert described the findings as “inconclusive.” If this were all that the expert had to say, the court’s exclusion of the tests as irrelevant would have been unremarkable.

But the DNA expert, Shaun Weiss, did have more to say. As the court explained the proffered testimony, Weiss would have added that Natson was “26 times more likely to be the father of the fetus than a random person” and that “there is a 96.30% probability that Defendant is the father.” Weiss down played these numbers, maintaining that “the statistical probability of paternity must be at 99.99% for the DNA scientific community to consider a DNA test to show a paternity match.” In other words, Weiss believed that unless an inference of paternity is all but certain (99.99%), the test results are not scientifically acceptable. In light of these expert asservations, the federal district court concluded that
It would be sheer speculation for a jury to determine from Weiss's testimony that Defendant is the father. Therefore, the Court finds that the testimony is not relevant and would not assist the trier of fact. Accordingly, it is not admissible under Federal Rules of Evidence 702, 401, and 402.
Furthermore, the court dismissed the 26:1 odds and the 96% probability as "significantly low." The testing was "not probative." Weiss could only "testify with certainty that Defendant was 'possibly' the father, along with thousands of other random persons."

This is a very strange application of the legal concept of relevance. Suppose the government had found an unused 9mm bullet near the body (that might have been dropped there by the killer). Would the court have dismissed as irrelevant evidence that Natson also owned a 9 mm pistol because it only shows that defendant's pistol might possibly have been the murder weapon, along with thousands of  random pistols?

The notion that identification evidence is not relevant unless it limits the class of possible perpetrators to a very small number has been called the "defense attorney's fallacy." The fallacy lies in equating weakly or modestly probative identification evidence to the complete absence of probative value. Such reasoning is inconsistent with the common-law definition of relevance expressed in Federal Rule of Evidence 401. Rule 401 defines relevance in probabilistic terms:
“Relevant evidence" means evidence having any tendency to make the existence of any fact that is of consequence to the determination of the action more probable or less probable than it would be without the evidence.
Whether Natson was the father of the fetus (and thereby might have had the motive the government ascribed to him) is surely of some consequence, and if offspring of Natson and Carter are 26 times more probable to possess the genotypes of the fetal bones than are the offspring of Carter and a randomly selected man, then the genotypes make it “more ... probable” that Natson is indeed the father. Thus, the DNA test results, although far less definitive than in the usual paternity case with copious, fresh samples, were unequivocally relevant.

This explanation of why the genetic data is relevant does not depend on the court’s claim that a likelihood ratio of 26 means that Natson is “26 times more likely to be the father of the fetus than a random person” or that “there is a 96.30% probability that Defendant is the father.” These expressions are statements of the odds or probability of the “fact that is of consequence.” Geneticists cannot deduce such values without a probability for the fact "without the evidence.” To arrive at probabilities for a claim of paternity, parentage testers typically assume that the “fact ... without the evidence” is equiprobable, then adjust it in light of the genetic evidence. The expert's choice, which is not usually based on scientific data, of 50% for the probability without the evidence, has been sharply criticized. With respect to the question of relevance, however, that issue is a red herring. In Natson, 26 is the ratio of (1) the probability of the genetic evidence when Natson is the father to (2) the probability of the same evidence when he is not. It is a likelihood ratio. As such, the evidence alters the probability of paternity, no matter what the starting probability might be. That makes the evidence relevant. It does not matter whether the probability without the evidence is 50%, 5%, or any other number (except for unrealizable prior probabilities of exactly 1 or 0).

Although the court excluded the statistical statements about the DNA evidence, the jury convicted Natson. Family members testified that Carter was pregnant and that she had told them that Natson was the father, but the most crucial testimony may have come from FBI firearm and toolmark examiner Paul Tangren. Tangren opined that a discharged ammunition cartridge recovered "from the scene of the alleged crime" exhibited toolmarks that "were sufficiently similar" to those on cartridges test-fired from a pistol owned by Natson "to identify Defendant's gun ... to a 100% degree of certainty."

The judge imposed a sentence of imprisonment for life without parole. Natson appealed the conviction on the ground that “the case investigators, intentionally and calculatingly, refused to develop information ... which might implicate” other suspects. In an unreported opinion, the U.S. Court of Appeals for the Eleventh Circuit denied this appeal.


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