A recent edition of The Economist reported on the perceived “CSI Effect.” [1] The following paragraph raised some eyebrows:

In reality, scientists do not deal in certainty but in probabilities, and the way they calculate these probabilities is complex. For example, when testifying in court, a fingerprint expert may say that there is a 90% chance of obtaining a match if the defendant left the mark, and a one in several billion chance of a match if someone else left it. In general DNA provides information of a higher quality or “individualising potential” than other kinds of evidence, so that experts may be more confident of linking it to a specific individual. But DNA experts still deal in probabilities and not certainties. As a result of all this reality checking, trials are getting longer and more cases that might previously have resulted in quick convictions are now ending in acquittals.

I am skeptical of the claim that television shows have converted quick convictions into acquittals, but even if that is true, how is DNA is more “individualising” than fingerprints? Fingerprints of identical twins are distinguishable. DNA profiles, in their current form, are not. The latter fact has been a problem in some investigations and prosecutions. Police investigating a notorious jewel heist in Berlin found DNA “in a drop of sweat on a latex glove discarded next to a rope ladder.” [2] When “they ran the material through the German [DNA] database, . . . [t]he computer identified two 27-year-old identical twins . . . .” Id. A German court released them, explaining that “[f]rom the evidence we have, we can deduce that at least one of the brothers took part in the crime, but it has not been possible to determine which one.” Id. According to a Time Magazine story:

Identical twins share 99.99% of their genetic information, and the tiny differences are impossible to isolate because of their nature; they tend to be spontaneous mutations limited to certain organs or tissues. “Identifying those [differences] would amount to dissecting the suspects,” says Peter M. Schneider, a University of Cologne forensic expert. “Our hands are tied in a case like this,” says criminal-law expert Hans-Ullrich Paeffgen of Bonn University. “The law doesn’t allow us to detain someone indefinitely just because he is suspected of a crime. This may be different elsewhere. But I’d rather live in a country where someone guilty is not convicted for lack of conclusive evidence than in a place where innocent people are locked up.”

This isn’t the first time an identical twin has proved impossible to pin down. The genetic material can thwart paternity tests if both twins claim — or deny — fathering a child. In the U.S., a jury in a rape trial in Houston deadlocked in 2005 when the DNA recovered at the crime scene matched identical twins who had kidnapped their victim together. A year earlier in Boston, a suspected rapist blamed his identical twin when confronted with the matching DNA. Although he was already serving a sentence for a rape conviction, the jury could not agree on a verdict, and the judge declared a mistrial. Earlier this year, an identical twin suspected of drug-smuggling and sentenced to death in Malaysia was set free when the court could not prove beyond doubt whether he or his brother had committed the crime.

Thus, fingerprint examiners like to claim that their information is more individualizing. But this overlooks the issue of whether today’s subjective comparisons of latent prints to clear, complete ones taken under controlled conditions are any more precise than STR profiling. Latent print examiners declare a match when they have a large subjective probability that the latent print and the exemplar originated from the same finger. In the United States, at least, the usual testimony for a subjective match is that the items came from the same finger. Do examiners ever testify to the likelihoods of competing hypotheses, as illustrated by The Economist‘s statement “that there is a 90% chance of obtaining a match if the defendant left the mark, and a one in several billion chance of a match if someone else left it”? Lacking the data to support statements like these and being trained to give categorical opinions about the source of a latent print, fingeprrint examiners do not seem to testify in terms of likelihoods. However, research supporting likelihood-based testimony exists, and the time is ripe for a change.

References

1. The “CSI Effect”, Economist, Apr. 22, 2010, http://www.economist.com/science-technology/displaystory.cfm?story_id=15949089

2. Claudia Himmelreich, Despite DNA Evidence, Twins Charged in Heist Go Free, Time, Mar. 23, 2009, http://www.time.com/time/world/article/0,8599,1887111,00.html