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May 6, 2008

My brother's DNA: Near-miss DNA searching

California has adopted an aggressive policy toward near-miss DNA searching -- something discussed in this blog before. The state is going to compare DNA profiles recovered from crime-scenes to those in its offender database (1) to see if there are any "cold hits" to convicted offenders and arrestees, and (2) to see if there are any almost-matching profiles that are likely to have come from a very close relative.

The first procedure has been upheld in case after case challenging its constitutionality (in the context of convicted offenders). Why would the second procedure be constitutionally defective? According to a Los Angeles Times article of April 26 on the California policy, some lawyers think it is an unreasonable search that might run afoul of the Fourth Amendment. The paper also quotes "Tania Simoncelli, science advisor to the American Civil Liberties Union," as asserting that "The fact that my brother committed a crime doesn't mean I should have to give up my privacy!"

This crie de coeur surely is sincere, and it may not be meant as a constitutional argument, but it is interesting to ask whether it supplies a plausible principle for applying the Fourth Amendment. Consider the following case: You have an identical twin brother. He robs a bank, is locked away in prison, and his DNA profile is put in an offender database. This can happen even though his DNA was not evidence in the bank robbery case and had nothing to do with that crime.

While your brother is out of circulation, you break into a house. cutting your hand on the glass of a window that you shattered to gain entry. A tiny bloodstain with your DNA on it is analyzed. The profile is compared to those in the database. It matches the one that is file perfectly -- your brother's -- because identical twins have the same DNA sequences. But the police know that your brother was in prison when the house was burgled. They scratch their heads until they realize that he might have an identical twin with identical DNA.

So the police investigate you and find plenty of other evidence against you. Now you are facing trial. You move to exclude evidence that your DNA matches that in the bloodstain on the ground that this discovery is the result of an unreasonable search, arguing that "the fact that my brother committed a crime doesn't mean I should have to give up my privacy!" Not only that, you contend that the rest of the evidence must be dismissed because all of it is the fruit of this illegal search.

I do not see how anyone (who agrees that convicted-offender databases that include bank robbers are constitutional) can argue that this search infringes the Fourth Amendment. It is too bad that you and your brother share the same DNA profile, but the police have not forced you to surrender your DNA, and you have no right to stop them from checking your brother's DNA to see if he might be responsible. By checking him, they learn something about you. You might not like it, but let's face it, this probably is not the first time that your brother got you into trouble.

Counter-arguments, anyone?

Of course, the California policy is not limited to identical twins. Furthermore, it involves partial matches and less complete information. All that I have tried to show is that the slogan that "the fact that my brother committed a crime doesn't mean I should have to give up my privacy!" does not settle any constitutional question. It states the conclusion of what must be a rather complex argument about (1) the privacy of information that identifies a class of individuals and (2) the power of the state to investigate one individual on the basis of information it legitimately obtains from another individual.


May 6, 2008 | Permalink | Comments (1) | TrackBack

May 5, 2008

Rounding Up the Usual Suspects II

Not long ago, I mentioned the DNA-database-trawl issue that has led to several confused court opinions. The evidentiary issue is whether a complete search through a database of DNA profiles that produces one and only one match is less probative than a simple match to a known suspect. Some researchers in the U.K. tendentiously call the former use of the database “speculative searching.” (Kaye 2006, 18).

Now, an article in the May 3 Los Angeles Times claims to have uncovered a national scandal of sorts. The reporters describe a recent “cold hit” case that they say

is emblematic of a national problem, The Times has found. [¶] Prosecutors and crime labs across the country routinely use numbers that exaggerate the significance of DNA matches in "cold hit" cases, in which a suspect is identified through a database search. [¶] Jurors are often told that the odds of a coincidental match are hundreds of thousands of times more remote than they actually are, according to a review of scientific literature and interviews with leading authorities in the field.

The article maintains that

[I]n cold hit cases, the investigation starts with a DNA match found by searching thousands, or even millions, of genetic profiles in an offender database. Each individual comparison increases the chance of a match to an innocent person. [¶] Nevertheless, police labs and prosecutors almost always calculate the odds as if the suspect had been selected randomly from the general population in a single try. [¶] The problem will only grow as the nation's criminal DNA databases expand. They already contain 6 million profiles.

This description portrays one approach to the issue as if it is the consensus in the scientific literature. It is not. There is disagreement about the need to adjust a random-match probability. Furthermore, if one counts the number of peer-reviewed articles on the subject, the dominant view is that adjustment is not necessary.

I won't present a full blown analysis here, but I will offer a thought on the statement that “[e]ach individual comparison increases the chance of a match to an innocent person.” It is true that if one searches a database of a million innocent people, all of whom are unrelated to the source of the crime-scene DNA, there are more opportunities for a match to an innocent person than if one searches a database of half a million innocent people, or than if one searches a database of one only one innocent person (i.e., the suspect). So sooner or later, searches of innocent databases will produce a false positive. Indeed, they already have.

But is the probability that an innocent database will contain a matching type the right question to ask? The probative value of a match depends on how much it shifts the odds in favor of the prosecution's claim that the matcher is the source of the crime-scene DNA. The enhancement in the odds grows progressively larger as the size of the database increases. The reason is simple. More and more people are definitively excluded as possible sources of the crime-scene DNA. This raises the probability that someone else in the population — including the matcher — is the source. In the limiting case of a database that includes every person on earth, the evidence of a single match in the database becomes conclusive (ignoring scenarios involving fraud or laboratory error).

It can be shown (and has been) that, due to this “exclusion effect,” the single match in the database raises the odds even more (at least slightly) than does testing a single person at random and finding that he matches. (E.g., Donnelly and Friedman 1999; Kaye 2008). Therefore, if there is any prejudice in the existing practice of reporting the random-match probability in the “cold hit” case, it is not because a cold hit in a large database is less probative than a cold hit in a small one!

In sum, searching large databases gives more information than searching small ones, and searching small ones is better than limiting a search to a single individual. The DNA evidence has more, not less, probative value in a database-search case than in a single-suspect case.


Donnelly, Peter, and Richard D. Friedman. 1999. “DNA Database Searches and the Legal Consumption of Scientific Evidence.” Michigan Law Review 97: 931–984.

Kaye, D.H. 2008. “Rounding Up the Usual Suspects: A Legal and Logical Analysis of DNA Trawling Cases.” (submitted for publication).

Kaye, Jane. 2006. "Police Collection and Access to DNA Samples." Genomics, Society and Policy. 2: 16–27.


May 5, 2008 | Permalink | Comments (0) | TrackBack