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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


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The odds of a non-twin source for the DNA in your example is so vanishingly close to zero that the probabilistic components of the inferences are masked. In the normal (non-twin) case, something should depend, I think, on whether the probabilities are so lopsided as to narrow the field to a similar extent. I do not know enough about DNA analysis and the databases to gauge whether that condition could be satisfied. If the databases include mtDNA, for example, then it might be possible for investigators to focus on persons of common matrilineal descent. But then it would matter, wouldn't it, how many of those there were? Probable cause might be conceded in a one-sibling case but more open to dispute in a 12-sibling case.

It does seem dangerous to me to open the door to probable cause determinations that are founded on nothing more than statistical correlations in the middling range -- though not so dangerous with DNA, maybe, as with other kinds of probabilistic analysis that may be more methodologically plastic.

Posted by: Peter Nordberg | May 6, 2008 7:21:57 AM

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