A DNA Test That Can Distinguish Identical Twins: Permissible Under the Fourth Amendment?

A very interesting report from the BBC report begins:

It's well known that identical twins are not totally identical - they can, usually, be told apart, after all. But up to now it has been almost impossible to distinguish their DNA. It's claimed that a new test can do it quickly and affordably, however - and this could help police solve a number of crimes.

The story goes on to detail several criminal investigations that were thwarted by identical twin suspects. And it points out that the technology can be used in paternity tests. The details and stories in the article are fascinating and I recommend that you read the whole thing.

The article points out that this new test is different from current DNA testing which compares only a few parts of people's DNA -- parts that are typically called "junk" DNA because the parts of DNA that are examined and compared in DNA testing do not have any known association with observable, physical traits.

What are the Fourth Amendment implications of this kind of test?

"Junk" DNA testing was the subject of Maryland v. King, where the Supreme Court held that police could test the DNA of arrestees against a national database. Because the DNA testing did not reveal any traits of the individuals, the Court noted that the testing was limited to determining the identity of arrestees -- an important government interest. This government interest was weighed against the intrusion of DNA testing on a person's privacy, which the Court noted was rather low. DNA testing involved a minimally invasive swab, and the information collected in carrying out the test was limited to parts of the DNA sequence that did not reveal any traits about the suspect.

For more of my commentary on King, see my previous posts here and here.

The type of DNA testing discussed in the BBC's report is far more involved than the testing in King. The tests used to distinguish identical twins looked at the entire DNA sequence. This would likely give researchers access to information about the subject's physical traits, meaning that the test could have uses beyond simple identification. The search carried out in scanning the DNA would arguably be more intrusive than the search in King because of the broader amount of information the government would collect.

If this alternate method of analyzing DNA is used, however, it will probably be reserved for those cases where traditional DNA testing reaches an impasse -- such as cases involving identical twin suspects. Once an investigation has gone far enough for the necessity of this test to be realized, the evidence obtained by regular DNA tests will likely be enough for police to obtain a warrant to carry out the more intrusive test.

While testing the entire DNA strand is distinguishable from the tests the Supreme Court addressed in King, this kind of testing will probably only be used in situations where the police have a warrant to carry out the search -- or at least after an independent fact finder has confirmed that there is probable cause that the suspect has committed a crime. In these situations, a test of the entire DNA strand will probably be permissible under the Fourth Amendment.