DNA swabs: Discrimination endorsed by US supreme court

“Taking and analyzing a cheek swab of the arrestee DNA is, like fingerprinting and photographing, a legitimate police booking procedure that is reasonable under the Fourth Amendment,” the majority of the US Supreme Court ruled on June 3rd.  This post explains how this decision endorses discrimination in the real world where the databases of such DNA do not include everyone and are not a random sample of all social groups.

1.  Picture the DNA of everyone as arrayed in two dimensions where people who are more closely related will be closer together on the plot.   Imagine that, in practice, when a DNA swab is taken of a person, relatives within, say, 1/4 inch of a person can be identified if they are in the existing DNA database.  If the relative—not the person swabbed— has left a DNA trail at a crime they committed, the match will be noted and police would then go searching for the relatives of the person swabbed and try to identify (using DNA or other means) who is a suspect for that crime.

RandomSquare2Groups

2. Also picture that society consists of two groups—the squares and the diamonds—but that crimes are committed by the same fraction of each group, at least crimes that leave a DNA trail.

3.  Now imagine that squares and diamonds are included in the existing database in the same ratio as in society as a whole.  (This would certainly be the case if everyone were in the database.)  This means that, if a square is arrested and swabbed, then because their relatives will tend to be squares, more squares will show up in any search of the database.  Conversely, if a diamond is arrested and swabbed.  The net effect is that, when an overlap is found with DNA left at a crime so police go searching for the relatives of the person swabbed, squares and diamonds are picked up in the same ratio as in society as a whole and square and diamond criminals are apprehended in the same ratio as in society as a whole.

4.  Feel free to test this for yourself (on paper or in your head): a) Color in a random sample of points as the criminals, b) randomly choose a sample of points and draw 1/4″ circles around them, and c) count the numbers of square and diamond criminals  apprehended.  The ratio should be similar to the 72: 38 in society as a whole.

5. But now imagine that squares and diamonds are NOT included in the existing database in the same ratio as in society as a whole, but diamonds are more frequently included—as shown by the odd-shaped region marked below.  Yet crimes that leave a DNA trail are still committed by the same fraction of each group.

RandomSquare2Groups2

The result will be that the ratio of square to diamond criminals  apprehended will be skewed towards the diamonds.  In other words, if you are a square criminal you are less likely to be caught by this DNA database approach than if you are a diamond criminal. In the best case scenario, the ideal of equal under the law would only apply to your fate after you are apprehended.

6.  Given that African Americans are disproportionately represented in existing DNA databases, as are poor people, the Supreme Court decision has the discriminatory effect of making it more likely that if you are a poor or African-American criminal you are more likely to be caught than if you are a well-off or Euro-American criminal.

7. I have not yet seen this effect discussed, either in the dissenting Supreme Court decision or commentaries.

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