What does it mean when you refuse to give a DNA sample?

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Adam Shostack is bothered by the Truro MA's effort to solve a crime using large-scale DNA sampling:
The city of Truro, Massachusetts is trying to collect DNA from all 790 residents to solve a crime, reports the New York Times. Its not clear why they believe that residents are more likely to be the criminal than non-residents, and it is clear that they don't get the 4th amendment, against dragnet searches, or the 5th, against self-incrimination:
Sgt. David Perry of the Truro Police Department and other law enforcement authorities here say that the program is voluntary but that they will pay close attention to those who refuse to provide DNA.
There are numerous crimes which could be solved by door-to-door searches. We don't allow them, and we shouldn't allow this, either.

Note the section Adam quotes about how "the program is voluntary but that they will pay close attention to those who refuse to provide DNA". This has some interesting implications.

Let's assume for the moment that we have some population of size N and we know that includes the perpetrator. Let's start with a premise I think we can all agree with: no sane perpetrator is going to hand over his DNA voluntarily, since it's sure to convict him. Now, of the remainder of the population, some fraction will agree to hand over their DNA (call these people Compliant and denote their number as N_c) and some won't (call them Noncompliant and denote their number as N_n).

Assume for the moment that the police have no information about who the perpetrator is other than that he's a member of the population (yes, I know this isn't true, but it's useful for illustrative purposes). So, any given person has a 1/N chance of being the perp. Now, once they've done the testing, they've narrowed the set down to the Noncompliant, and any Noncompliant person has a 1/N_n chance of being the perp. The higher a fraction of compliance that the police can get, the more information they get from the fact that someone isn't compliant. In the limit, if everybody but the perp complies than the probability that the single Noncompliant person is the perp is unity.

This effect is enhanced by the fact that compliance and noncompliance aren't randomly distributed. It's true that some people will refuse to comply on principle, but a substantial fraction of the Noncompliant probably have less noble reasons. Even if they're not the perp, they may have other reasons not to have their DNA collected--for instance they've committed another crime that their DNA might match to. (The police say they're only going to use the information for this particular crime, but criminals don't typically trust the police). From this perspective it makes sense for the police to single out the Noncompliant for extra scrutiny.

There's a positive feedback loop here. If the fact that someone doesn't comply increases the probability that they're a bad guy, and singles you out for extra scrutiny, it becomes pretty unattractive to refuse to comply. Only the people who have a real reason to refuse will do so, which means that Noncompliance is an even stronger signal, increasing the incentive to comply. Similarly, this gives the police a real incentive to hassle the Noncompliant in order to increase the strength of the signal. If they're successful, the only people who won't comply are criminals and a few privacy nuts, who can be hassled pretty much at will.

The converse of this is that if enough people value their privacy and refuse to comply, the value of Noncompliance as a signal diminishes drastically and the police will find it very hard to lean on the Noncompliant. If you don't like this kind of search, your best bet is to convince other people to refuse.

Acknowledgement: This post came out of extensive discussions with Kevin Dick.

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In responding to my comments about Truro's DNA dragnet, with a fascinating discussion of signaling, Eric Rescorla writes: Even if they're not the perp, they may have other reasons not to have their DNA collected--for instance they've committed another... Read More

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See http://education.guardian.co.uk/schooltrips/story/0,10621,1233668,00.html
about what is still the most interesting case of this kind. In 1997 a 13-year-old British schoolgirl was raped and murdered in rural France. The cops took some 4,000 DNA samples "voluntarily" from the local male population aged 18-35 -- and the perp wasn't among them. He was eventually identified much later by conventional police work and brought to trial. Of course, his DNA sealed his conviction.

A more fairly worded piece of advice, I think, would have been, "if you like this kind of search even less than you like letting the perpetrator of this crime go free, your best bet is to convince other people to refuse." No doubt there are crimes (say, parking violations) whose enforcement most everyone would consider not worth the unpleasantness of a mass collection of DNA. But I suspect that most people would gladly give a DNA sample to help catch a murderer, as in the case under discussion. (Okay, most non-Americans, anyway. Note that the technique has been used successfuly in Europe.)

And the self-reinforcing nature of both mass compliance and mass non-compliance will thus likely produce a fairly sharp cutoff between those crimes for which this tactic is effective, and those for which it's not worth the police's trouble. Sounds to me like a wonderfully democratic case of spontaneous collective decision-making. What's not to like?

What happens to the DNA samples and the digitally processed "DNA fingerprints" of all the "innocent" people who have given a "voluntary" DNA sample ?

In the United Kingdom, for example, these are retained forever
c.f. http://www.spy.org.uk/spyblog/archives/genetic_profile_databases/index.html

The gentiles of Denmark, I hear, had the proper response to this sort of governmental "request".

Chris, I assume you're referring to the story of the Gentiles of Denmark all wearing yellow stars in response to the Nazi order that Jews wear a yellow star at all times. In fact, the story is apocryphal, but wartime Danes certainly did undermine their Nazi occupiers' measures against the Jews, and the Danish underground resistance eventually managed to smuggle almost all the Jews of Denmark to neutral Sweden in 1943 to save them from deportation.

Of course, there's scarcely a hint of similarity between the Nazi occupiers' orders, on pain of severe punishment, to round up all Jews for mass slaughter, and the police force of a democratically accountable jurisdiction asking for the community's voluntary cooperation in bringing a murderer to justice. I highly doubt that the citizens of Denmark would have the slightest difficulty--then or now--distinguishing between the two cases. Indeed, I believe you have grossly insulted the Danish people by suggesting otherwise.

As for what it says about you that you are unable to see such obvious, fundamental differences--well, I'll leave that to readers to judge.

Dan, I think it's somewhat disingenuous to say that the police are asking for "voluntary" cooperation. In a truly voluntary situation, there wouldn't be any consequences for refusing to provide a sample, but that's pretty clearly not the case here, and not only because the police believe that everyone who refuses is likely to really be the murderer.

Dan, it disturbs me that you are so majoritarian. Americans have already firmly said no to such measures--see the 4th & 5th ammendments.

You are also presuming that a sharp line CAN exist between different cases. "Will you give you DNA to catch a jaywalker?" "No." "Will you give it to catch a murder?"

Once the state has the data, it is not going to relinquish it easily. (Because it is a form of power.) Massive, strong resistance is the only way to keep this from becoming pervasive, and it's not likely to succeed for long.

Dan -

I am aware that the story is apocryphal. Hence my use of "I hear".

Perhaps the analogy is strained, and feel free to mentally invoke Godwin's law and tune me out. However, when a government official with a gun tells me I'd better spit in a cup or else he'll have his eye on me as a possible murder suspect, I think my rights are being violated. In fact, I would say that those people who choose to exercise their rights are being singled out and are having them violated en masse.

You are quite right that this situation is not at all like Denmark, but not in any way you discuss. In this case, doing the right thing and refusing to comply comes with absolutely no penalty at all if (as in apocryphal Denmark) everyone does it.

Eric--yes, allowing the police to investigate crimes impinges on people's freedoms. For example, if you go into a store and consider buying something--say, a length of pipe--then you are risking the possibility that the police, finding a murder victim apparently bludgeoned to death with a piece of pipe, will learn about your purchase, and consider you a suspect. In principle, this makes your decision to buy the piping a little less "voluntary".

And the more heavily correlated with actual criminal activity your behavior becomes, the more the police are likely to want to investigate you. For example, few people stage fake crimes for fun, in part because they don't want the hassle of the police investigating them. One could therefore conclude that these decisions are ever-so-slightly "involuntary", because of the police consequences.

But one could also play this same game with any other (non-police) consequence of a voluntary action. I can choose not to wear skunk oil, but is my decision really "voluntary", if deciding the opposite will cause nobody to want to go anywhere near me? If the perfectly legitimate choices and actions that others take as a result of my choices and actions suffice to make my decisions "involuntary", then no decision can be said to be voluntary, and your entire libertarian outlook collapses.

Of course, you could always claim that it's a matter of degrees of voluntariness, and we could debate at length the merits of this particular tradeoff between a slight reduction in "voluntariness" and a possibly significant increase in police effectiveness. But even if we end up disagreeing on this point, you might still concede that my use of "voluntary" to describe the collection of DNA in Massachusetts was actually quite reasonable in this quantitative sense, given that I was comparing it with the Nazi orders regarding the treatment of Jews in wartime Denmark.


I'd be happy to concede that the police in Truro, MA are being quantitatively far less coercive than the Nazis in Denmark were.

However, I think the degree of coercion in both cases is decidely non-trivial, and hence it is fair to say that what is being "requested" is not at all voluntary.

I do not like being threatened with harm for exercising my rights. However, if the police in Truro, MA were to (say) offer me a free dozen donuts for a DNA sample (which they would destroy iff I was ruled out as a suspect), I might be happy to provide one. I am sure that dissertations are being written on this, but I haven't looked at any of the literature beyond Kahnemann and Tversky many years ago.

From a purely economic point of view (that is, the behavior of the system in equilibrium under the "punish for noncompliance" vs. "reward for compliance" regimes) I am not sure a difference should be expected.

That the two situation should be viewed differently by the courts is an entirely different matter, however.

(feel free to take it to email if you want -- I won't be commenting further in this forum since I think the broad outlines are clear enough!)

But one could also play this same game with any other (non-police) consequence of a voluntary action. I can choose not to wear skunk oil, but is my decision really "voluntary", if deciding the opposite will cause nobody to want to go anywhere near me? If the perfectly legitimate choices and actions that others take as a result of my choices and actions suffice to make my decisions "involuntary", then no decision can be said to be voluntary, and your entire libertarian outlook collapses."

Seeing as the implicit question on the table is whether the police threatening you with increased scrutiny for failing to provide a DNA sample is in fact legitimate, I'd say you're rather assuming your premise here.

Anyway, I'm not particularly interested in having this discussion with you at this point, since I think our respective positions are pretty clear. My original post was purely about the tactical implications. If you'd care to discuss those, then we of course can. If you just want to debate what the police should or should not be allowed to do, I'll let you have the last word.

What a curious claim that "there's scarcely a hint of similarity between the Nazi occupiers' orders, on pain of severe punishment, to round up all Jews for mass slaughter, and the police force (...) asking for the community's voluntary cooperation in bringing a murderer to justice."

From what I recall, first the Jews had to identify themselves with their stars. Then they were rounded up.

Now we have the population being asked to identify themselves with their DNA. Whatever comes next will be much easier.

Eric, I was under the impression that you were questioning the legitimacy of the police asking an entire population for DNA samples--not the legitimacy of their drawing perfectly valid conclusions from the results, and doing their duty accordingly. Are you asserting that it would not be perfectly legitimate for the police to conclude that a particular resident is more likely to be the murderer--and hence to investigate him more thoroughly--given that he was the sole resident to refuse a DNA sample, and that all those who offered a sample turned out not to match the killer's DNA? And if you are indeed making such an assertion, then would it apply regardless of how the police happened upon the DNA--say, even if they'd happened upon all the samples by accident--or only when the police specifically ask for samples?

If your answer is, "only when the police ask for samples", then does the same distinction apply in the skunk oil case? If you mention to lots of your friends that you've bought a vial of skunk oil, and we all say that we tend to avoid people who smell like skunks, has our decision to avoid you if you wear skunk oil now become "illegitimate" (and your decision to not to wear skunk oil "involuntary") because we warned you in advance of the (otherwise completely legitimate) choices we would make, and their adverse consequences for you?

Sorry about all the hypotheticals--I'm really having trouble understanding your reasoning, here.


I think you're misreading me entirely. My original post was purely about the tactical implications. I didn't say anything about legitimacy or otherwise.

My first comment (in response to your comments) merely made the point that I didn't think that people who provided samples under the assumption that the police would hassle them if they didn't were truly doing so "voluntarily". No comment on legitimacy here either.

My second comment is the only one that addresses the topic of legitimacy at all, and all I did was note that you're argument about whether people were behaving voluntarily assumed that it was legitimate for the police to ask people for samples--with the implied threat that they would be hassled if they didn't comply. Since that's what the posters here are mostly arguing is illegitimate, I think you're kind of assuming your premise. I think we can agree that if the police told you you would be shot on the spot if you didn't comply that would be neither voluntarily nor legitimate. On the other hand, if they told you that they would give you a sticker if you complied, that would clearly be voluntary. It's the gray area in between that I think people here are arguing with you about.

As I said previously, I don't think there's much point in discussing the moral/political dimension here because I think our underlying positions are diametrically opposed, so I was more or less confining myself to nitpicking about terminology. As I indicated previously, I'm primarily interested in the tactical implications.

Okay, then--I'll just clarify what I meant by "legitimate". I was referring to the police drawing conclusions from the results of their request for DNA samples--not the request itself (whose legitimacy I obviously can't yet assume). Obviously, given the post-collection circumstances--a bunch of samples, and some information about who did and didn't provide them--shooting the non-providers would not be standard police procedure (absent significant changes in the law). However, evaluating suspects in light of the new information would be standard police procedure, and hence "legitimate".

Now, you might argue that even this sort of standard police procedure would be illegitimate if based on information that was illegitimately obtained, i.e., if the request for DNA samples was itself illegitimate. (This is the famous "fruit of the poisoned tree" doctrine of the "exclusionary rule". I consider it an outrageous abomination, but that's beside the point.) However, if you argue that the request for DNA samples was illegitimate because it wasn't entirely voluntary, then you've set up a completely circular argument: the request for DNA samples was illegitmate because it was involuntary, because it was based on the threat of actions that were illegitimate, because they would have been based on the request for DNA samples, which was illegitimate....

Again, if I take the police out of the picture, it becomes even clearer: threatening to shoot you if you wear skunk oil may impinge on the voluntariness of your decision, but if it somehow makes your decision involuntary that I'll react the way I naturally would to anyone who smells like a skunk, then in what sense can any decision be said to be voluntary?

The problem is that the police aren't necessarily behaving the way they naturally would. Indeed, they have an incentive to harass people that they don't think are plausible suspects in order to increase the overall rate of compliance, and hence the quality of the signal.

Perhaps a hypothetical will illustrate this more clearly. Someone invents an automatic speed governor which automatically knows the current speed limit and physically prevents your car from speeding. The police suggest that you install this governor. They then randomly sample the people who don't and follow some of them around. When they break any traffic law (which we all do on a daily basis) they are then ticketed. Now, it's a reasonabel inference from the fact that people chose not to install the governor that they were more likely to speed. However, the police aren't really acting directly on that inference but are rather using vigorous enforcement as a tool of coercion. Would you consider people's choice to install the governor in this case to be voluntary?

Now you're discussing an entirely different problem. It's obviously inappropriate for the police to gratuitously harass completely law-abiding citizens whose free choices happen to have inconvenienced the police. Not giving DNA is only one exotic example--not donating to the policemen's benevolent fund, not showing up at a pro-police rally and not offering policemen free donuts are all activities that the police have an "incentive" to punish with harassment. But we don't therefore forbid the police to set up charities or eat donuts--rather, we try to identify and punish instances of undue coercion-by-harassment of this kind.

All of this, however, is completely different from sensibly focusing a murder investigation on those people who have not been eliminated by a DNA test. If the number of such people is small, so much the better--nobody should be surprised to find the police checking into those people very carefully. If you think that's harassment, then I can't imagine what you'd classify as legitimate police activity.

And if the number of people who refuse to have a speed governor installed is small, nobody should be surprised if the police check into those people's driving quite closely. As far as I can tell, you've failed to elucidate any difference between these two cases.

You appear to be saying that you don't see any way of distinguishing between the amount of surveillance of non-speed-governed cars required to ensure society's desired level of compliance with speed limits, on the one hand, and an amount so totally beyond that level as to constitute harassment, on the other. (Or, to put it another way, you don't see a distinction between police "harass[ing] people they don't think are plausible suspects in order to increase the rate" of speed governor adoption--or the rate of policemen's benevolent fund contributions, for that matter--from police simply enforcing speed limits by "check[ing] into those people's driving quite closely" who don't have speed governors.)

The obvious next question is, do you believe there are any limits on police behavior? If they can disguise harassment of inconvenience cases (non-suppliers of DNA, non-installers of speed governors) as legitimate police work, then why would they not just go all the way and run a full-fledged protection racket, demanding whatever they want from whomever they want, in return for not hounding them till they find something to nail them on? And in that case, why would you bother to worry about something as minor as this DNA collection business, when they can have your ass in a sling whenever they please anyway?

On the other hand, if you believe that the police aren't a full-fledged protection racket, then why can't whatever's preventing them from shaking you down for cash also prevent them from shaking you down to get you to give DNA, or install a speed governor, or whatever?

I'm surprised you find this confusing. It's precisely putting limits on the reasons that the police can subject you to intensive scrutiny (you know, things like warrants, probable cause, etc.) that makes it difficult for the police to run a protection racket.

In any case, I'm finding myself bored with this conversation. As I've noted several times, the post you're responding to was solely about the tactical implications of widepread DNA collection. We've now wondered well beyond that, so I'll let you have the last word if you like.

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