Posts Tagged ‘dna’

Taking DNA Samples at Arrest? Not a Problem.

Wednesday, August 4th, 2010

dna

On May 8, 2005, we were having a party.  It was our birthday, and our firstborn had just turned 1 a few days before, so it called for a big celebration with friends and family.  For us, it was a time of new beginnings.  But for Jerry Hobbs, May 8 2005 was the end.  He found his 8-year-old daughter and her 9-year-old friend brutally stabbed to death, in a park in Zion, Illinois.  He immediately called the police, who immediately made him their number-one suspect.  He’d just gotten out of jail in Texas, after all, so why investigate further?  He was subjected to a long, intense interrogation, and eventually made a statement that sounded like a confession.  He recanted the statement, saying it was coerced, but that didn’t matter, and he was charged with the murders.

Shortly after his interrogation, the police found DNA on the girls’ bodies that didn’t match Hobbs.  The DA discounted it, saying it must have been cross-contamination and couldn’t have been relevant to the crime.  But the DNA was in semen found on the girls’ bodies — and inside one girl’s vagina — and that’s not cross-contamination.  The DA insisted that it was still irrelevant, and that the semen must have been on the ground before the attack.  Seriously.  Hobbs remained in custody, charged with the double murder, for more than five years, though his case never went to trial.

He was in jail until a couple of hours ago, that is.  As it happens, that DNA on the girls’ bodies was extremely relevant.  Jorge Torrez, who had lived in Zion at the time, was arrested in Arlington, Virginia a few months back, and charged with the abduction and repeated rape of one woman as well as attacking another woman.  Virginia, unlike Illinois, takes DNA samples along with fingerprints when someone is arrested.  The DNA taken at Torrez’s arrest went into the database, and popped up as a match to the DNA found on the girls.  The Illinois prosecutors dithered for weeks, but this morning they finally released Hobbs from prison (though they refused to issue an apology, insisting they and the police had done everything right).  Still, an innocent man went free at last.

And if Torrez’s DNA had not been swabbed on arrest?  Hobbes’ coerced, false confession might well have resulted in yet another wrongful conviction.

This raises a lot of issues.  There’s the misuse of DNA evidence, and there’s the false confession, but those are topics for another time.  (If you’re interested in learning ways to defend such cases, you can check out our “Hope for Hopeless Cases” CLE series, particularly lectures IV and V.)

Today, however, our beef is with the civil liberties argument against taking DNA samples at arrest.

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The argument is that people who haven’t yet been convicted of a crime should not be compelled to give DNA samples.  It smacks of “Big Brother” and “Minority Report.”  The government might conceivably (more…)

Bacterial Fingerprinting? Don’t Hold Your Breath

Wednesday, March 17th, 2010

bacteria

Over the past couple of days, the news has been filled with stories about using microbes to identify suspects. Everyone has all kinds of bacteria all over their bodies, of course, and whenever you touch something you leave a smudge of your bacteria behind. On Monday, researchers at CU-Boulder published a study where they swabbed computer keyboards, tested the DNA of the bacteria they found, and saw that those bacteria’s DNA more closely matched the bacteria on the computer users’ skin than the bacteria on other people’s skin.

That’s all the study found. The bacteria on your keyboard have DNA that more closely matches the DNA in the bacteria on your fingers, than that of bacteria on other people’s fingers. Frankly, although that’s a nifty result and the scientists deserve to be praised for their work, it’s really a very modest finding. Not exactly earth-shaking.

But as usual, the media took this modest finding and blew it way out of proportion. The study’s authors insist that the project “is still in its preliminary stages.” The media make it sound like we’ll be seeing this stuff in court before we know it. The fact is that using microbial DNA to link a suspect to a crime scene is not going to be a reality any time soon, if ever.

For one thing, there is as yet no reason to conclude that your particular bacteria are as unique as your fingerprints or your personal DNA. Bacteria do not use sexual reproduction, after all, and so their DNA is less diverse than human DNA. The uniqueness of your bacteria is very much an open question.

We don’t even have a baseline of what bacteria are even normal to find on human bodies. A single person will have a huge variety of different microbial populations on different parts of his skin — the microbial mix on his fingertips is not the same as the mix on his nose or his toes. All the various types of bacteria people can have will need to be isolated, all the different DNA each kind of bacterium can have will need to be sequenced, all the various combinations will have to be analyzed, and a massive amount of comparisons will have to be made.

In other words, there will need to be many more studies, based on way more data, plus some pretty robust statistical analyses of large populations, before any scientist can reach the same conclusions as those you’ve been reading in the news. That’s going to take a very long time, even with the accelerating advances in DNA sequencing technology.

Still, it really is an intriguing idea. After all, a perpetrator may not leave behind any blood, sweat or tears. Fingerprints may not be obtainable from fabric or wiped surfaces. But he may still leave behind a smudge of (more…)

Coming Soon: Full-Genome DNA Analysis

Tuesday, March 9th, 2010

This is amazing. Sequencing an entire human genome is now going to be cheap and fast. We predict this will be a game-changing technology for the use of DNA technology.

Right now, DNA evidence is looked at much like fingerprint evidence. With fingerprints, law enforcement doesn’t compare every single ridge and whorl to see if there’s an exact match. Instead, particular locations are compared, to see if those locations are the same. And there’s a lot of subjective interpretation that is needed to make that call. DNA evidence is no different. The entire DNA sequence is not compared. Instead, a handful of locations are compared, to see if the DNA at those locations is the same. And there’s a lot of subjective interpretation that is then needed to make that judgment call. This can call DNA evidence into question, particularly when there are mixtures, degraded samples or equipment glitches that create room for errors of judgment.

If one were to compare the entire genome, however — all 3.3 billion base pairs of it — there would be much less room for interpretation and error.

The problem is that sequencing an entire genome has, to date, been prohibitively expensive. The first genome was a massive undertaking. As of 2009, only 7 people’s genomes had ever been sequenced. The time and expense needed to compare the genomes from a bit of evidence and from a single suspect would take forever and cost a huge amount of money. It’s just not practical.

But if the new technology announced in this WSJ video is for real, all that could change very soon.

If the technology is available to analyze and compare all 3.3 billion base pairs rapidly and cheaply, the current system of comparing 9 or 10 or 13 loci will be woefully inadequate. The current system relies on interpretations that can be incorrect, and statistics that can very easily be (more…)

DNA Evidence: Good Science, Bad Results

Monday, March 8th, 2010

electropherogram-mixture

A couple of weeks ago, we taught another CLE course for the good folks at West Legal Ed Center, in our “Hope for Hopeless Cases” series. This one was on ways to defend cases where the government is going to use DNA evidence to prove your client’s guilt. (Here’s a link.)

DNA evidence can be just devastating. The science is good, after all. And to a lot of potential jurors (and judges and lawyers, unfortunately), “science” is another word for “magic.” Which is another word for “I don’t have to understand how it works, all I know is that it must be so.”

This can often be a wonderful thing, when the science is used correctly, and for the limited purposes to which it is suited. When used correctly, DNA evidence can free the innocent, and help ensure that we really are only punishing the guilty.

The problem is, DNA evidence is all too often used wrong.

And when that happens, the wrong people can get convicted.

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And now today we read a good article in the latest Washington Monthly called “DNA’s Dirty Little Secret: A forensic tool renowned for exonerating the innocent may actually be putting them in prison.” (Link here.)

It’s a good article, about the case of John Puckett, who was convicted in 2008 of an old murder from 1972. It was a brutal rape and murder, with about 20 suspects at the beginning, but the case went cold. Then in 2003 the police tested the DNA found in the evidence. It was old DNA and degraded, and it was also a mixture of multiple people’s DNA. The results were compared to California’s DNA database, and there was a possible match with Mr. Puckett. He hadn’t been a suspect in 1972, but based on this apparent match — and on nothing else — he was prosecuted and ultimately convicted. Jurors have since said that they convicted because of the statistical odds quoted to them at trial, and that if they had known the stats of false positives — which were one in three — they never would have trusted the government’s stats like that.

The article highlights the fact that DNA evidence may be based on good science, but by the time it gets to a jury it can be seriously flawed. Contrary to popular belief, DNA evidence is not objective. It involves a huge amount of subjective interpretation and judgment calls. And whenever human beings have to interpret data and make judgment calls, there is a lot of room for reasonable doubt.

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Contamination, of course, can be a huge issue. Cops screw up when they collect biological evidence, when they stick it in evidence bags, and when they ship it off to the lab. There’s all kinds of opportunities in the real world for a suspect’s DNA to get mixed up with the evidence sample. We’ve worked on at least two cases in the last six months where that is exactly what seems to have happened.

But leaving aside contamination, there are all kinds of ways that experts can look at DNA evidence results and draw the wrong conclusion.

There are technical errors in the lab, for one thing. Sometimes they analyze the wrong evidence. Sometimes the machines doing the analysis aren’t working properly. Sometimes the lab doesn’t test control samples and negative controls, to see if the machines are working right, and whether they’re giving false positives. They almost never do double-blind analysis. Often, analysts will manually adjust the data results, adding or deleting data (!) when it doesn’t look right to them.

There are analytical errors all the time, too. They’re comparing (more…)

Ninth Circuit Bungles Math, Can the Supremes Fix It?

Tuesday, September 1st, 2009

Prosecutor's Fallacy

The “Prosecutor’s Fallacy” is one example of why we think Statistics should be a required course in college. Let’s say the police have the DNA of a rapist. Only 1 in 3,000,000 people chosen at random will match that DNA sample. Your DNA matches. At your trial, the DNA expert testifies that you have only a 1 in 3,000,000 chance of being innocent. That is not correct, however. That’s an example of the Prosecutor’s Fallacy.

Yes, there is a very small chance that someone’s DNA would match if they were innocent. But that is not the same as saying there’s a very small chance that someone is innocent if their DNA matches.

This is basic conditional probability. And if you think about it, it’s just common sense. What you’re doing is switching the conditions around, and leaving the result unchanged. You can’t expect to change the conditions and not change the result.

To illustrate with an extreme example, we drew the picture you see above. A black circle indicates a DNA match. All guilty people are going to have a DNA match, obviously. And a tiny fraction of innocent people are going to have a DNA match, as well. But if the number of innocent people is large enough, then the number of innocent people whose DNA matches could actually be larger than the number of guilty people. Someone whose DNA matches is actually more likely to be innocent in that scenario.

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Prosecutors and DNA experts aren’t the only ones who get this wrong. Courts do, too. The Ninth Circuit recently made a hash of it in their decision in McDaniel v. Brown, which will now be one of the first cases to be heard by the Supreme Court at the start of this year’s October term.

In McDaniel v. Brown, Troy Brown was prosecuted for the alleged rape of a little girl. The facts are pretty gruesome, but irrelevant here. What’s relevant is that, at his trial, the DNA expert testified that Brown’s DNA matched the DNA in the semen found on the girl, that there was a 1 in 3,000,000 chance that someone’s DNA would match, and that therefore there was a 1 in 3,000,000 chance that Brown was innocent.

Brown got convicted. He later brought a habeas petition to the District Court. He introduced a professor’s explanation of how the prosecution had screwed up. The District Court expanded the record to include the professor’s explanation, and found that the DNA expert had engaged in the Prosecutor’s Fallacy. In part because of that (there was also a chance it could have been his brother’s DNA), the District Court found there wasn’t sufficient evidence to convict.

The government appealed to the Ninth Circuit.

Now, the Ninth is known for being touchy-feely. It’s not known for its analytical prowess. Posner, they ain’t. But they bravely tackled this statistical conundrum. And they screwed up.

In trying to deal with the prosecution’s error, the Ninth swung too far in the other direction, finding that the DNA evidence at Brown’s trial couldn’t establish guilt, period. No jury could have found Brown guilty.

So the government took it to the Supreme Court, making two arguments. One is procedural — that the habeas court shouldn’t have been able to consider the professor’s explanations, but only the trial record, in determining the sufficiency of the evidence before the jury. The other argument is that even though the chances of Brown being innocent weren’t 1 in 3,000,000 they were still pretty damn low, and the DNA evidence is still plenty sufficient.

Brown’s lawyers, to their credit, don’t seem to be arguing that the Ninth Circuit did it right. Instead of characterizing the decision below as ruling on the sufficiency of the evidence, Brown’s attorneys argue that it was really a Due Process ruling. The testimony wasn’t so much insufficient as it was incorrect. It was unreliable. This is bolstered by the fact that the Ninth Circuit ordered a new trial (which Double Jeopardy would preclude after a finding of insufficient evidence, but which is standard after a Due Process finding of unreliable evidence.)

That’s not the way the Ninth characterized its ruling, however, so Brown wisely suggested that the Supreme Court might simply kick the case back for the Circuit to explain its ruling better.

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Oral arguments are scheduled for October 13. We haven’t made any predictions yet about the upcoming term, so we’ll start here.

We think the state will convince Chief Justice Roberts and Justices Scalia, Kennedy, Thomas, Ginsberg and Alito of the following:

(1) The Ninth Circuit improperly remanded for a new trial, which is improper after a finding of insufficiency; and

(2) At any rate, the Circuit improperly found the evidence to be insufficient, when there was plenty of evidence of guilt.

We think that Justices Stevens and Breyer (we have no clue about Sotomayor) will dissent, arguing that the jury was totally thrown by the DNA expert’s mischaracterization, that this was a Due Process violation at the very least, and that the DNA evidence probably should have been thrown out entirely, so the Ninth Circuit should be reversed and the District Court’s original ruling should be reinstated.

What are the odds that we’re really right? Who wants to do the math?

Mandatory DNA Sampling Constitutional. Expect Ruling to be Upheld.

Friday, May 29th, 2009

dna.png

In a decision sure to be fought before the 9th Circuit, a federal judge in the Eastern District of California yesterday upheld mandatory DNA collection from people merely arrested for federal felonies, regardless of the nature of the crime charged.

Obviously, this raises eyebrows in certain circles. Taking DNA from people who haven’t even been convicted yet? Taking DNA from people who aren’t suspected of committing crimes where DNA would even be relevant? Doesn’t this violate basic principles of our jurisprudence?

Well… and this is a defense attorney talking here… no.

The case is U.S. v. Pool, decided by Judge Gregory G. Hollows. The defendant was charged with possession of child porn, and was released on bond. One of the conditions of release was that he provide a DNA sample.

This requirement was mandatory under two federal laws: the Bail Reform Act, 18 U.S.C. § 3142(b) and (c)(1)(A), which mandates it for pre-trial release; and the DNA Fingerprinting Act of 2005, 42 U.S.C. § 14135a, which mandates it for everyone arrested on a federal felony charge.

DNA is usually collected by dabbing a cotton swab in the person’s mouth or something similar. Rarely, it is collected by a blood test. The DNA is to be used solely by law enforcement for identification purposes.

Pool argued that this warrantless DNA sampling violates the Fourth Amendment. It’s a search, there’s no warrant, and there’s no special need for the testing for nonviolent arrestees.

Judge Hollows rejected that argument, stating that every Circuit to consider the issue has held there to be no Fourth Amendment violation here, and that the criterion is not “special need” but rather the “totality of the circumstances.” The reasonableness “is determined by assessing, on the one hand, the degree to which it intrudes upon an individual’s privacy, and on the other, the degree to which it is needed for the promotion of legitimate governmental interests.”

Pool argued that pre-conviction sampling is improper, based on the Supreme Court cases Ferguson v. City of Charleston, 532 U.S. 67 (2001)(unconstitutional search for law enforcement to use hospital’s diagnostic test of pregnant patient to obtain evidence of drug use), and City of Indianapolis v. Edmond, 531 U.S. 32 (2000)(vehicle checkpoint unconstitutional when primary purpose was to detect evidence of drug trafficking). Those cases relied on the “special need” analysis he suggested.

Judge Hollows rejected that as well, as those searches involved police fishing for evidence, before anyone was formally charged with a crime. The statutes at issue here subject people to DNA testing after a finding of probable cause by a judge or grand jury. After someone’s been indicted, courts can impose all kinds of restrictions on liberty. The situation is much more like that of people who have been convicted, than of people who have not yet been charged with anything, and so the “totality of the circumstances” test is more appropriate.

For more than 45 years, it’s been well-settled that someone who’s been arrested has a diminished expectation of privacy in his own identity. He can be compelled to give fingerprints, have his mug shot taken, and give ID information. DNA is no different than fingerprints — a unique identifier that helps law enforcement find the right suspect, and eliminate the wrong suspect. In fact, DNA is more precise than photos or fingerprints, so the government interest in obtaining it is even stronger.

Meanwhile, the invasiveness is minimal. Even blood tests are considered “commonplace, safe, and do not constitute an unduly extensive imposition on an individual’s privacy and bodily integrity.” Oral swabs are considered no more physically invasive than taking fingerprints.

The judge also rejected arguments that DNA evidence, once taken, might possibly be stolen and put to an impermissible use. That risk applies to everything, and there are criminal penalties to deter it. Just because someone might break the law doesn’t mean the setup is improper.

Judge Hollows pointed out that all the same concerns being raised about DNA were raised in the early part of the 20th Century with respect to fingerprints. And since at least 1932 it’s been understood that the public interest far outweighs the minimal burden to the individual being fingerprinted. The same reasons that justify post-arrest fingerprinting without a warrant justify post-arrest DNA sampling without a warrant.

Pool also argued that this violates Fifth Amendment procedural due process, because it’s mandatory, and thus precludes an opportunity to be heard. But that only applies if the defendant’s privacy rights outweigh the government interest, and it’s the other way around here. Pool argued that there is a risk of erroneous deprivation of his privacy interest, for arrestees who are not ultimately convicted. But the system is set up to expunge DNA records if the person is exonerated or the charges are dismissed. So the risks are minimal, and the government interests are compelling, and that means there is no procedural due process problem.

Pool also argued that this violates the Eighth Amendment protection against excessive bail. Bail conditions have to be proportionate to the perceived government need requiring the condition. But the Supreme Court case that set this rule, U.S. v. Salerno, 481 U.S. 739 (1987), specifically rejected any idea that this “categorically prohibits the government from pursuing other admittedly compelling interests through regulation of pretrial release.” This being nothing more than a booking procedure, and not comparable to conditions of release that actually have to do with the concerns arising from letting someone out on bail, there’s no reason to consider it excessive.

Pool also argued that the statutes violate the Separation of Powers, as Congress has intruded on judicial decision-making in the setting of bail conditions. But here, Congress didn’t direct any judicial findings. It merely directs what the judge needs to do after a certain finding has been made. That’s what Congress is supposed to do. There’s no problem there.

Poole finally argued that this is an unconstitutional extension of power, because the Commerce Clause doesn’t authorize DNA sampling. But the Commerce Clause lets the government make conduct a federal crime. The resulting government powers, such as incarceration and terms of release, have nothing to do with it, and don’t need to be independently authorized under the Commerce Clause.

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What to make of this?

Pool’s arguments stem from a presumption that a person out on bail is more like a pre-arrest suspect. Judge Hollows’ decision stems from the opposite conclusion, that a person out on bail is more like a person on post-conviction supervised release. Any arguments before the 9th Circuit will have to focus on which it is, and we are inclined to believe that the Circuit will side with Judge Hollows here.

Central to the distinction is the fact that there has already been a judicial determination here, separating the defendant from the class of unarrested individuals. Either a judge or a jury has found that it is more likely than not that a federal felony was committed, and that this person did it. Once that has happened, a person’s rights are substantially changed. Society has an interest in ensuring that they come back to court to be judged. Society has an interest in ensuring that they don’t cause more harm in the meantime. These interests outweigh a defendant’s interests in liberty and property, to varying degree depending on the individual. That’s why we have bail and bail conditions.

What is odd, however, is that Congress made DNA sampling a mandatory bail condition, when it has nothing to do with pre-trial release.

Judge Hollows correctly points out that, conceptually, DNA sampling is no more invasive than fingerprinting, and is used for the same purposes. It’s a booking procedure, not a release consideration. Congress could just as easily have made DNA sampling a mandatory part of post-arrest processing, along with the mug shot and fingerprints. It would have been just as constitutionally sound.

By calling it something that it’s not, Congress subjected DNA sampling to this exact challenge.

Now, the ACLU differs with us, and calls the ruling “an incredible threat to civil liberties.”

“We think this ruling is incorrect,” ACLU attorney Michael Risher told reporters. “It ignores the presumption of innocence and it does not pay enough attention to the protections of the Fourth Amendment.” He also opined that police now have an incentive to make pretext arrests, just to get people’s DNA to help them solve crimes. How this changes things from the already-existing incentive to make pretext arrests to get fingerprints is unclear to this defense attorney. And anyway, police don’t need to arrest someone to get DNA or fingerprints — they can be collected by pretext in any number of ways, without a warrant, and often are.

With respect to the Fourth Amendment, what is clear here is that this is not a search for evidence. The crime has already been charged. It’s very clearly an administrative tool for establishing the identity of the defendant. Evidentiary consequences are merely hypothetical, if the person should somehow commit a violent crime in the future and leave behind DNA that gets compared to the database. That’s no different from mug shots, and unlike mug shots (where the chances of a false positive are unreasonably and embarrassingly high, given their variety and the innate unreliability of eyewitness recognition) DNA has an insignificant risk of identifying the wrong person. Mug shots aren’t a Fourth Amendment issue, neither are fingerprints, and neither is DNA, really.

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One issue, however, is when the DNA is being taken for the purpose of gathering evidence, in the investigation of a crime.

That’s not the case here, and it’s sort of off point, but should a warrant even be involved then?

Well, isn’t it a Fifth Amendment violation then? You’re making someone incriminate himself against his will, right?

Wrong. Self-incrimination doesn’t enter into it, because what’s important there, the underlying policy of the right, is that we don’t want the government overriding people’s free will, and making them convict themselves out of their own mouths. We don’t want another Star Chamber. We don’t want the government using its overwhelming power to extort unwilling confessions, whether by thumbscrews, lead pipes, or simple custodial interrogation.

But taking blood samples has been held not to involve the right against compelled self-incrimination. Nobody’s being forced to say “I did it.” All they are being forced to do is provide physical evidence. There is no free will involved in the creation of that physical evidence — it exists whether the person wants to hand it over or not — but there is free will involved in the creation of confessions and incriminating statements.

But that brings us back to the Fourth Amendment. If someone is being compelled to give a swab or blood sample, then the government is seizing pre-existing evidence just as if they were seizing drugs from someone’s home. So shouldn’t a warrant be required after all?

Yes it should. But that’s only when the evidence is being sought as evidence. Constitutional rights really do depend on what’s going on. An administrative requirement is not the same thing as a criminal investigation. A DNA sample for administrative ID purposes is not the same thing as one taken to identify a potential suspect.

That’s the big difference here. And even given the 9th Circuit’s pro-defendant tendencies from time to time, we have a hard time predicting anything but an affirmation of Judge Hollows’ decision when this comes up on appeal.

As Technology Improves, Solving Murders Gets Harder (fractal weirdness)

Thursday, January 8th, 2009

Homicide Clearance Rates

In 1963, the first year of comparable recordkeeping, 91% of murders were solved. In 2007, the number was only 61%.

At the same time, the technological ability to solve murders increased dramatically. Scientific crime scene investigation significantly increases the amount of useful evidence that can be found. Digital crime labs and computerized analysis make it easier to interpret that evidence. And of course, modern DNA techniques enable police to make unbelievably accurate identifications from the smallest particle of hair or fluid. Today’s reality would have been a science fiction fantasy twenty years ago.

So what gives?

For one thing, the kinds of murders have changed. In previous generations, murder was almost always a personal matter. The victim and the killer knew each other, had a relationship. Husbands killed wives. Friends killed friends. Rivals killed each other. To begin a successful investigation, a detective would paint a bull’s-eye on the victim. The closer a suspect was to that bull’s-eye, the more likely they were to be the killer. Cases were solved not so much by technology and physical evidence, as by getting people to talk or confess. Acquaintance homicides were, and still are, often solved because the killer contacted the police or surrendered himself.

But now, a significant number of murders are committed by gang members. Gang members and drug dealers get killed by their own groups, who aren’t likely to talk lest they be killed themselves. They get killed by members of rival gangs, and may not even know their killers. Killers may even kill completely unrelated, innocent people, through mistaken identity or reckless “drive-by” shootings. Witnesses are intimidated by the threat of being killed themselves if they come forward. So relying on people to talk or confess is not as likely to solve these crimes.

For another thing, technology only gets you so far. DNA only identifies someone if you have a sample of their DNA to compare. Gunshot residue only helps if you have the suspect’s fingers in the first place. Fingerprints are harder to find than people think, and even then can only be compared to known fingerprints. In other words, technology helps you confirm that you have the right suspect, but first you have to get that suspect. And getting the suspect in the first place often means an old-fashioned investment of shoe leather — hitting the streets, talking to possible witnesses, and conducting skilled interrogations.

Because of the advances in technology, acquaintance homicides are truly being solved at a greater rate than they were in previous decades. The suspects are known, or easily found, so the DNA and other scientific tests make identifying the killer much more certain. The scientific identification also helps get confessions.

But stranger-to-stranger homicides have increased dramatically. And despite the technological advances, these continue to have a high probability of never being solved. Motive is hard to figure out. The killings are often part of a planned crime, so that less evidence will be left behind for law enforcement to find. And any connection between killer and victim is going to be hard or impossible to identify.

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So what can be done?

Studies find no correlation between the number of available police officers, or the amount of their budget, and the ability to clear homicide cases. So shoving more officers on the street, or shoveling more money at the problem, is not a solution.

Studies do show, however, that cases get cleared when detectives are ambitious and they are held accountable for the success or failure of their investigations. Cases get cleared more often when the detectives have the necessary time to devote to the investigation, and when they are part of a specialized unit where everybody is focusing on the same kind of crime.

How do you get ambitious detectives? Study after study shows this to be a huge factor. Media attention can help, when there is a lot of pressure to solve a high-profile case. But in urban areas the media is often antagonistic, media praise of police rare, and so is an underdeveloped tool. Better P.R. by the police could improve ambition. Increased internal attention, status and reward for greater clearance rates would help, as well.

Solving stranger or gang-related murders requires witnesses to come forward. They fear retribution, or being punished themselves for their own crimes. Most murders, even stranger murders, are witnessed. So a critical need is to overcome witness fears.

Studies have found that most witnesses were actually involved with the crime. They either took part in some way, they brought the killer and victim together, or they tried to stop the murder from taking place. “Innocent bystanders” only make up 9% of witnesses.

Civic pride is not likely to cause the majority of witnesses to come forward. Gang culture, and the culture of the communities where such gangs flourish, teaches witnesses to do the opposite. Cash rewards sometimes help, but the amounts commonly offered are simply too small to justify the risks a witness would run if he came forward.

Ensured anonymity is a must. But in a judicial system that properly allows the accused to see and confront his accusers, anonymity cannot be ensures. Witnesses know this. Only a real and system-wide practice of concealing the appearance and identity of witnesses to violent crimes is likely to inspire the necessary confidence. And in our legal culture, we as Americans simply value the confrontation rights of the accused more than we value the evidence we might gain by limiting those rights. That’s just the way it is.

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Reducing gangs themselves, and changing the culture in which they flourish, is the long-term solution.

Gangs arise within subcultures where there is little other societal bonding and community for young males, where those young males lack (or do not see) the ability to gain status and women otherwise, and where there is a general lack of control over one’s life. Entertainment media have a huge impact on perceptions of the world. These factors create perverse incentives, so that gang membership and codes of behavior can seem to be the right choice to make.

Common factors of such communities are a lack of value placed on education, a reliance on government or others, a lack of ownership, and a xenophobic relationship with the larger community. Undervalued education minimizes earnings and options in adulthood, as the lack of parental involvement kills schools and a thou-shalt-not-do-better-than-us attitude among peers kills student ambition. Reliance on welfare, the police, programs and others to take care of life’s needs leads to an endemic lack of personal responsibility, which kills family ties and any bond to a larger civic society. Illiteracy, immersion in the skewed reality of television and musical entertainments, and a perception that the rest of society is foreign and irrelevant, further impact perceptions of how the world works.

These problems have often been many generations in the making, and are not susceptible to overnight changes. Policy changes would be required that strengthen the family bond, rather than giving incentives to father children from multiple mothers without requiring any long-term ties and responsibilities. Policy changes would be required that lead community members to see themselves as part of the larger society, and not separate from it, subject to separate rules. Policy changes would be required that create incentives for parental involvement in schools, and pave the way for cultural views of education as the means to success.