{"id":2254,"date":"2006-04-14T23:14:40","date_gmt":"2006-04-15T06:14:40","guid":{"rendered":"http:\/\/www.amptoons.com\/blog\/?p=2254"},"modified":"2006-04-14T23:14:40","modified_gmt":"2006-04-15T06:14:40","slug":"experts-answer-what-does-dna-evidence-prove","status":"publish","type":"post","link":"https:\/\/amptoons.com\/blog\/?p=2254","title":{"rendered":"Experts answer: What Does DNA Evidence Prove?"},"content":{"rendered":"

(Note: I did the research and much of the writing for this post before Blac(k)ademic posted her critique<\/a> of writing which treats rape “like a damn sports event where we are taking sides and rooting for each side based on DNA samples.” I’ll don’t really disagree with Blac(k)ademic, but I still think that engaging with the particulars is sometimes necessary, and is possible to do without treating rape like a sports event. I intend to post more on this later, but for now I gotta get away from the computer for a while.) <\/em><\/p>\n

Now that DNA evidence has not shown a connection between Mary Doe, the alleged victim, and any of the Duke Lacrosse players, many people are saying that the case should be closed. Some are even saying that the DNA proves that no rape happened, and Mary Doe made a false accusation.<\/p>\n

To tell you the truth, until I began researching this post, I knew nothing about DNA evidence beyond what I’ve seen on TV detective shows. The public comments about DNA in this case have all come from defense lawyers, whose statements may be more about what they want potential jurors to hear than about what the most truthful analysis is. (The same is true for public statements made by the DA, of course).<\/p>\n

In a comment<\/a> at Ginmar’s livejournal, Fiona64<\/a> identified herself as a “forensic science geek” who, while not an expert, at least knows more about DNA evidence than the average CSI watcher. I emailed her with a bunch of questions, which she was kind enough to answer.<\/p>\n

That gave me the idea of emailing evidence analysts – experts who write technical papers and testify in courtrooms – and asking them background questions. I cut down my list of questions to just four (I thought there was a better chance of getting responses if there were fewer questions asked) and emailed several DNA evidence experts. To my surprise, several were kind enough to email me back.<\/p>\n

So here I’m going to summarize the answers. Then, below the fold, you’ll find the full text of all the responses I got, along with a brief description of each expert’s background (the background descriptions are quoted from this website<\/a>). I’ve tried to be honest in how I’ve quoted folks, but I’ve also been fitting in writing this post around Passover and work, so my advice is to go ahead and read the full answers.<\/p>\n

Keep in mind that these answers are only general background comments about what DNA evidence can and cannot prove, not specific comments about the Duke rape case.<\/p>\n

On the whole, the most striking thing to me about the responses is the lack <\/em>of unanimity between the experts. It’s pretty clear that DNA evidence is a field in which experts can, and do, disagree.<\/p>\n

Question one: 1. If DNA evidence fails to prove that an accused person raped an alleged rape victim, does that prove that the accused person could not have raped the alleged victim?<\/p><\/blockquote>\n

Here, all experts agreed that it is possible for DNA to not be left behind by a rapist, but different experts seemed to disagree about how likely <\/em>that is.<\/p>\n

Jennifer Friedman, of the Los Angeles County Innocence Project, gave an answer which implied that it’s extremely unlikely that anyone could rape and not leave DNA behind (unless they used a condom). “In general, if there is a sexual assault with penetration either vaginal or anal and no condom is used one would expect DNA to be present. Even if the perpetrator did not ejaculate, his epithelial cells will often times be left behind. In order to answer this question most accurately, I would need to know specifically what is alleged to have occurred. Occasionally, DNA may be left on the alleged victim and yet the person who swabbed the area may have missed the area with the DNA, but this is rare.”<\/p>\n

On the other hand, Elizabeth Johnson flatly answered “no” (perhaps because she was in a rush), and William Thompson wrote that “absence of evidence is not necessarily evidence of absence.”<\/p>\n

2. Is it possible for a rape to happen and for no useful DNA evidence to be left behind? Put another way, if someone says she was raped, but no DNA evidence supports her claim, does that prove she made a false rape report?<\/p><\/blockquote>\n

Not one expert was willing to say that absence of DNA evidence is proof of a false rape accusation. But how strongly they said “no” to this question varied significantly.<\/p>\n

On the “weak no” side, Dan Krane wrote: “It is worth noting that DNA tests are amazingly sensitive (DNA profiles can be generated from as little material as that left behind in a fingerprint) and Y-STR tests have the potential of determining a male’s DNA profile even when a female’s DNA is present in hundreds or thousands of times greater quantities. Scientists are always wary of asserting that the absence of evidence is not proof of absence but it certainly is reasonable to expect to find a rapist’s DNA associated with a victim when the victim presents herself to investigators within hours of an attack and when she has not bathed, the rapist did not use a condom and ejaculation occurred.”<\/p>\n

On the other hand, Simon Ford believes that CSI has given the public an inflated idea of how certain and quick DNA tests are, and writes “If the claim is such that one would expect to see biological material and none is found, then sure it may be an indication of a false claim, but there are really so many other potential explanations, particularly when just dealing with the first round of DNA testing, such as issues like condom use, vasectomy, choice of test (autosomal STR v. Y-STR), choice of samples to test, many other things like this can all play a part.”<\/p>\n

And William Shields wrote “unless the victim stated that she was sure there no condoms used the absence of DNA could not prove she was mistaken much less that she made a false report.”<\/p>\n

Finally, Fiona64 writes “It may, however, mean that the proverbial net needs to be thrown wider in order to obtain a different pool of suspects.”<\/p>\n

3. I’ve seen it claimed that mixed DNA from multiple rapists will not necessarily match the individual DNA of any of the rapists, so multiple rapists not using condoms may be unidentifiable. This seems dubious to me. Do you know if this is true?<\/p><\/blockquote>\n

The answers given by experts to this question were truly all over the place. Several experts do not think there is any danger of a false exclusion in such a scenario. For instance, William Thompson of the University of California wrote “It depends on the way the analyst chooses to interpret the mixed profile. In the cases I look at, the analyst are usually quite lenient about what they will call a ‘match.’ A mixture of DNA from three or more men can often be interpreted in a manner that allows a very substantial fraction of the male population to be ‘included’ as a potential contributor.”<\/p>\n

Another expert, Jennifer Friedman, writes “If someone is excluded, he is definitively excluded.”<\/p>\n

But William Shields of the State University of New York wrote, “If three, four, or more people donate DNA then there will be so many alleles in a mixture that very few if any people can be excluded as potential contributors. In such an event the evidence does become useless.”<\/p>\n

4. Is it possible for a condom to be used, without physical evidence of condom use (traces of latex, etc) being left behind?<\/p><\/blockquote>\n

A few experts I contacted didn’t feel they knew enough to answer this question.<\/p>\n

Of the experts who answered this question, however, all agreed that condoms could be used and not necessarily detected. Elizabeth Johnson wrote, “Testing for these substances is not typically done, despite what you see on CSI. There has been some research done re spermicides on condoms, but none of this is done as part of a typical test and validated methods for casework aren’t there yet.” William Shields wrote “This question is better asked of a forensic chemist but I do know that such traces are often but not always left behind.” And William Thompson flatly answered “yes.”<\/p>\n

So what do I conclude from all this?<\/p>\n

Despite the DNA evidence, from what the experts say it’s possible that condoms were used, preventing DNA evidence from being left behind. It’s also possible that the rapists were party guests but not members of the lacrosse team. The idea that this case has now been settled, or that Mary Doe has been proved a liar, rests on weak and inconclusive evidence. It is clear that those who have say the DNA evidence proves no rape took place are vastly exaggerating what DNA evidence (or the lack of DNA evidence) can conclusively show.<\/p>\n

(The other evidence I’ve seen put forward to support the “it was a false rape accusation” arguments – 911 calls, photos, Mary Doe’s past arrest, etc. – I don’t see as even a tiny bit persuasive, for reasons I’ll describe in an upcoming post.)<\/p>\n

I don’t deny that it’s possible no rape took place, and clearly this possibility is supported (but far <\/em>from proved) by initial DNA results. Still, given all the evidence available so far, I continue to believe Mary Doe’s claim that she was raped at the lacrosse player’s party. And although there may not be enough evidence for “guilty beyond a reasonable doubt,” that in and of itself doesn’t prove that no rape took place. Rape is, contrary to popular belief, an extremely <\/em>difficult crime to prove in a courtroom; there are many more rapists than there are tried and convicted rapists.<\/p>\n

****Important note for comment-writers****: Comments on this post are for “feminist and feminist-friendly posters” only. If you are a poster who is unknown to me, and you leave a comment that is not clearly coming from a feminist point of view, I will probably NOT <\/em>let the comment through. However, everyone is welcome to post comments on the exact same post at Creative Destruction<\/a>. So if you’re not clearly a feminist, and you want your comment to be seen, I strongly advise you to post it over there, rather than on “Alas.”<\/strong><\/p><\/blockquote>\n

* * *<\/p>\n

Below the fold are the full answers given by each of the experts. I think all the answers are pretty interesting, and recommend reading them all. If you only have time for one, though, I think Simon Ford’s email is especially valuable both for the background information it contains and for the picture it paints of how complex questions of DNA evidence can be.
\n
\nFiona64<\/strong><\/a><\/p>\n

I will do my best to answer your questions, with the caveat that I am *not* an evidence analyst; I work for the lab and am majoring in forensic anthropology (I will eventually work *in* the lab). I can find source material to answer the things about which I’m uncertain, though, so please feel free to ask away.\n\n

If DNA evidence fails to prove that an accused person raped an alleged rape victim, does that prove that the accused person could not have raped the alleged victim?<\/p><\/blockquote>\n

Nope. Lack of DNA evidence is not automatically exculpatory. Really, the vast majority of crimes do not have DNA evidence.<\/p>\n

Is it possible for a rape to happen and for no useful DNA evidence to be left behind?<\/p><\/blockquote>\n

Most assuredly. I can think of a number of scenarios in which that might be the case; the obvious one is when a rapist uses a condom.<\/p>\n

Is it possible for no conclusive DNA evidence to be left behind even if the rapist(s) doesn’t use a condom?<\/p><\/blockquote>\n

This is a little trickier; rape with an object, or by an assailant who shaves his body hair, or in a “drug-assisted” situation (i.e., GHB, Rohypnol) are just a couple of scenarios in which this might be the case.<\/p>\n

Other circumstances in which rape may have occurred without DNA being present in the victim include the assailant having had a vasectomy, failed to ejaculate, or is azoospermic (a shmancy word that means his semen does not contain spermatozoa).<\/p>\n

If DNA evidence fails to prove a connection between a group of suspects and an alleged rape victim, does that prove that no rape took place?<\/p><\/blockquote>\n

Not at all. It may, however, mean that the proverbial net needs to be thrown wider in order to obtain a different pool of suspects.<\/p>\n

You’ve said that “Mitochondrial DNA (from hair samples) and epithelial DNA (from shed skin cells on clothing, for example) takes significantly longer to process. We have come along way with DNA analysis; a sperm sample used to take 3 months to analyze, and we can now do it in 48 hours. However, mitochondrial and epithelial samples still take as much as 3 weeks just to replicate an adequate sample to test.”<\/p>\n

Can you explain the significance of this? Is it possible for the mitochondrial and epithelial DNA to have different results than the initial DNA analysis?<\/p><\/blockquote>\n

Nuclear DNA is what most people think of when it comes to evidence — that’s what comes from blood, semen, pulled hair (with root bulb, which is where the nuclear DNA is) and buccal (cheek) cells. Mitochondrial DNA can come from bones, teeth or cut hair … and it has to be replicated by PCR (polymerase chain reaction) most of the time in order to have an adequate sample to look at. The difference comes in with mDNA in that it is matriarchal in nature; that is, your mDNA is virtually identical to your mother’s, grandmother’s, etc. Without a matriarchal sample for comparison, mDNA is not particularly useful. Furthermore, if two potential assailants are siblings, their mDNA will be nearly identical. :-\/ BTW, the reason that mDNA could be used to identify the corpse of Laci Peterson (which is how they did it) was that they had bone samples *and* the availability of Laci Peterson’s mom for additional samples.<\/p>\n

Epithelial cells can only be tested for DNA if they contain glycogen … and not all epithelials do.<\/p>\n

So, as you can see, it’s not always a straightforward thing. Ideally, all cases would be awash with nuclear DNA … from a forensic point of view, anyway. It would make it very easy to get the right person every single time, and it’s just not like that.<\/p>\n

I’ve seen it claimed that mixed DNA from multiple rapists will not necessarily match the individual DNA of any of the rapists, so multiple rapists not using condoms may be unidentifiable. Do you know if this is true?<\/p><\/blockquote>\n

I don’t know whether it’s true, but it stands to reason strictly from a physical science perspective. You have liquid (semen) from more than one source, and it’s going to have pretty much the same density. An analyst could say “there’s more than one source here,” but not be able to tease out which cells go with which source. It’s like pouring a bottle of Aquafina and a bottle of Dasani into the same pitcher; you know it’s two different kinds of water, from two different sources, but you can no longer tell once they’ve intermingled which one is which.<\/p>\n

Is it possible for a condom to be used, without physical evidence of condom use (traces of latex, etc) being left behind?<\/p><\/blockquote>\n

Again, I don’t know the exact answer here. The theory is that anything you touch could potential transfer trace evidence; the best test I’ve ever seen for this is suggesting that one walk in stocking feet from one room to the next, and then see what you took with you. However, I could envision a scenario in which non-latex (lambskin) condoms might have been used … and those typically also do not have spermicide.\n<\/pracut>\n

Jennifer Friedman<\/strong>
\nJennifer Friedman, J.D., Deputy Public Defender and Forensic Science Coordinator with the Los Angeles Public Defender’s Office. She has litigated several high-profile cases where DNA testing played a pivotal role and is also the founder and former head of the Los Angeles County Innocence Project.\n\n

1. If DNA evidence fails to prove that an accused person raped an alleged rape victim, does that prove that the accused person could not have raped the alleged victim?<\/p><\/blockquote>\n

It does not absolutely prove that the accused could not have committed the crime. However, in general, if there is a sexual assault with penetration either vaginal or anal and no condom is used one would expect DNA to be present. Even if the perpetrator did not ejaculate, his epithelial cells will often times be left behind. In order to answer this question most accurately, I would need to know specifically what is alleged to have occurred. Occasionally, DNA may be left on the alleged victim and yet the person who swabbed the area may have missed the area with the DNA, but this is rare.<\/p>\n

2. Is it possible for a rape to happen and for no useful DNA evidence to be left behind? Put another way, if someone says she was raped, but no DNA evidence supports her claim, does that prove she made a false rape report? <\/p><\/blockquote>\n

See above.<\/p>\n

3. I’ve seen it claimed that mixed DNA from multiple rapists will not necessarily match the individual DNA of any of the rapists, so multiple rapists not using condoms may be unidentifiable. This seems dubious to me. Do you know if this is true?<\/p><\/blockquote>\n

I would disagree with this. If DNA is left behind, particularly in an intimate sample (so that the male and female DNA may be separated)even if there is a mixture, individuals will either be included or excluded based on the alleles present. If someone is excluded, he is definitively excluded. If someone is included, this might be significant or insignificant depending upon how many others are also included. What is difficult about mixtures, is determining which genetic profiles are actually present. Thus, if someone is excluded, he is excluded. But if someone is included, he may not necessarily be the person who left the DNA.<\/p>\n

4. Is it possible for a condom to be used, without physical evidence of condom use (traces of latex, etc) being left behind?<\/p><\/blockquote>\n

This is not my area of expertise but I have seen a number of cases where the lab tests for the presence of spermicide and concludes based on its present that a condom was recently used.\n<\/pracut>\n

Simon Ford<\/strong>
\nTrained primarily in molecular biology and biochemistry, Dr. Ford is the Founder and President of Lexigen Science and Law Consultants, a firm that specializes in providing advice to lawyers about genetic evidence since 1988. He has personally reviewed the DNA evidence from thousands of criminal investigations and has conducted numerous workshops for agencies on the analysis of STR test results.<\/p>\nBarry:<\/p>\n

For your information, I’ve not been following the reports of the Duke rape case allegations, all I know is that the incident allegedly involves multiple potential assailants, and that the initial DNA report failed to establish a link with any of the individuals tested from the Duke team — that’s about all I know.<\/p>\n

One important issue underlying the media coverage of a story like the Duke incident is that TV shows like CSI have raised unrealistic expectations in the general public of what forensic science, and specifically DNA testing, can do. People get the idea that you run a test, it takes just a few hours, and you get a definitive answer yes or no. In truth DNA testing takes much longer, can produce ambiguous results and has always got to be considered in the framework of the specific question asked and in the context of other biological tests, such as microscopic observation of cell types (for example sperm in a rape case) or serological tests for body fluids (such as semen). Also there are different types of DNA tests. Many labs start with the standard autosomal STR test on sex assault cases. This test looks at regions of DNA which differ from person-to-person and are scattered across the non-sex chromosomes. In some instances, for example samples which contains large amounts of (female) victim DNA, the assailant’s DNA may get swamped out, and so the lab can use one of the Y-STR tests, which homes in on variable regions only found on the Y-chromosome (which men have and women do not). In cases of this type, the initial (autosomal) STR DNA report may fail to report a match with the suspect but a later Y-STR DNA report may incriminate. There can also be other simpler explanations for a negative initial report being followed by a later incriminating report, such as refining the choice of samples to test. I would not read too much into a negative report until the whole testing scenario has been completed.<\/p>\n

With regard to your specific questions. I don’t know much about testing for latex and detection of condom use. The question regarding mixtures is easiest to deal with. A mixed DNA profile will consist of the DNA profiles of the contributing individuals superimposed, one on top of another. It can be quite complex, because not all contributions are going to be in equal amounts, causing unevenness in the profile, and the DNA profiles of the individuals who contribute least to the mixture may well drop below detectable levels. So the first potential problem is that minor contributors may be missed. Beyond that though, it is still possible to answer the question as to whether a specific profile can be excluded as being a potential contributor to the mixed profile.<\/p>\n

Once you have determined that a particular profile cannot be excluded the problem is how to express the significance of that observation; and this is were statistics comes in. The statistical calculations for mixture cases can be quite complicated. In single source cases labs often report stats in the trillions and quadrillions but in mixture cases the stats can be dramatically eroded — the more contributors, the less powerful the stats — it is not uncommon to see stats in the tens or hundreds in mixture cases. The presence of multiple contributors can erode the statistical power of the test to the point whereby, if there are enough contributors, the test really doesn’t have any useful meaning any more. All the lab would be able to say is that there were a large number of contributors, but no further conclusions could be drawn.<\/p>\n

Your two remaining questions are more difficult to answer, because they deal with DNA evidence without giving the context of the specific question being asked for that case or information about other biological testing. Every case is different and has its own nuances. With regard to DNA it all hinges on whether the specific allegations claimed by the victim are amenable to being proven right or wrong by DNA. If the claim is such that one would expect to see biological material and none is found, then sure it may be an indication of a false claim, but there are really so many other potential explanations, particularly when just dealing with the first round of DNA testing, such as issues like condom use, vasectomy, choice of test (autosomal STR v. Y-STR), choice of samples to test, many other things like this can all play a part.<\/p>\n

Sorry I can’t be more definitive, but I hope these comments help illustrate the difficult challenges that labs and attorneys face with regard to the real world usage of DNA evidence in criminal cases.<\/p>\n

All the best,<\/p>\n

Simon Ford<\/pracut>\n

Elizabeth Johnson<\/strong>
\nElizabeth Johnson, Ph.D. has been a practicing forensic scientist for the past thirteen years, specializing in forensic biology and DNA issues. Dr. Johnson established and directed the DNA laboratory at the Harris County Medical Examiner’s Office in Houston, Texas from 1992-1996. She then worked at Technical Associates, a private criminalistics laboratory in Ventura, California, for six years. After leaving Technical Associates, Dr. Johnson has gone into solo practice, based in Thousand Oaks, California, providing testing, review, consultation, testimony, and education to those in need of assistance with forensic DNA.\n\nBarry,<\/p>\n

Sorry for the brevity of the answers…must scram for court.<\/p>\n

Elizabeth Johnson<\/p>\n

1. If DNA evidence fails to prove that an accused person raped an alleged rape victim, does that prove that the accused person could not have raped the alleged victim?<\/p><\/blockquote>\n

No<\/p>\n

2. Is it possible for a rape to happen and for no useful DNA evidence to be left behind? Put another way, if someone says she was raped, but no DNA evidence supports her claim, does that prove she made a false rape report? <\/p><\/blockquote>\n

It is possible that penetration occurs w\/o ejaculation therefore not leaving sufficient biological material in her to be detected in typical DNA tests.<\/p>\n

3. I’ve seen it claimed that mixed DNA from multiple rapists will not necessarily match the individual DNA of any of the rapists, so multiple rapists not using condoms may be unidentifiable. This seems dubious to me. Do you know if this is true?<\/p><\/blockquote>\n

You cannot identify an individual from a mixture. You can exclude or fail to exclude a person from a mixture. Still very useful information.<\/p>\n

4. Is it possible for a condom to be used, without physical evidence of condom use (traces of latex, etc) being left behind?<\/p><\/blockquote>\n

Testing for these substances is not typically done, despite what you see on CSI. There has been some research done re spermicides on condoms, but none of this is done as part of a typical test and validated methods for casework aren’t there yet.\n<\/pracut>\n

Dan Krane<\/strong>
\nDan Krane, Ph.D., Associate Professor of Biological Sciences at Wright State University, his research lab generates thousands of DNA profiles each year as well as the lead author of the best selling undergraduate textbook on bioinformatics. A leading authority on forensic DNA evidence, he is founder and CEO of Forensic Bioinformatics, Inc. and has testified as an expert witness in approximately fifty cases.\n\n

1. If DNA evidence fails to prove that an accused person raped an alleged rape victim, does that prove that the accused person could not have raped the alleged victim?<\/p><\/blockquote>\n

There is no “pat answer” to your question. The circumstances of the investigation (particularly the victim’s recounting of the events when it is available) almost always need to be taken into consideration. For instance, consider a situation in which a woman asserts that she was raped by a single individual and that she had not had sexual contact with any other individual for many days. If investigators identify a suspect and then find that his DNA profile is not the same as the source of semen from the victim, then those test results can be construed as proof that the suspect was not the perpetrator.<\/p>\n

2. Is it possible for a rape to happen and for no useful DNA evidence to be left behind? Put another way, if someone says she was raped, but no DNA evidence supports her claim, does that prove she made a false rape report? <\/p><\/blockquote>\n

It is possible for no DNA profile to be generated during a rape investigation even though a rape had occurred. The greater the amount of time between the rape and evidence collection, the greater the likelihood that no DNA will be recovered. Other factors such as: did the victim bathe or not, did the rapist use a condom or not, and did the rapist ejaculate or not also need to be considered. However, it is worth noting that DNA tests are amazingly sensitive (DNA profiles can be generated from as little material as that left behind in a fingerprint) and Y-STR tests have the potential of determining a male’s DNA profile even when a female’s DNA is present in hundreds or thousands of times greater quantities. Scientists are always wary of asserting that the absence of evidence is not proof of absence but it certainly is reasonable to expect to find a rapist’s DNA associated with a victim when the victim presents herself to investigators within hours of an attack and when she has not bathed, the rapist did not use a condom and ejaculation occurred.<\/p>\n

3. I’ve seen it claimed that mixed DNA from multiple rapists will not necessarily match the individual DNA of any of the rapists, so multiple rapists not using condoms may be unidentifiable. This seems dubious to me. Do you know if this is true?<\/p><\/blockquote>\n

Mixed DNA samples are generally much more difficult to interpret than unmixed samples are. The statistics associated with mixtures are typically much less impressive than those associated with unmixed samples as a direct result. It is possible for large amounts of one or two people’s DNA relative to other contributors to a mixture to mask or obscure the presence of DNA from secondary contributors. However, the presence of one person’s DNA will not change the DNA profile of another contributor to a sample.<\/p>\n

4. Is it possible for a condom to be used, without physical evidence of condom use (traces of latex, etc) being left behind?<\/p><\/blockquote>\n

I don’t think that my opinion on this would be any more expert than your own so I’ll let you come to your own conclusion!\n<\/pracut>\n

William Shields<\/strong>
\n Dr. Shields is a Professor of Biology at the State University of New York, Syracuse. Dr. Shields has become one of the most easily recognized experts in the field of DNA profiling through his participation as a testifying expert in countless highly publicized trials including the on-going CA v. Peterson. He has published widely in the areas of population genetics and mitochondrial DNA testing.<\/p>\nDear Barry: It doesn’t take long to answer your questions so here goes:<\/p>\n

1. If DNA evidence fails to prove that an accused person raped an alleged rape victim, does that prove that the accused person could not have raped the alleged victim?<\/p><\/blockquote>\n

It depends. If condoms were used then the rapist might not leave DNA evidence during the commission of the crime. If the victim said no condoms were used then no DNA would be evidence that she might be lying. If DNA was found but did not match the accused this would be evidence that the accused did not leave the DNA. The entirety of the evidence is needed to know what any result means.<\/p>\n

2. Is it possible for a rape to happen and for no useful DNA evidence to be left behind? Put another way, if someone says she was raped, but no DNA evidence supports her claim, does that prove she made a false rape report?<\/p><\/blockquote>\n

I answered this above but unless the victim stated that she was sure there no condoms used the absence of DNA could not prove she was mistaken much less that she made a false report.<\/p>\n

3. I’ve seen it claimed that mixed DNA from multiple rapists will not necessarily match the individual DNA of any of the rapists, so multiple rapists not using condoms may be unidentifiable. This seems dubious to me. Do you know if this is true?<\/p><\/blockquote>\n

Mixed DNA from more than one individual never “matches” a single person. Instead if all of a person’s DNA types are found in the mixture then they cannot be excluded as potential contributors. If two people donate DNA then all of their “alleles” or markers will be there so as many as four per locus (genetic site) of which there at least 15 that can be tested. If three, four, or more people donate DNA then there will be so many alleles in a mixture that very few if any people can be excluded as potential contributors. In such an event the evidence does become useless.<\/p>\n

4. Is it possible for a condom to be used, without physical evidence of condom use (traces of latex, etc) being left behind?<\/p><\/blockquote>\n

This question is better asked of a forensic chemist but I do know that such traces are often but not always left behind<\/p>\n

Hope this helps,<\/p>\n

Bill Shields<\/pracut>\n

William C. Thompson<\/strong>
\nDr. Thompson is a Professor in the Department of Criminology, Law and Society at the University of California, Irvine, and a member of the California Bar. He has been a member of the NACDL DNA Strikeforce and has represented a number of defendants in cases that utilized DNA evidence. His work into flaws with the Houston, TX Police Department Crime Laboratory has recently been featured prominently in stories by the television news show 60 Minutes.\n\n

1. If DNA evidence fails to prove that an accused person raped an alleged rape victim, does that prove that the accused person could not have raped the alleged victim?<\/p><\/blockquote>\n

NO. ABSENCE OF EVIDENCE IS NOT NECESSARILY EVIDENCE OF ABSENCE. ON THE OTHER HAND, IF THE ALLEGED VICTIM SAYS “THIS SEMEN STAIN WAS DEPOSITED BY THE DEFENDANT” AND THE TEST REVEALS THAT THE SEMEN STAIN WAS IN FACT FROM SOMEONE OTHER THAN THE DEFENDANT, OR THAT IT IS NOT A SEMEN STAIN, THAT WOULD RAISE DOUBTS ABOUT THE VERACITY OF THE ALLEGED VICTIM.<\/p>\n

2. Is it possible for a rape to happen and for no useful DNA evidence to be left behind? Put another way, if someone says she was raped, but no DNA evidence supports her claim, does that prove she made a false rape report?<\/p><\/blockquote>\n

NO. A RAPIST WHO FAILED TO EJACULATE OR USED A CONDOM MIGHT LEAVE TOO
\nLITTLE BIOLOGICAL MATERIAL BEHIND FOR IT TO BE DETECTED.<\/p>\n

3. I’ve seen it claimed that mixed DNA from multiple rapists will not necessarily match the individual DNA of any of the rapists, so multiple rapists not using condoms may be unidentifiable. This seems dubious to me. Do you know if this is true?<\/p><\/blockquote>\n

IT DEPENDS ON THE WAY THE ANALYST CHOOSES TO INTERPRET THE MIXED PROFILE. IN THE CASES I LOOK AT, THE ANALYST ARE USUALLY QUITE LENIENT ABOUT WHAT THEY WILL CALL A “MATCH.” A MIXTURE OF DNA FROM THREE OR MORE MEN CAN OFTEN BE INTERPRETED IN A MANNER THAT ALLOWS A VERY SUBSTANTIAL FRACTION OF THE MALE POPULATION TO BE “INCLUDED” AS A POTENTIAL CONTRIBUTOR.<\/p>\n

4. Is it possible for a condom to be used, without physical evidence of condom use (traces of latex, etc) being left behind?<\/p><\/blockquote>\n

YES\n<\/pracut>\n

****Important note for comment-writers****: Comments on this post are for “feminist and feminist-friendly posters” only. If you are a poster who is unknown to me, and you leave a comment that is not clearly coming from a feminist point of view, I will probably NOT<\/em> let the comment through. However, everyone is welcome to post comments on the exact same post at Creative Destruction<\/a>. So if you’re not clearly a feminist, and you want your comment to be seen, I strongly advise you to post it over there, rather than on “Alas.”<\/strong><\/p><\/blockquote>\n","protected":false},"excerpt":{"rendered":"

(Note: I did the research and much of the writing for this post before Blac(k)ademic posted her critique of writing which treats rape “like a damn sports event where we are taking sides and rooting for each side based on … Continue reading →<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[123],"tags":[],"class_list":["post-2254","post","type-post","status-publish","format-standard","hentry","category-duke-rape-case"],"_links":{"self":[{"href":"https:\/\/amptoons.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/2254","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/amptoons.com\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/amptoons.com\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/amptoons.com\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/amptoons.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=2254"}],"version-history":[{"count":0,"href":"https:\/\/amptoons.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/2254\/revisions"}],"wp:attachment":[{"href":"https:\/\/amptoons.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=2254"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/amptoons.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=2254"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/amptoons.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=2254"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}