It will not be easy to dispute the prevailing wisdom, fed by CSI-style media fantasies, that forensic science is virtually infallible. Yet, the intellectual weaknesses of many of the “forensic sciences” are now becoming increasingly apparent.
—William Tobin and William Thompson1
It can justifiably be argued that, to date, the greatest contribution to civilization arising directly from the genetics revolution has been the exoneration of hundreds of falsely convicted individuals who have logged thousands of years in prison, some on death row. The failure to match the DNA of the alleged perpetrator with the DNA at the crime scene has, in most cases, trumped other physical and circumstantial evidence, including eyewitness testimony, that juries deemed “beyond a reasonable doubt” in deciding the guilt of an innocent individual. For many, DNA exculpatory evidence has come to mean “beyond a conceivable doubt.”
Although DNA exonerations are hailed as one of the most important achievements of forensic science, the primary goal of the government in applying DNA to criminal justice has been one of finding guilt, not innocence. District attorneys have deployed DNA as indisputable evidence to argue that a suspect was the perpetrator of a rape or was at the scene of another type of violent crime. DNA evidence has sent thousands of individuals to prison, but it has only helped exonerate around 250 falsely convicted individuals.2 However, in the process of vetting suspects and evaluating alibis, police also use DNA to exclude individuals in routine casework before they are drawn too deeply into the web of investigation and surveillance. Early exclusions are a win-win situation for criminal justice and those individuals who might otherwise have to experience protracted periods under the cloak of suspicion and with fear of prosecution.
When DNA evidence is used to support a hypothesis either of guilt or of innocence, and all other hypotheses that contradict the proffered one are either patently false, are highly improbable, or lack any substantial evidentiary support, DNA evidence takes on authoritative preeminence. It is under these conditions that DNA evidence is presented in the popular Crime Scene Investigation (CSI) vernacular as infallible. But in reality, ideal circumstances are rarely the way in which criminal cases get played out. DNA infallibility is a myth, even though DNA evidence can be highly authoritative and effective in identifying and prosecuting criminals. The following describes eight central myths of forensic DNA evidence. These myths have been identified from media accounts and in exaggerated claims made by individuals who view DNA as the ultimate and incontestable authority within forensic science.
Myth of DNA Consistency
DNA Is DNA Is DNA. All DNA Evidence Is Strong Evidence; There Is Not Much Difference in Quality from One Sample to the Next.
Although DNA is revered as the “gold standard” of forensic science, not all DNA is the same. William C. Thompson has pointed out that there is considerable case-to-case variation in the nature and quality of DNA evidence.3 For example, it is not uncommon that biological evidence collected at the scene of a crime does not provide a full DNA profile when analyzed. Sometimes only a few cells are picked off a piece of clothing or an object. Also, in many cases DNA is analyzed or reanalyzed many years after a crime has been committed, and by that time it has further degraded. DNA, like any other chemical, will break down over time. The rate of degradation depends on the type of cells (saliva, blood, semen, skin), as well as the conditions under which the samples are stored. Improper storage or exposure to sunlight, moisture, bacteria, or other unfavorable conditions can accelerate degradation rates. Where significant degradation has occurred, the DNA analysis may not result in a full DNA profile.
Some DNA evidence is also less probative than other DNA evidence. For example, a DNA sample taken from a cigarette butt on the street where a crime was committed is less likely to have come from the perpetrator than DNA extracted from a vaginal swab of a rape victim.
DNA evidence presented in a case is only as good as the DNA found at the crime scene. When DNA evidence is compromised because the biological sample produces less than a full profile or because it may be unrelated to the crime in question, that should be acknowledged from the start. Partial DNA matches that occur as a result of degradation must be understood to carry far less weight than full matches where DNA is being used to establish that an individual was at the scene of a crime. Likewise, matches with DNA that might have been left by an innocent passerby should not carry the same relevancy as those where the DNA evidence is more likely to have come from the perpetrator.
Myth of Infallible Matches
There Are No False Positives in DNA Testing. If Two Samples of DNA Are Found to Match, Then the Samples Must Have Come from the Same Individual.
Although current scientific consensus supports the conclusion that, except possibly for identical twins, no two individuals have an identical genome, the conclusion of individuated genomes does not imply that a match of two DNA profiles indicates that the samples came from the same individual. False positives can and do occur in forensic DNA analysis. They can happen because of error, contamination, interpretation of the output of DNA analyzers, and chance profile matches that can be expected in a sufficiently large population.
We have explained in some detail in chapter 1 the stages involved in performing DNA analysis for identification. The reliability of the process depends on the quality of the DNA obtained at the crime scene; the care with which it is collected, labeled, and transported; the standards and quality-control procedures of laboratories performing the DNA profile analysis; and the interpretation of the DNA analyzer data, including whether a partial profile or a mixed profile is obtained. There are a number of opportunities where errors can occur in the collection, handling, and storage of DNA samples that can result in false positives and, therefore, constitute a risk of incriminating an innocent person.
Sample mix-ups and mislabeling in rape cases, where biological evidence that is being compared contains mixtures of the perpetrator’s DNA with that of the victim, can lead to the incrimination of the wrong person. If the reference DNA samples are switched, then the DNA that is believed to have come from the suspect (but in actuality is that of the victim) will invariably be found to be included in a vaginal swab. In 2000 the Philadelphia City Crime Laboratory admitted to having accidentally switched the reference samples of the defendant and the victim in a rape case. As a result of the sample switch, the lab issued a report that stated that the defendant’s profile was included in a mixed sample taken from vaginal swabs. The report also stated that the defendant was a potential contributor of what the analysts took to be “seminal stains” on the victim’s clothing, which they later realized were in fact bloodstains from the victim.4
BOX 16.1 The Case of Lazaro Soto Lusson
In 2002 it was discovered that 26-year-old Lazaro Soto Lusson was mistakenly charged with multiple felonies because the Las Vegas Police crime lab switched the labels on two DNA samples. While in jail on an immigration hold, Lusson’s cellmate, Joseph Coppola, accused him of rape. Police took DNA samples from both men to investigate the allegation. While they were conducting the analysis, they ran the samples against the state database and matched Lusson’s mislabeled DNA to two unsolved sexual assaults. Lusson faced life in prison and was incarcerated for over a year before this mistake was discovered.
Source: Glenn Puit, “Wheels of Justice Turn Slowly,” Las Vegas Review-Journal, July 6, 2002.
DNA samples can also be contaminated, either before or after collection, especially if they are not stored under proper conditions. Samples can be contaminated by the inadvertent transfer of trace amounts of DNA. Ironically, this error type is of increasing concern as DNA tests become more sensitive.5 Lab analysts have cross-contaminated samples by not properly sterilizing lab equipment between cases or with their own DNA by not wearing the proper protective clothing while conducting DNA analyses.
Even trace amounts of outside DNA can complicate a DNA analysis. Where more than one source of DNA is present in a mixture, the results of the DNA analysis can be difficult to interpret. It can be difficult to tell how many individuals contributed to the source of the DNA sample, let alone which alleles are associated with each of those contributors.6 The presence of one source of DNA can also mask another, and degradation might cause one source to go undetected. Any of these situations can lead to a false positive match.
BOX 16.2 The Case of Timothy Durham
In 1993 Timothy Durham was convicted of raping an 11-year-old girl and sentenced to 3,000 years in prison despite having produced 11 alibi witnesses who placed him in another state at the time of the crime. The prosecution’s case rested almost entirely on a DNA test, which showed that Durham’s genotype matched that of the semen donor. Postconviction DNA testing showed that Durham should have been excluded as a possible suspect, and reanalysis of the initial test showed that the misinterpretation arose from the difficulty of separating mixed samples. The lab had failed to separate completely the male and female DNA from the semen stain, and the combination of alleles from the two sources produced a genotype that could have included Durham’s. Durham was released in 1997 after serving four years in prison.
Source: W. C. Thompson, F. Taroni, and C. G. G. Aitken, “How the Probability of a False Positive Affects the Value of DNA Evidence,” Journal of Forensic Sciences 48, no. 1 (January 2003): 47–54, information at 50.
Sample cross-contamination appears to be a surprisingly common lab error. Thompson has found that these errors are chronic and occur even at the best-run DNA labs.7 Under a guideline issued by the FBI, DNA laboratories are required to maintain corrective-action files to keep track of discrepancies that arise in casework. Many laboratories do not adhere to this guideline, but Thompson has reviewed corrective-action files for some labs where a file is maintained. A small laboratory in Bakersfield, California, for example, documented
multiple instances in which blank control samples were positive for DNA, an instance in which a mother’s reference sample was contaminated with DNA from her child, several instances in which samples were accidentally switched or mislabeled, an instance in which an analyst’s DNA contaminated samples, an instance in which DNA extracted from two different samples was accidentally combined into the same tube, falsely creating a mixed sample, and an instance in which a suspect tested twice did not match himself.8
Thompson worries that this and other similar examples are only “the tip of the iceberg,” especially since these represent only the errors that the lab itself has caught, corrected, and documented.
In 2004 the Seattle Post-Intelligencer reported that forensic scientists at the Washington State Patrol Laboratory had made mistakes while handling evidence in at least 23 major criminal cases over three years. Most of these mistakes involved contamination by DNA from unrelated cases, from the lab analysts themselves, or between evidence in the same case.9
Cross-contamination of DNA samples in laboratories has led to false cold hits in several cases. For example, in Washington State a cold hit turned up when DNA from a rape case was compared with the state database. However, the juvenile offender who appeared to be the source of the DNA would have been only 4 years old at the time the rape was committed. In realizing that this individual could not have been connected with the crime, the Washington State Patrol Laboratory came to understand that the juvenile’s sample had been used as a training sample by another analyst when the rape case was being analyzed.10
Processing DNA samples requires that humans collect and handle biological samples, which are then subjected to laboratory techniques run by human technicians. DNA testing is only as reliable as are the people overseeing each of these processes, and infallibility simply cannot be achieved. Therefore, forensic scientists must depend on quality control, retesting, troubleshooting, and transparency of every decision made in the process to achieve reliable, trustworthy forensic evidence every time.
BOX 16.3 Gary Leiterman: Murderer or Victim of Cross-Contamination?
In March 1969 Jane Mixer, a 23-year-old University of Michigan law student, was murdered. No leads were generated for the case until 2002, when the Michigan State Police Crime Laboratory in Lansing processed the DNA evidence from the crime and found DNA from two men. A drop of blood from Mixer’s hand was found to match an individual named John Ruelas, who was in the database because he had been convicted of killing his mother. DNA taken from the pantyhose of the victim was found to match Gary Leiterman, who was in the database for having previously been convicted of fraud involving prescription drugs. Ruelas, it turned out, was only 4 years old at the time of the murder. Police could not find any link between the child Ruelas and Mixer, and no explanation was provided why his DNA was found on the victim’s hand 33 years after the crime was committed. At the same time, a review of the lab records revealed that DNA samples from both Ruelas and Leiterman were being processed for submission to CODIS in connection with other cases on the same day on which the old samples from the Mixer case were being analyzed. In addition, DNA from the victim was barely detectable on her pantyhose, indicating that significant degradation had occurred over the 33 years since the crime had been committed. However, Ruelas’s and Leiterman’s profiles did not show a similar level of degradation, indicating that they were unlikely to have been deposited at the same time as the victim’s DNA.
When these facts are taken together, it seems plausible that the detection of DNA from both Ruelas and Leiterman could have been due to contamination of the Mixer crime-scene evidence that occurred in the laboratory while the DNA samples were being processed. Certainly this seems the only likely explanation for the presence of Ruelas’s DNA. Nonetheless, Leiterman was convicted of Mixer’s murder in 2005.
Sources: Amalie Nash and Art Aisner, “DNA Evidence Key in 1969 Slaying Trial,” Muskegon Chronicle, July 22, 2005; William C. Thompson, “The Potential for Error in Forensic DNA Testing (and How That Complicates the Use of DNA Databases for Criminal Identification)” (paper produced for the Council for Responsible Genetics [CRG] and its national conference, “Forensic DNA Databanks and Race: Issues, Abuses and Action,” New York University, June 19–20, 2008); Theodore Kessis, “Report of Findings, People v. Gary Leiterman, No. 04-2017-FC,” http://www.garyisinnocent.org/web/CaseHistory/NewDNAFindings/tabid/58/Default.aspx (accessed April 28, 2010); People of the State of Michigan, Plaintiff-Appellee v. Gary Earl Leiterman, Defendant-Appellant, Court of Appeals of Michigan, no. 265821, decided July 24, 2007.
Myth of Objectivity
Two DNA Samples Either Match or They Do Not Match;
DNA Analysis Is Not Subject to Interpretation.
Even if DNA samples are collected, handled, and processed with the greatest care and errors are minimized to the highest extent possible, the results of DNA analysis are still subject to interpretation. The subjectivity of DNA analysis is largely unacknowledged in popular accounts.
In the early development of DNA forensic analysis, the output came in the form of an autoradiogram, which had some resemblance to a supermarket bar code. The bars, which represented the appearance of an allele at a specific locus, showed up in different intensities. When bar segments of the autoradiogram were very light, some interpreters of the data might neglect the bar, believing that it was an imperfection or an artifact. By neglecting the faded bar, the forensic DNA specialist could conclude that there was an exact or partial match; in the latter case he might report that the individual’s DNA profile was consistent with the profile of the crime-scene sample.
Advances in DNA testing methodology replaced autoradiograms with electropherograms that generally give a cleaner visualization of the alleles in a DNA profile. These graphs have peaks and numbers associated with each peak that identify the locus where the peak is found and give the number of STRs within an allele at the peak, and the height of the peak indicates the amount of DNA associated with the peak (see chapter 1). Although the new output from the DNA analyzers is a marked improvement over the output in autoradiograms, discretionary factors remain in the interpretation of the data.
First, there is no standard rule of thumb for how an analyst should report ambiguous results of DNA analyses. For example, where degradation has occurred, peak heights might be very low, and the profile might be considered incomplete. One analyst might decide that these measurements are spurious and unreliable and might report this result as “inconclusive,” while another might report a partial profile. Partial matches may not provide sufficient information to support the conclusion that it is extremely unlikely that someone other than the suspect would have the identical partial match, but they may provide sufficient information to exclude a person from a crime. If even a subset of alleles is found not to match that of a suspect, he or she cannot be the source of that sample. Unfortunately, there have been cases where lab analysts have failed to report analysis information that might have led to a different outcome in a case.
BOX 16.4 The Case of Robin Lovitt
In September 1999 Robin Lovitt was convicted and sentenced to death for the murder of a pool-hall manager in Arlington, Virginia. His conviction rested heavily on DNA evidence. A bloodstain was found on a jacket that Lovitt was wearing when he was arrested, several days after the crime was committed. The lab analyst reported that the results of the analysis of the stain were “inconclusive,” but the prosecutor argued that the stain came from the victim. In addition, bloodstains on the murder weapon matched the blood of the victim but also contained a single, additional allele that was shared by Lovitt. A DNA expert testified that this allele was shared by 19 percent of African Americans.
A closer look at the DNA evidence revealed that the lab analyst failed to report that the DNA analysis of the sample from Lovitt’s jacket produced a partial profile of five loci. All five of those loci matched Lovitt’s DNA. Therefore, there was no evidence that the blood on his jacket matched that of the victim, and if anything, the evidence that it matched Lovitt was exculpatory. In addition, the single extra allele that was found on the murder weapon was far more common in the population than was reported to the jury; according to the FBI’s population data, that allele is found among 33 percent of African Americans, 46 percent of Caucasians, and 40 percent of Hispanics.
Lovitt’s death sentence was reduced by Governor Mark Warner in 2005 to life imprisonment without the possibility of parole. The problems with the interpretation and presentation of the DNA evidence were never fully considered in any of the appeals.
Source: William C. Thompson and R. Dioso-Villa, “Turning a Blind Eye to Misleading Scientific Testimony: Failure of Procedural Safeguards in a Capital Case,” Albany Law Journal of Science and Technology 18 (2008): 151–204.
Cases that involve mixtures of DNA from two or more sources provide the most opportunity for ambiguity. In the face of a mixture, a forensic analyst usually attempts to separate the alleles so that the profiles of each of the contributors can be determined. Even for experienced forensic analysts, however, there may be several ways to sort the alleles among two or more contributors. If all the contributors are available for DNA testing, the sorting process can be carried out with a reasonable degree of accuracy. Some forensic scientists believe that the sorting of the alleles must be done before one has information about the profiles of possible suspects. To do otherwise could bias the interpretation.11 When the contributors to the DNA profile of interest are not all available or are in dispute, then certain assumptions can be made about the likelihood that the suspect’s profile is included among the evidence. The forensic investigator may choose one hypothesis that fits the suspect’s profile in the assemblage of alleles without disclosing to the jurors the other possible allele assortments that do not match the suspect. As Thompson and colleagues have noted:
By their very nature mixtures are difficult to interpret. The number of contributors is often unclear. Although the presence of three or more alleles at any locus signals the presence of more than one contributor, it often is difficult to tell whether the sample originated from two, three, or even more individuals because the various contributors may share many alleles.12
Misinterpretation of mixtures has resulted in false cold hits and even wrongful convictions. In analyzing mixed samples it is critical that the person engaged in the DNA analysis not have an interest or a stake in the outcome of the case and not be seeking to find a match among the alleles with a preexisting suspect’s DNA profile.
BOX 16.5 The Case of Josiah Sutton
In 2004 Josiah Sutton was exonerated after spending four and one-half years in prison for a rape he could not have committed. Sutton’s conviction rested almost entirely on the basis of DNA tests performed by the Houston Police Crime Laboratory. The lab claimed that a semen stain found in the back of the car where the rape occurred contained two profiles—Sutton’s and that of an unidentified man. In addition, the lab analyst testified that the DNA found in the sperm fraction of vaginal swabs and on the victim’s jeans “matched” Sutton’s. Reanalysis of the lab report showed that the semen sample came from a single source, and not from Sutton. In addition, the lab analyst exaggerated the significance associated with the inclusion of Sutton’s DNA profile in the mixed evidentiary sample by not reporting any statistics and repeatedly testifying about the uniqueness of each DNA pattern. It turned out that the chance of a coincidental match in this case was quite high: the frequency in the African American population of men who would be “included” in the vaginal sperm fraction was 1 in 15. Exposure of the errors in Sutton’s case led to a full-scale investigation of the Houston Crime Lab and the review of hundreds of cases involving DNA evidence.
Source: William C. Thompson, review of DNA evidence in State of Texas v. Josiah Sutton (District Court of Harris County, Case No. 800450), February 6, 2003.
Myth of Individuality
No Two People Can Have the Same DNA Profile. The Probability of a Coincidental Match Is Zero or Infinitesimally Small.
When the DNA profiles of two pieces of biological evidence “match,” it is often presumed that they must have come from the same source. But could the match have been coincidental? Do the police have an innocent person whose DNA profile happens to match perfectly the profile of the DNA left at the crime scene? What are the chances that two or more people share the same DNA profile and that the matching profiles do not represent DNA from the same individual?
There is a generally recognized assertion that, except possibly for monozygotic (identical) twins, no two people can have identical sets of 3 billion base pairs of DNA.13 In forensics, however, no person’s complete DNA is sequenced. In the United States 13 loci, as well as markers on the X and Y chromosomes for gender determination, are selected for DNA analysis. The question about coincidental matches reduces to this: what are the chances that more than one person (such as sibling pairs) will have the exact number of short tandem repeats (STRs) in the 26 alleles of the 13 loci used to profile their DNA?
There are three important principles used in developing the probability statistic that prosecutors use in court. The first principle states that the 13 loci are independent and thus are not linked in the population. This is based on a testable assumption that the loci are assorted randomly. The second principle, derived from the first, is that the probability that any individual has a particular array of STRs is given by the product of the frequency with which each allele appears in the population. The third principle states that individuals who are identified with similar racial, ethnic, or ancestral groups have a greater likelihood of allelic similarity in their DNA profiles, including the number of STRs at a locus in the chromosome, than individuals associated with other population groups. When a DNA profile match is found, the population frequencies of the alleles are most commonly determined from one of three population reference groups, Caucasian, African American, or Hispanic, on the basis of the perpetrator’s closest “racial” identity. The third principle allows forensic scientists to estimate the likelihood that two unrelated individuals have the same DNA profile for 13 loci or fewer.
Dan Krane describes a three-stage process that forensic laboratories use to determine the probability that the DNA taken from a random, unrelated individual in the population has the same profile as the evidence sample (the random-match probability, RMP).14 In step 1 the frequency of each allele in the DNA profile of interest is estimated in the reference database. As an example, at a particular locus, allele 1 (7 STRs) appears at a frequency of 3 percent, and allele 2 (12 STRs) occurs at a frequency of 4 percent. In the second step the frequency of each genotype is calculated by the formula 2 times p times q, where p and q are the frequencies of the two alleles in the genotype. The multiplier 2 comes from the fact that each allele can come from either the mother or the father. In the example here, the frequency of the genotype with alleles of frequencies .03 and .04, respectively, is 2 × .03 ×.04 = .0024. The frequency of the overall genotype of 13 loci is obtained by multiplying the frequencies of each locus.
To highlight the principles of probability underlying forensic DNA, consider the following example. Suppose that we have an urn filled with 1,000 balls, some red and some blue. Now imagine that we have selected 10 balls and found that 7 were red and 3 were blue. Can we assume that our next pick of 10 balls would give us the same number of red and blue ones? Obviously not. For one thing, we do not know whether we took a random sample of the balls in the urn. We also do not know whether the balls are distributed homogeneously, or whether all of the red balls are stacked at the bottom of the urn. But if we repeatedly selected 10 balls, we could calculate the average number of blue and red picks. If there were in fact 700 red balls and 300 blue ones, then our ratios in the picks of 10 would cluster around a mean of 7 red and 3 blue, although we would not get that ratio in every selection.
How do scientists know what the allele frequencies are in the population? How do they know how many people have a particular allele at locus 2? There is no direct way because neither the government nor scientists have the DNA profile of everyone in the world, and therefore they cannot calculate the exact frequency of the STR alleles of interest that exist in the entire population. Instead, scientists use convenience databases rather than a random sample of the population. The databases from which forensic scientists draw allelic frequencies could be a few hundred people whose DNA happened to be on hand when allele frequencies first needed to be determined. Because the databases are not a random sample of the population, in theory the probability estimate could either overestimate or underestimate the frequency of the alleles and thus give a false value for the chances of a coincidental match. As in the urn example, even without a random sample of the population, if we kept taking samples of allele frequencies from the population, we would eventually get an average that approaches the real frequency of the allele in the entire population.
But, unlike the example of balls in the urn, where we may know the distribution of red and blue balls, we do not know what the exact allele frequencies are in the population. Forensic scientists infer the actual allele frequencies from the small sample of people who do not represent a random sample of the population. If we drew the allele frequencies from a completely different population, it is likely that we would get a different set of frequencies. As in the case of the urn, if we chose enough population samples, we would expect to obtain a distribution of allele frequencies whose average would approach the average of the entire population.
Some forensic scientists argue that a random selection of the population for the purpose of obtaining allele frequencies is not necessary because even small reference groups will have allele frequency distributions at specific loci that are similar to those of the larger population. The analogy for the urn is that with sufficient mixing we can select 10 balls from the top (not random, but a convenience sample) and get 7 red and 3 blue balls.
The racial or ethnic background of someone who is the source of an evidence sample is often unknown or in dispute. As a result, allele frequencies from the three common racial groups named earlier are commonly used to attach a weight to any matches to such a profile. It is plausible to assume that the greatest chance of a coincidental match of a DNA profile would come from individuals who have similar phenotypes and therefore come from the same “racial” lineage. When an exact match is found between a DNA profile obtained from a biological specimen left at the crime scene and a DNA profile obtained from a suspect, the forensic investigator then determines the likelihood that some randomly chosen unrelated person whose DNA was not left at the crime scene would exhibit an identical DNA profile (the RMP).
Even with frequencies for each STR sequence in the range of 1 in 10 (1 person in 10 has the same number of repeats), the product rule rapidly yields a very low probability [(1 ⁄ 10)n, where n is the number of alleles]. The conventional wisdom within the forensic field is that the likelihood of a random match from 13 loci is inconceivable (perhaps one in a trillion) so long as the DNA is properly handled, no laboratory errors occur, and the individuals involved are not identical twins.
For example, if the frequency of each of the 26 alleles in a DNA sample as determined by the relevant population database is 1 out of 10, then the RMP that two nonidentical twins would have the identical profile is 2 × (1 ⁄ 10)26, or 1 in 50 septillion (1 septillion = 1024 or a trillion trillion). This point was made in Discover Magazine:
Reports on DNA matches . . . include scientifically rigorous probabilities of the likelihood of finding the same DNA profile in a random, unrelated individual. The chances are typically far less than 1 in 10 billion for a full DNA profile from a single individual. It is that degree of improbability that forms the basis for the common perception that DNA testing is foolproof.15
Suppose that the crime-scene sample is profiled and compared with that of the suspect, and on the basis of the population database used by police, the RMP is determined to be 1 in 10 million. The prosecution tells the jury that the suspect is the likely offender because if we choose an unrelated person at random in the population, the chance that this individual would have the exact profile of that found at the crime scene is 1 in 10 million. But with 6 billion people on the earth, there could, on average, be 600 people with the identical profile. The defense can justifiably say that the suspect is 1 out of 600 people who could have the same DNA profile as that found at the crime scene.16 An RMP of 1 in 10 million does not necessarily mean that you will find one and only one such profile in a population of 10 million. On average an event with a probability of 1 in 10 million occurs once in every 10 million trials, but in some instances it might occur more than once in 10 million trials.17
But what about people who are related or who are from a relatively isolated or highly inbred population? Could there be an exact match for a DNA profile of 13 loci of two individuals who are not identical twins? To date, no such case has been recorded. According to Dan Krane,
The crux of the problem is simply that the RMP delivers pretty much what it says that it will (the chance that a randomly chosen, unrelated individual from a particular population has a perfectly matching DNA profile) and that it is completely silent on the chance that a close relative (or that one of a very large number of relatively close relatives) would have [identical] DNA profiles.18
Krane notes that the assumptions behind the product rule (random assortment of all alleles) do not apply for relatives of individuals. The chances for a coincidental match, then, even if small, are not zero.
Myth of the Infallibility of a Cold Hit
A Cold-Hit Match Made Against a Large Database Has the Same Weight as a Match Between a Person Suspected of a Crime and Evidence from a Crime Scene.
Generally there are two ways in which police seek to find DNA profile matches with crime-scene evidence. First, when they have a suspect, they obtain a biological sample from that individual and compare it with the profile derived from the crime-scene sample. If police get an exact match (all 26 alleles are identical), it usually comes with other evidence linking the suspect to the crime. Otherwise they would not have had reason to obtain the DNA profile of the suspect in the first place.
Second, when police have no suspect, they may compare the DNA profile from the crime scene with all the profiles that have been entered into a DNA offender database or a DNA database consisting of offenders, arrestees, and/or volunteers. This is a fishing expedition using computer technology to make comparisons between one DNA profile (from the crime scene) and the more than 8 million profiles that have been banked in the national Combined DNA Index System (CODIS).19 If they get a match in this case, it is called a “cold hit” because they are operating blindly, without any evidence linking a suspect to the crime or any a priori suspicion.
Do both of these kinds of DNA profile matches—a match that occurs by comparing a known suspect’s DNA with that of the crime scene and a match that occurs as a cold hit—merit the same statistical weight? Keith Devlin, a statistician at Stanford University, argues that a 13-locus match would be a definitive identification provided that “the match is arrived at by comparing a profile from a sample from the crime scene with a profile from a sample from a suspect who has been identified by means other than his or her DNA profile.”20 Devlin argues that the chance that the match is coincidental is higher, however, when a given sample is compared with many samples in a database. In cold-hit cases the investigation involves searching a database of hundreds of thousands or even millions of genetic profiles for a match. Each individual comparison increases the chance that a match will occur with an innocent person.
David Kaye uses the “birthday problem” in statistics to illustrate this point.21 If you are in a room with a group of people and you choose one, then the chances that the two of you have the same birthday is 1 out of 365. But if we ask what the chances are that you have the same birthday as anyone in the room, that will depend on how many people are in the room. Moreover, if you asked what the probability is of one birthday match (not necessarily yours) in the room, the probability would even be greater because you are making pairwise comparisons with everyone in the room.
This example has been used to illustrate the point that RMPs can underestimate the chances of a coincidental match in a cold-hit case, where no other evidence but a DNA profile match is found. Even with their aggressive collection of DNA from citizenry, good practice guidelines adopted by the British police state clearly that because of chances of a coincidental match and other limitations of DNA evidence, individuals should not be convicted exclusively on DNA evidence (i.e., a cold match in a database).22
The National Academy of Sciences recognized that the method of determining the RMP from a suspect sample (where there is prior evidence of suspicion) should not be identical with that from a cold hit (where there is no prior evidence of suspicion). In the latter case the RMP should depend on the size of the database. The chance of finding a random match is greater with a very large database. The academy wrote: “If the only way that the person becomes a suspect is that his DNA profile turned up in a database, the calculations [of RMP] must be modified. . . . Multiply the match probability by the size of the database searched. This is the procedure we recommend.”23
Although it is true that the larger the database, the greater are the chances of finding a match, including a random match, for crime-scene DNA, it is also true that finding a single match of a suspect in a large database improves the chances that the suspect was at the crime scene because it rules out all the other people in the database.
The reliability of the calculation of the RMP is dependent on the reliability of the independence of the genetic loci used in the calculation. But the independence principle remains an assumption or idealization. Devlin has argued for an empirical method of calculating RMPs that requires large data sets and not simply 200 to 400 data points. If we use the product model for calculating RMPs, we could validate it by comparing its results with the frequency of matches found in a large database.
One such test was run in 2005 on the Arizona convicted-offender database containing approximately 65,000 entries, which was analyzed for profile similarities. Approximately 1 in every 228 profiles in the database matched another profile in the database at 9 or more loci; approximately 1 in every 1,489 profiles matched at 10 loci; 1 in 16,374 profiles matched at 11 loci; and 1 in 32,747 matched at 12 loci (both were siblings). Devlin opined: “How big a population does it take to produce so many matches that appear to contradict so dramatically the astronomical, theoretical figures given by the naive application of the product rule?”24 About 1 in 1,489 profiles matched at 10 loci. If we calculated the RMP based on STR frequencies of a very conservative 1 of 5, the theoretical answer would be 1 in 11 million, a much lower probability than was actually found in Arizona. On the basis of this empirical result Devlin concludes:
It is not much of a leap to estimate that the FBI’s national CODIS database of 3,000,000 entries will contain not just one but several pairs that match on all 13 loci, contrary (and how!) to the prediction made by proponents of the currently much touted RMP that you can expect a single match only when you have on the order of 15 quadrillion profiles.25
The debates among statisticians and forensic scientists on RMPs play out in the courtroom as well. The same information can be packaged and presented differently to a panel of jurors, one framing that supports the prosecution and another that supports the defense.
Let us suppose that the calculated frequency of an individual’s 26 alleles is 1 in 6 billion. This means that when you multiply the frequencies of the individual alleles in the relevant population, the product of the frequencies yields a frequency of 1 in 6 billion. This could be presented to the jury as follows: “There is only one person in 6 billion with this DNA profile and that is our suspect, because there are only 6 billion people on the earth. If there were 12 billion we would have to conclude that there might be another person with the same DNA profile.”
But “1 in 6 billion” is a theoretical calculation based on databases that have not been chosen randomly to determine allele frequencies. So there is still a chance that more than one person on the planet will have the same DNA profile. If we had a DNA profile for every living person on the planet, we could ascertain definitively whether more than one exact profile match occurs.
In cold-hit matches the profile is uploaded to a database where, let us assume, one match is found. There are two ways of thinking about the probability of this being a coincidental match. On the one hand, if the database is very small, we might think that this could be a coincidental match because we have not seen a large-enough population from which to judge the profile. On the other hand, since the database is small, the likelihood of getting a coincidental match should be small because it increases with the size of the comparison population (a world database would increase the chances of a coincidental match). Even though the theoretical calculation gives us an RMP of 1 in 6 billion, we know that the assumptions behind the calculation do not take account of close family relations; those have to be analyzed using kinship statistics. Thompson notes, “These estimates understate the probability of a coincidental match in actual cases because they take no account of the possibility that the pool of possible suspects contains the relatives of the perpetrator, who would be more likely to have the same profile due to common ancestry.”26
Now suppose that the database from which police obtained the cold hit was very large. An actual DNA database of 6 million profiles that yielded one cold hit tells us that 5,999,999 people have been excluded from the crime-scene match. The larger the database, the more confidence we can have that our cold hit—with no other evidence—is not a false match because we are approaching the true population size. By imagining a database with 60 million people and one cold hit, we gain even more confidence, given that 59,999,999 people are excluded. But if we found 1 match in 60 million people, then there could be 100 matches in 6 billion people (1 match per 60 million, using a kind of inductive logic).
So another narrative that could be presented to the jury is that the chance of a coincidental match for a cold hit in a database of 60 million people is 1 out of 100. Telling a jury that there could be another 99 people on the planet with the same DNA profile presents a very different statistic that could change its psychology when it is trying to determine the grounds for “beyond a reasonable doubt.”
Would higher probability statistics in cold-hit cases make a difference in their probative value or how juries relate to the evidence? Thus far, in cold-hit cases the courts have opted for the RMP estimates from forensic statisticians over the mathematical statisticians. Juries typically do not get to hear the controversy because it is often resolved before experts appear before the jury.
BOX 16.6 The Case of John Puckett
In 1972 a 22-year-old nurse was sexually assaulted and stabbed to death in San Francisco. More than 30 years later a swab that had been taken from the victim’s mouth in 1972 containing a degraded sperm sample and at least one other person’s DNA produced a partial DNA profile of 7 markers. When the profile was compared with California’s DNA databases of 338,000 profiles, it matched with that of John Puckett. Puckett, then 70, denied ever knowing the victim, and there was virtually no other evidence linking him to the crime, aside from the fact that he lived in San Francisco in 1972 and had a previous rape conviction. During his trial the jury was provided a random-match probability of 1 in 1.1 million, based on population statistics. During pretrial hearings Bicka Barlow of the San Francisco Public Defender’s Office argued that this figure did not take into account the size of the database. Following the NAS recommendation to multiply the RMP by the number of profiles in the database, she argued that the chances were in fact 1 in 3 that the database search had resulted in linking an innocent person to the crime. The judge did not allow this statistic to be presented to the jury. Puckett was convicted and sentenced to life in prison.
Source: Jason Felch and Maura Dolan, “DNA Matches Aren’t Always a Lock,” Los Angeles Times, May 4, 2008.
Increasingly, police are trolling their databases for partial matches of DNA profiles. This means that they might be interested in a cold hit with 20 out of 26 matched alleles. It is possible to generate a fairly high RMP with fewer than 13 loci that could sound convincing to a jury. In 1999 police in the United Kingdom found an exact match of 6 loci between the profile of crime-scene DNA from a burglary and a profile logged into the United Kingdom’s national databank. The frequency of a random match was calculated by law enforcement to be 1 in 37 million, which is persuasive evidence in a country of 60 million people. When the suspect was arrested, it soon became obvious that the match was a coincidence because the man was disabled and was physically incapable of carrying out the crime. The coincidental match could have been corroborated by testing more alleles in the biological samples.27
According to Thompson, “The British Home Office has reported that between 2001 and 2006, 27.6 percent of the matches reported from searches of the U.K. National DNA Database (NDNAD) were to more than one person in the database,”28 largely because police were uploading partial DNA samples where degradation of the crime-scene sample had taken place or because a number of individuals were entered into the database more than once. The current interest in familial DNA searching has resulted in greater interest among criminal investigators in partial matches. Although forensic scientists have made efforts to develop statistical models that predict the probability that a partial match of an individual implicates that person’s family as the source of the DNA, the results have been highly problematic and contested (see chapter 4).29
Myth of Infallible Rape Evidence
If the DNA of a Suspected Rapist Is Found in the Vaginal Smear of the Victim, Then the Suspect Must Be the Rapist.
DNA testing has been responsible for a high conviction rate in crimes involving rape. It is widely assumed that if a suspect’s DNA is found in a vaginal smear of the rape victim, then the suspect’s guilt has been established beyond a reasonable doubt. There are two separate issues. First, does the DNA of the suspect in the vaginal smear prove beyond a reasonable doubt that the suspect had sexual intercourse with the victim? Second, does the DNA match prove that the suspect raped the victim?
The answer to the first question must most probably be in the affirmative. It seems extremely unlikely that a suspect’s DNA could enter the vaginal canal without intercourse. The victim could surely set the record straight in such an event. That said, there was one case where a woman implanted a sperm sample in order to thwart law enforcement. In 1999 a convicted rapist named Anthony Turner smuggled a sample of his semen out of prison, concealed in a ketchup packet. Turner’s family members paid the woman $50 to use the sperm to stage a phony rape as a way of casting doubt on the DNA evidence that placed him in prison.30
The second question asks whether a DNA match implies a rape. There are cases where a victim has had multiple sexual partners, one or more of whom may have been consensual, where a vaginal smear by itself may not reveal the actual rapist. This is where forensic investigators can use elimination samples in mixed DNA samples where there have been consensual partners. It is also possible that in violent crimes against women involving more than one man, one of the perpetrators did not penetrate the victim or did not ejaculate. Thus the nonappearance of sperm is not by itself conclusive evidence that the suspect was not involved in violence or a rape against the woman. DNA evidence, separated from its context, is never solely definitive for either conviction or exoneration, although the burden for the former is much higher.
Myth of DNA Detection Equaling Physical Presence
When the DNA of a Suspect Is Found at a Crime Scene, Then the Suspect Must Have Been Present at the Crime.
The fact that an individual’s DNA is found at the scene of a crime does not indicate that he or she committed the crime in question or even was present at the crime scene. There are many ways in which a person’s DNA can wind up at the scene of a crime. As discussed in “Myth of DNA Consistency,” DNA in the form of a vaginal swab found on a rape victim might be far more useful to investigators than DNA lifted from a cup or a cigarette butt.
Moreover, even if a person’s DNA is reportedly found at the scene of a crime, it is not necessarily the case that the person deposited it there. There is always the possibility that the DNA could have appeared as result of secondary transfer, that the DNA could have been planted, or that the results of the DNA analysis were fabricated.
Secondary transfer refers to the phenomenon where DNA deposited on one item winds up on another. The individual does not have direct contact with that item (primary transfer); instead, his or her DNA is transferred by way of an intermediary, which could be either another person or another object. For example, if person A shakes person B’s hand, they are each likely to have trace amounts of the other’s DNA on their hand. If A then takes out a kitchen knife and cuts vegetables, it is quite possible that the DNA of both A and B could be found on the knife handle, even though B never touched the knife. Ironically, the potential for inadvertent transfer of DNA to muddy an investigation has increased over time as DNA testing techniques have become more sensitive and able to type the DNA of samples of only a few cells.
BOX 16.7 Massachusetts v. Greineder
In 1999 Mabel Greineder was found beaten to death in a wooded area in Wellesley, Massachusetts. Her husband, Dr. Dirk Greineder, a prominent physician and adjunct Harvard professor, was arrested after a DNA profile similar to his was found, mixed with his wife’s profile, on gloves and a knife found near the scene of the crime. Some of Greineder’s alleles were not found, and additional alleles that did not belong to him or his wife were also found on the items. Greineder challenged the DNA evidence in the case. He argued that his DNA could have appeared on those objects as a result of tertiary transfer. He claimed that because he and his wife had shared the same towel that morning, his DNA could have been transferred from his face to the towel, and then from the towel to his wife’s face. Then, in the process of her murder, his DNA could have been transferred again to the knife and the gloves. This theory, he claimed, was also consistent with the fact that additional alleles had been found on the gloves and the knife that matched neither him nor his wife.a Greineder hired a private DNA lab to test his hypothesis. The lab ran an experiment and presented testimony in the case that tertiary transfer could indeed have occurred as he described it.b The jury ultimately convicted Greineder of murder in 2001. In 2005 Greineder’s lawyers filed a motion with the Supreme Judicial Court requesting a new trial. They argued that DNA testing crucial to the prosecution’s case had been conducted improperly and that Greineder had been deprived of effective legal counsel. Arthur J. Eisenberg, the director of the DNA Identity Laboratory at the University of North Texas Health Science Center and then chairman of the U.S. DNA Advisory Board, submitted an affidavit stating that the genetic testing conducted for the trial by the forensic laboratory Cellmark “was contrary to what is generally accepted in the science community. . . . There was no scientifically reliable evidence that Dirk Greineder was a potential contributor to the DNA obtained from any of the three key pieces of evidence.” Eisenberg said that too little DNA was found on the items to obtain reliable results and that, furthermore, the profiles of both Dirk and Mabel Greineder were ascertained by Cellmark before interpreting the key evidentiary samples, potentially biasing the analyst’s interpretation of the results. The motion was denied. In October 2009 his lawyers filed another motion for a new trial.
a William C. Thompson, Simon Ford, Travis E. Doom, Michael L. Raymer, and Dan E. Krane, “Evaluating Forensic DNA Evidence, Part 2,” The Champion (April 2003): 16–25, at 24.
Sources: Rachel Lebeaux, “Greineder Appeals Murder Conviction,” Wellesley Townsman, August 3, 2005; Denise Lavoie, “Wellesley Doctor Seeks New Murder Trial,” Associated Press, October 8, 2009, http://www.boston.com/yourtown/news/wellesley/2009/10/wellesley_doctor_seeks_new_mur.html (accessed April 28, 2010).
In science, misconduct, including outright fraud, rises to the level of high crimes and misdemeanors. A special federal office called the Office of Research Integrity was established in March 1989 to investigate scientific misconduct. Among the most blatant and reviled forms of misconduct is the “cooking of data,” a term that means that the investigator discards or fabricates data to conform to a hypothesis. There have been cases of data fabrication so egregious that even seasoned observers found them difficult to comprehend. In one case biologist William T. Summerlin used a felt pen to mark a mouse and claimed that it expressed a skin transplantation without immunosuppression.31 Others have been known to doctor photographs or reuse old photographs.32 Arthur Koestler’s classic book The Case of the Midwife Toad tells the story of the highly acclaimed early twentieth-century biologist Paul Kammerer, who used india ink to fabricate darkened nuptial pads in the toad in order to support a Lamarckian theory that inherited characteristics can be acquired from environmental conditions.33
The idea of fabricating evidence is not unique to science. It is a well-documented practice in law enforcement, where criminal investigators are either so confident that the suspect is guilty or are so pressured to solve a crime that they feel justified in “cooking the evidence” by planting drugs, a gun, or other incriminating items in the home or car of a suspect. In 1995, six Philadelphia police officers pleaded guilty to charges of planting illegal drugs on suspects, the theft of more than $100,000, and the falsification of reports. The investigations into the officers’ actions led to the release of hundreds of defendants whose convictions were overturned by the appeal courts. Also in 1995, two other officers from Philadelphia received prison sentences of 5 to 10 years for framing young men. Since 1993 the city of Philadelphia has paid out approximately $27 million in more than 230 lawsuits alleging police misconduct.34
In one analysis of the O. J. Simpson case the author noted, “Evidence presented later at the trial showed that the officer had used racist language in an interview with a writer, that he described police beating a Black suspect and that he asserted that the police planted evidence against Black suspects.”35 Merrick Bobb reports that the Los Angeles Police Department (LAPD) suffered “embarrassment and opprobrium” when it was disclosed that “LAPD officers were shown to have planted evidence and guns and wrongfully shot young Latinos suspected of gang activity.”36 If police can plant drugs, they can certainly plant DNA. Another account of the Simpson case noted that sloppy handling of DNA evidence—including an inability to account for missing blood from a reference sample collected from Simpson and the discovery of several bloodstains at the crime scene several weeks after the crime had been committed—supported a theory that Simpson’s blood had been planted after the murders had taken place.37
DNA can also be planted at a crime scene by a criminal in an attempt to thwart the police or to frame someone else for a crime. Several instances have already been reported where criminals have planted or tampered with evidence or have paid inmates to take DNA tests as a way of confusing investigators or evading prosecution. Prisoners have also been overheard coaching each other on how to plant biological evidence at a crime scene and how to avoid leaving their own DNA behind.38 We have seen how DNA was smuggled out of prison to cast doubt about a conviction.39 An elaborate scheme is hardly needed, of course; more simply, DNA evidence can be deposited at a crime scene by way of discarding DNA-carrying items, such as used cups, cigarette butts, a hair sample, or other items likely to contain testable amounts of DNA.
In 2009 a study published by scientists in Israel demonstrated that a somewhat more motivated criminal with access to a single hair strand, cigarette butt, or dry saliva stain and some basic laboratory equipment (a polymerase chain reaction [PCR] analyzer and a testing kit that is commercially available) could amplify DNA and spread it around a crime scene. Similarly, an artificial DNA profile could be assembled and amplified on the basis of a reference profile, without the need for any source DNA. In either case the amplified DNA can then be applied to objects and planted at the crime scene.40 Although neither of these approaches is likely to be pursued by an average criminal, neither of them would require significant resources or more than a basic knowledge of molecular biology. Dan Frumkin, lead author of the article “Authentication of Forensic DNA Samples,” has stated, “You can just engineer a crime scene. Any biology undergraduate could perform this.”41 Laboratories often update their equipment and sell off their PCR analyzer machines on the Internet; when the authors last checked, there were two such analyzers for sale for approximately $500 each on eBay.
Finding someone’s DNA at a crime scene may be a prima facie reason to consider that the person was at the location at some time, but it is certainly not definitive or infallible evidence of this. Given the history of misconduct in criminal justice, planting of DNA evidence by police seeking to close the case or perpetrators seeking to divert police cannot be left out of the equation. Finding someone’s DNA at a crime scene is not infallible evidence either that they were there or that they committed the crime. DNA typing helps determine the source of the biological material at a crime scene; other evidence is needed to determine whether the true donor of the sample committed a crime.
Myth of the Infallible Mismatch
If Two Samples of DNA Are Found Not to Match, Then the Samples Cannot Have Come from the Same Individual.
This claim appears, at first glance, to be well grounded in science. Textbooks report that all our cells contain the identical string of DNA molecules. If two DNA samples do not match, then it would seem that they surely cannot come from the same individual.
It is true that a nonmatch is more definitive than a match. In other words, a nonmatch offers more conclusive evidence that two samples did not come from the same individual than does a match in showing that the source of DNA of two samples is the same. As an analogy, a single black swan falsifies the statement “all swans are white,” whereas a white swan (or many white swans) supports the statement but does not prove it. Nonetheless, even a nonmatch has its limitations.
Lydia Kay Fairchild, a resident of Washington State, was pregnant at age 26 with her third child in 2002. She had an on-and-off relationship with the putative father of her children, a man named Jamie Townsend. They were separated during her pregnancy. Without a job or means of support, Fairchild applied for welfare benefits. The state welfare agency required proof that Townsend and Fairchild were indeed the biological parents of the children. DNA tests were performed. The results confirmed that Townsend was the children’s father. But there was a wrinkle. Fairchild’s DNA was found not to match that of her children. Ordinarily there would be a 50 percent similarity between the DNA of a child and each biological parent. The court ruled that Fairchild was not the biological mother of her two children, a son aged 4 and a daughter aged 3, as she had claimed, and considered this a case of welfare fraud. The judge discounted the hospital birth records as forgeries and accepted the DNA evidence as indisputable.42
The state prosecutor for the case wanted Fairchild’s two children to be placed with guardians while the investigation continued. She was charged with attempting to defraud the state and was denied public assistance. Her insistence that she was the biological mother of her children convinced the judge to offer her a last opportunity to prove her case. The judge ordered someone to be present during the birth of Fairchild’s third child; the court-appointed witness would take blood samples of the newborn immediately after delivery and have them analyzed. Like his siblings, the newborn’s DNA was found to be different from that of his mother. The state could no longer claim that, despite the DNA conundrum, Fairchild did not gestate her children. Other explanations were sought. Fairchild’s lawyer characterized the response of the prosecutor to this result:
The questions that have gone through the prosecutor’s mind include whether or not she [Fairchild] was involved in being a surrogate mother. If the egg and sperm had been planted then she wouldn’t have a [genetic] relationship to the child. Maybe she’d abducted the children from somewhere or was involved in some other criminal activity.43
The explanation for the dissimilarity of DNA between mother and child was eventually solved: Fairchild is a chimera. This means that some of her cells have one DNA type, while other cells have an entirely different DNA type. Fairchild’s skin and hair-root DNA did not match that of her children, while the DNA from her cervical cells did match their DNA.
Chimerism occurs during the development of a blastocyst in the womb. Two fertilized eggs, either implanted by in vitro fertilization or dropped from the ovaries, fuse in the early stages of development, creating an embryo with cells that have different DNA profiles. Another route to chimerism is the vanishing-twin thesis. Somewhere between 20 and 30 percent of pregnancies start out as fraternal twins but end up as single babies. One of the early-stage fraternal embryos is absorbed by the mother, while some of its cells enter the body of the remaining embryo and remain there throughout development. These embryonic anomalies occurring after in vitro fertilization are sometimes referred to as embryo amalgamation.44 Alternatively, chimerism can also arise from cells that routinely pass from mother to fetus and get integrated into the fetus.
There are no clear estimates of the rate of chimerism in the population. Howard Wolinsky, who estimates as many as 1 in 8, believes that it is “not rare, but rarely discovered, because it seldom generates any observable anomalies.”45 Catherine Arcabascio reports chimerism figures ranging from as high as 1 in 10 to 1 in 2,400 persons.46 New York Times science writer Gina Kolata reported the following on chimerism:
Dr. Ann Reed, chairwoman of rheumatology research at the Mayo Clinic, who uses sensitive DNA tests to look for chimerism, finds that about 50 to 70 percent of healthy people are chimeras. The more scientists look for chimerism, the more they find it. It seemed not to exist in the past, she said, because no one was explicitly looking for small amounts of foreign cells in people’s bodies. “Some believe that if you look hard enough you can find chimerism in anybody,” said Dr. Reed. . . . It is so common that she thinks there must be a biological reason for it. It also may cause problems, she and others say.47
There are insufficient empirical data to narrow the uncertainty about chimerism incidence. Chimerism can have serious implications for individuals undergoing blood transfusions or organ transplantation. It has also emerged as a defense on the part of professional athletes who have been accused of transfusing themselves to boost their endurance.48 The implications for paternity testing and forensic analysis are significant:
Take, for example, the hypothetical case of a chimeric criminal who leaves DNA at the scene of the crime. The suspect may leave a sample of hair, semen, saliva, perspiration, urine, ear wax, mucus, bone, fingernail scrapings, blood, or skin. He may even leave a combination of those forensic clues at the scene. If he is a chimera, however, the DNA from his saliva could, in theory, differ from the DNA in his semen, skin, blood, or some other sample left at the scene.49
Criminal chimeras could be mistakenly exonerated if DNA served as the definitive evidence. In addition, those who are falsely convicted of a crime and whose only chance at exoneration is the submission of the crime-scene DNA for a cold hit in CODIS could also be stymied by the actual perpetrator if he or she were a chimera. If chimerism occurs at a higher rate than the lower estimates predict, the entire project of forensic DNA would have to be reconsidered for fallibility of identification.
Where does this leave us? There is nothing infallible about DNA. DNA evidence can be strong or weak or anything in between. Human error can and does occur in the collection, analysis, and interpretation of DNA results. Samples can be switched, cross-contamination can occur, analyses can be improperly interpreted, and the results can be poorly communicated. Any errors of this sort can lead to the false incrimination and wrongful conviction of an innocent person. The possibility that chimeras are a rule rather than a rare exception could undermine the very basis of the forensic DNA system.
In the meantime, are the myths or exaggerations of infallibility obstructing the cause of justice? Is too much power attributed to DNA as truth telling? Would higher probabilities in the estimate of RMPs in coldhit cases make a difference in their probative value or how juries relate to the evidence? Is contamination of evidence going unnoticed? How often are people being wrongfully arrested, tried, and convicted of crimes on the basis of flawed DNA evidence? These questions illustrate the human dimension in the use of forensic DNA. Human judgment is notoriously fallible, but it remains our only guide as long as we understand its limitations.