When Science Goes Wrong: Twelve Tales From the Dark Side of Discovery - Simon LeVay (2008)

FORENSIC SCIENCE: The Wrong Man

ON FRIDAY, OCTOBER 30, 1998, a 16-year-old African American youth by the name of Josiah Sutton walked to a neighbourhood convenience store at the corner of Fondren Road and West Bellfort Street on the southwest side of Houston, Texas. He was accompanied by his friend Gregory Adams. Although they were behaving innocently enough, the two youths were stopped by police, handcuffed and placed in the back of a squad car. Sutton didn’t see freedom again for four and a half years.

Five days earlier, sometime after midnight on the night of October 25, a 41-year-old rape victim arrived at a Houston hospital. After medical treatment and the taking of forensic samples, she gave investigators the following account of what had happened: at about 11pm, she was parking her car outside her apartment complex on Fondren Road when she was approached by two African American teenagers, one of whom was armed with a gun. They forced her back into the vehicle and drove it to a deserted location where they forced her to engage in oral and vaginal intercourse with both of them. She said that two other men approached the parked vehicle and witnessed the rape, but did nothing to help her and in fact chatted casually with the attackers. They even suggested at one point that the rapists should take care not to leave fingerprints. After the rape, the two attackers drove her to the southern outskirts of Houston and left her in a field.

The woman described her two attackers to police. They were black, and no more than 20 years old. The one who had held the gun to her head, and who had driven her car, wore a baseball cap with the peak turned sideway; he was about 5ft 7in tall and weighed about 9st 6lbs - significantly smaller than herself (she was 5ft 10in tall and weighed more than 14 stones. The other man, the woman said, was about the same height as the first, but even skinnier - about 8st 5lbs. He had been wearing a skullcap.

After several days spent recovering at another location, the woman returned to her home. On the Friday, as she was driving her car in her neighbourhood, she saw Josiah Sutton and Gregory Adams on their way to the convenience store. She noticed that Sutton was wearing a baseball cap turned sideways and Adams a skullcap. Relying in part on these features, she recognised them as her attackers and drove to the Fondren police station to report the sighting. A police officer immediately located the two men and arrested them. Then, while they were sitting handcuffed in the back of a police car, the police brought the woman by. She viewed the men from inside her own car, which was stopped about 10ft from the police car, and positively identified Sutton as the man with the gun, and Adams as her other attacker.

This was despite the fact that Sutton was six feet tall and weighed 14st 2lb - five inches and 65lbs more than her previous estimate.

The two youths were detained, and the next day they were charged with kidnapping and rape. Both protested their innocence. They volunteered to give blood samples for DNA testing, and these were taken. It took the Houston Police Department’s Crime Lab several months to process the DNA; during this waiting period, Josiah Sutton was ordered to stand trial as an adult and was moved from juvenile hall to the county jail.

This wasn’t Josiah’s first time in trouble. According to an account in the Houston Chronicle, he had quite a difficult upbringing. His father had left home when Josiah was about six years old and was, at the time, in jail on drugs charges. His mother, Carol Batie, had to raise Josiah and his four younger siblings on her own with a low-income job, supplemented later by a small amount of money that Josiah earned by cutting hair. In 1997, Josiah failed his freshman year in high school. The following July, he was arrested for illegal possession of a gun and sentenced to probation, but he violated the terms of his probation by failing to meet with his probation officer. He switched to a new school but was suspended for fighting, and later he dropped out of school altogether. His girlfriend became pregnant. By October, Josiah seemed to be on a downward spiral that could easily have culminated in a serious brush with the law. Nevertheless, during his time in the county jail he continued to maintain his innocence and he expressed confidence that he would be exonerated by the results of the DNA analysis.

Meanwhile, in the Crime Lab, the DNA study was being done by Christy Kim, who had 20 years’ experience as a forensic analyst. Kim had four samples that might contain the perpetrator’s DNA. These were a swab from the victim’s vagina, combings from her pubic hair, a stained portion of her jeans and a stain - identified as semen - from the back seat of her car, where the rape had occurred. In addition, Kim had blood samples from the victim as well as from the two suspects, Sutton and Adams. Thus, her first task was to determine whether the DNA present in Sutton’s and Adams’s blood samples matched that found in any of the four evidentiary samples. If it did, her second task was to determine, using statistical techniques, how strongly the match pointed to Sutton or Adams as the perpetrator.

Kim used a standard procedure to perform the analysis. First, she extracted DNA from the samples. In the case of the vaginal sample, she performed chemical procedures to isolate any sperm DNA that might be present, thus removing the victim’s own DNA. Next, she amplified the DNA in the various specimens, meaning that she increased the amount of DNA in the sample by using the polymerase chain reaction (PCR) technique. Because the PCR technique can amplify even a very few molecules of DNA, it was necessary to take care that each sample was not cross-contaminated with DNA from the other samples or from other sources within the Crime Lab.

Finally, Kim did the actual DNA typing. Using commercially-available test kits, she examined seven different sites in the genome (called ‘loci’), at each of which there are two or more different DNA sequences (or ‘alleles’) present in the population. These differences consist of varying numbers of repeats of a short DNA sequence: one allele might consist of three repeats, another of four repeats, and a third of five repeats, for example.

When six or seven loci are tested, the odds that an innocent suspect’s alleles will match those of the perpetrator merely by chance are very small - the odds are at least a few hundred thousand to one against this happening, and they may be much slimmer than that. Since there are only about 60,000 black teenagers in Houston, these kinds of odds made it unlikely, in principle, that an innocent person would be misidentified as the perpetrator of a crime such as the one Sutton was accused of.

There is one slight complication. It’s common for an individual to possess two alleles rather than just one allele at a given locus. That’s because people possess two versions of each chromosome - one inherited from their mother and one from their father. It may happen that the alleles on these two chromosomes are the same, but quite commonly they are different. If a suspect has two different alleles at a given locus, those two alleles should be present in the evidentiary sample also if the suspect is the actual perpetrator.

When Christy Kim examined the test results from the rape case, she immediately made a couple of important observations. With regard to the sperm sample from the victim’s vagina, the printout showed more than two alleles at some loci. Since no single individual possesses more than two alleles at a given locus, the sperm sample had to contain a mixture of DNA from more than one individual.

By itself, this finding was consistent with the victim’s account of having been raped by both Sutton and Adams. However, Kim also saw that, at several loci, Greg Adams possessed at least one allele that was not present in the vaginal sperm sample. Thus, none of the sperm found in the victim’s vagina came from Adams. The sperm sample on the seat of the car also failed to match Adams’s DNA.

Of course, Adams might have raped the victim without leaving sperm in her vagina or on the car seat. But in that case only one man’s DNA should have been present in the vaginal sample, rather than two, because the victim had stated that she had not had intercourse with any man, other than her two attackers, within the previous week. Given these facts, the DNA findings ruled Adams out as one of the rapists; evidently, the victim had misidentified him. Perhaps she had relied too strongly on that one item of identification, the skullcap - which is, after all, a popular form of headgear among African American teenagers.

When Christy Kim passed on the results of her DNA analysis to the prosecutor’s office, the DA quickly realised that the negative DNA findings trumped the positive identification by the victim. He dropped the charges against Adams, who was released. Nevertheless, he had been in custody for five months before this happened. Adams wasn’t the first person to suffer in this fashion: in 1996, the Houston Chronicle reported that another sexual assault suspect was held for nearly nine months while the Crime Lab performed a DNA analysis that ultimately proved him to be innocent. This was despite the fact that tests of this kind can be completed in two to three days.

For Josiah Sutton, the outcome of the DNA testing was quite different. With regard to the sperm sample taken from the victim’s vagina, Kim found that Sutton’s alleles were present at all seven loci that the machine tested. (There were also other alleles present that didn’t match Sutton’s alleles, but that was to be expected: they presumably belonged to the other, unidentified attacker.) And, Kim reported, Sutton’s alleles were also present in the sample of pubic hair combings, as well as in the sperm sample retrieved from the car seat where the rape had occurred.

Kim then went on to calculate how common Sutton’s DNA profile was by consulting reference tables that listed the prevalence of the various alleles at each locus for the demographic group that Sutton belonged to. She calculated that Sutton’s pattern of alleles occurred in only one in 694,000 black males. Josiah Sutton, it seemed, had left his genetic fingerprints all over the crime scene.

With positive identifications by both the victim and the Crime Lab, the DA was not about to let Sutton walk. He did enter into plea bargaining with Sutton’s attorney, but his offer - 50 years’ imprisonment - was hardly calculated to encourage a guilty plea, especially while Sutton was still loudly proclaiming his innocence. Carol Batie hired a different attorney, Charles Herbert, for her son. Encouraged by Sutton and Batie, Herbert requested that DNA samples be sent to an independent lab for testing. Kim did send the samples, but somehow they never got tested. By the time the trial got under way, in July 1999, there had been no independent review of Christy Kim’s analysis.

The OJ Simpson trial it wasn’t. The entire proceedings lasted just three days. The victim described her ordeal and pointed to Sutton as the man who had abducted her at gunpoint and raped her. Christy Kim described her analysis of the DNA. Herbert and the trial defence attorney made some efforts to challenge the believability of the witnesses - Herbert, for example, pointed out that the Houston Crime Lab was not accredited by the American Society of Crime Lab Directors, as mandated by Texas law - but it didn’t help. The prosecutor, Joseph Owmby, described Sutton as ‘evil and dangerous’, and the jury agreed. They took two hours to find him guilty.

The judge, a career prosecutor named Joan Huffman who had just been appointed to the bench, sentenced Sutton to 25 years’ imprisonment.

In prison, Sutton witnessed murder, rape, and suicide. He was injured in fights and he experienced solitary confinement. But he also converted to Islam, earned a high-school equivalency diploma and studied legal issues relevant to his case. He appealed his conviction, but lost. He wrote to the trial judge, asking for retesting of the DNA samples, but she refused.

Sutton also wrote to the Texas Innocence Network - an organisation based at the University of Houston. His application for legal assistance was rejected because the Network had a policy of not challenging DNA-based convictions. At that time, the Innocence Network, like other similar projects around the nation, viewed DNA testing as the gold standard - the ultimate arbiter of the truth that might get innocent people out of prison but would never put them there in the first place.

The first glimmer of a change in Josiah Sutton’s fortunes came in the spring of 2001, almost two years after his conviction, when the Texas state legislature enacted a revision to the criminal code that allowed convicted criminals to apply for retesting of DNA evidence under certain conditions. Early in 2002, Sutton filed a request for retesting, but his request was just one of hundreds that flooded the system. Nothing much happened, except that Sutton was moved back from prison to the county jail, where persons awaiting trial are usually held.

In the course of 2002, two reporters for KHOU-TV, the local station affiliated to the national CBS network, began to investigate the Houston Crime Lab. The reporters, Anna Werner and David Raziq, had been alerted to potential problems by defence attorneys. The reporters examined the Crime Lab’s records for numerous cases and sent seven of them for review by outside experts. One of these experts was William Thompson, a professor of criminology, law and society at the University of California, Irvine. Thompson had achieved some celebrity as a member of OJ Simpson’s defence team; he had critiqued the handling of DNA samples in that case.

When I met with Thompson in 2006, he told me that he was shocked when he studied the material he received from Werner and Raziq. ‘The first seven cases I looked at, there were egregious problems. There were outright misrepresentations of lab results: analysts would tell the jury that sample A matched sample B, and I would look at the underlying lab work and sample A did not match sample B. And they were failing to run proper control samples. When DNA testing is done, it’s important to run a control called a “reagent blank” [a test with no DNA] in order to make sure that you’re not contaminating these samples with foreign DNA. When you’re extracting the evidence samples along with the references samples, which Houston was doing, you could accidentally contaminate the bloodstain from the crime scene with the suspect’s DNA and thereby get him mixed in. In my own work, I always look closely at the reagent blanks to make sure they’re clean. Well, there weren’t any reagent blanks. In seven cases I didn’t see reagent blanks in any of them.’

Another problem had to do with the way the statistics were presented to the juries, especially in cases where the evidence sample contained a mixture of DNA from more than one person. The appropriate statistic to report is the fraction of the population whose alleles are represented in the mixture of alleles found in the evidence sample - that is to say, the fraction of the population who could have donated DNA to the sample. ‘But they were not providing that statistic,’ said Thompson. ‘They were providing the frequency in the population of people who would exactly match the suspect. Most other labs had figured out that in a mixture case you use what are called “mixture statistics”. So this lab was not following general practices.’

On November 11, 2002, KHOU ran the first of a series of investigative news reports about the deficiencies at the Houston Crime Lab, based in part on Thompson’s analysis. The reports drew a great deal of attention, because Harris County, where Houston is located, has long had a reputation as the death penalty capital of the world - it has executed more people since 1982 than any other city, or indeed any state except Texas itself and Virginia. Many of these cases have involved physical evidence that was analysed by the Crime Lab.

In response to the television reports and the public outcry that they provoked, the Houston Police Department commissioned an audit of the DNA lab, which revealed numerous deficiencies. In December, the lab was closed down indefinitely.

One of the people who saw the KHOU reports was Josiah Sutton’s mother, Carol Batie. She immediately contacted Werner and Raziq and told them about her son’s case. They collected the lab reports and court transcripts from the case and sent them off to Thompson for his opinion.

Thompson recounts that he and his wife, Claudia, worked on the boxful of reports together over the breakfast table one Saturday morning. Thompson studied Christy Kim’s lab report while his wife read the transcripts. One passage in the report read as follows:

A mixture of DNA types consistent with J. Sutton, the victim, and at least one other donor was detected on the vaginal swabs, unknown sample #1, debris from the pubic hair combings, and the jeans based on PM, DQA1, D1S80 typing results.

Thompson didn’t know what ‘unknown sample #1’ referred to, but by reading the transcript Claudia deduced that it was the semen stain found on the car seat where the rape had occurred. This meshed with what Christy Kim had testified in court - that the semen on the car seat could have come from Sutton. But when Thompson looked at the actual test results, he saw that this couldn’t be true: at the locus named ‘DQA1’ Sutton possessed the alleles known as ‘1.1’ and ‘2’, whereas the semen stain contained the alleles ‘2’ and ‘3’. Sutton’s allele ‘1.1’ was not present in the stain, and thus he could not have contributed to it.

Kim had actually read the tests correctly, because her lab notes recorded the mismatch between Sutton’s DNA and the semen on the car seat. But the report she wrote up for the police ignored the mismatch and wrongly fingered Sutton as a possible source of the semen. Questioned about this later, Kim said that her misstatement was caused by a ‘transcription error’.

The fact that Sutton wasn’t the source of the semen on the car seat didn’t let him off the hook, of course, since the semen could have come from the other attacker. So Thompson turned his attention to the sperm sample from the victim’s vagina. It was true, as Kim had reported, that the DNA from this sample contained Sutton’s alleles at every locus tested. At the DQA1 locus, however, the sample contained a total of four alleles: 1.1, 2, 3, and 4.1. If the other attacker’s alleles were 2 and 3, as indicated by the semen on the seat, Sutton must have contributed the remaining alleles, 1.1 and 4.1, if he was one of the rapists. But that was impossible, because Sutton didn’t possess the allele 4.1.

‘We had this sudden realisation over the breakfast table,’ said Thompson. ‘“Wait a minute,” we thought. “This makes no sense!”’

By themselves, Thompson’s deductions didn’t absolutely prove Sutton’s innocence, because they depended on two assumptions. One was that the semen sample on the car seat did indeed come from one of the rapists, rather than being the result of another sexual encounter that just happened to have taken place at the same location. Also, it was theoretically possible that Sutton was one of the rapists but a third man - neither Sutton nor the person who deposited semen on the car seat - was the source of the unexplained 4.1 allele in the vaginal sperm sample. Still, that seemed unlikely, since the victim said she hadn’t had intercourse with anyone other than the two attackers in the relevant time frame. Thus the most likely conclusion was that Sutton was in fact innocent.

Thompson also took issue with Christy Kim’s presentation of the statistics. Just as had happened in the previous cases that Thompson reviewed, Kim had simply reported the odds that a randomly-chosen black person would match Sutton’s alleles at the loci she tested. She gave that figure as 1 in 694,000. She also cited this figure in court. The jury was therefore left with the impression that there were astronomical odds against the possibility that Sutton’s DNA matched the vaginal sperm sample purely by chance. Yet, as Thompson demonstrated, the fact that the sample was a mixture of at least two individuals’ DNA shortened the odds tremendously. In fact, at three of the seven loci, any man on earth would have matched the vaginal sperm sample, because at each of those loci only two possible alleles exist, and the sperm sample contained both of them. When Thompson used the appropriate procedure to calculate the odds that a randomly-chosen black man would match the mixed sperm sample at all seven loci, he came up with a figure of not 1 in 694,000 - but 1 in 15.

And considering that the police had two shots at finding a match (once with Josiah Sutton and once with Gregory Adams) the odds that one of the men would match the sample were twice as high - l in 7.5. Thus, even if one ignored the fact that Sutton was positively excluded as a suspect (with the caveats mentioned above), the match of his DNA to the vaginal sample could easily have been a mundane coincidence of the kind that a busy crime lab could expect to encounter every day.

When Werner and Raziq received Thompson’s report, they aired Sutton’s case in a news special on KHOU-TV. A few days later, the Houston Police Department ordered a new test of the DNA evidence, this time from an independent laboratory named Identigene. The laboratory used a more recently developed test named Profiler Plus, which uses a special-purpose machine, combined with analytical software, to test nine different loci simultaneously. These loci are different from the ones tested by Christy Kim, and they are more informative because there are more possible alleles (7 to 14) per locus. The results proved that two men contributed to the vaginal sperm sample and that neither of them had the DNA profile of Josiah Sutton. Sutton was innocent.

A few days later, on March 12, 2003, Sutton was released on bond. Now 21 years old, his life as a free man took an erratic course: he tried various jobs and even enrolled in college, but nothing seemed to stick. Meanwhile, the director of the Texas Innocence Network, David Dow, finally agreed to take on Sutton’s case, with the aim of getting the parole board to recommend to Governor Rick Perry that he should issue a pardon.

Texas has two kinds of pardon. A ‘full pardon’ leaves the conviction standing: it simply terminates the sentence and restores the person’s civil rights, without saying anything about whether he actually committed the crime. A ‘pardon for innocence’ is much rarer: it annuls the original conviction and opens the door for the person to claim compensation for time spent in prison, at a rate of up to $25,000 per year. To obtain a pardon for innocence, the district attorney, the police chief and the judge must all send letters to the parole board recommending the pardon, and the letters much be accompanied by evidence that the person did not commit the crime for which he was convicted. In Sutton’s case, the district attorney, Chuck Rosenthal, seemed incapable of letting go of his belief that Sutton was one of the rapists, and he therefore petitioned for a full pardon rather than a pardon for innocence. When asked by journalists for his reasons, he mentioned the fact that the victim had positively identified Sutton as one of her attackers (though she had also positively identified Gregory Adams, who was unquestionably innocent). As for why Sutton’s DNA was not found in the evidence samples, Rosenthal suggested that he had used a condom or failed to ejaculate. This theory required the victim to have been raped by three men rather than the two she testified to. This kind of reasoning - adding extra, unknown perpetrators - has become a staple among prosecutors trying to rescue a case in the face of exculpatory DNA evidence. Peter Neufeld, co-founder of the New York Innocence Project, has famously dubbed it the ‘unindicted co-ejaculator hypothesis’.

Dow’s efforts to obtain the pardon for innocence dragged on for months. Rosenthal initiated yet another independent round of testing of the DNA evidence on the Sutton case, and the results confirmed Identigene’s earlier report. Finally, in May 2004, Governor Rick Perry issued Sutton a pardon for innocence. Sutton petitioned for compensation and was declared as being entitled to $118,000. He received his first payment of $60,000 in 2004. He went through the money in six months by buying three cars, partying and giving money to his family, according to the Houston Chronicle. He told the newspaper that he planned to be more careful with the next instalment. ‘It was a learning experience, and I have grown up,’ he said.

‘After he was exonerated, I went down to testify before a grand jury hearing about the lab,’ said Thompson, ‘and I took him and his mother out to dinner. I was kind of relieved to meet him, because I was thinking he would be some thug, but he struck me as sort of a gentle soul. He’s into meditation; he’s very serious about religion. He seemed confused and rootless and he didn’t quite know what to do with himself.’

While Sutton was seeking his pardon, the city of Houston turned its attention to the Crime Lab. In early 2003, the former police chief recommended that the lab’s director be fired, and he retired before that could happen. Nine employees of the lab were disciplined. Christy Kim and another analyst were given 14 day suspensions, but they appealed to the Civil Service Commission and the suspensions were overturned. Then, in December, the mayor fired Kim, but again the Commission reinstated her on the grounds that she had merely followed the laboratory’s standard practices. Eventually Kim resigned, and several other employees joined the director in retirement.

In early 2005 the city of Houston commissioned a new and completely independent review of its Crime Lab by Michael Bromwich of the Fried Frank law firm. As US Inspector General during the Clinton administration, Bromwich had conducted an investigation of the FBI’s Crime Lab that led to the disciplining of several agents.

His review of the Houston lab, published in several instalments in 2005 and 2006, was a damning indictment of the lab’s practices, not just in DNA work but in serology (blood group analysis) and other departments. Out of 67 DNA cases reviewed, Bromwich found that 40 per cent had deficiencies serious enough to raise doubt as to the reliability of the work, the validity of the results or the correctness of the analysts’ conclusions. Three of these were capital murder cases that had ended with the defendant being convicted and sentenced to death, though none had proceeded to actual execution.

In numerous instances, the analysts seemed to have shaded their findings to support the prosecution, often by failing to report findings that contradicted the prosecution’s case. In one serology case, for example, a bloodstain found at the scene of a double murder failed to match either the victims or the accused - a finding that was clearly exculpatory - but the DNA lab chief, James Bolding, concealed the mismatch by marking the stain’s blood type as ‘inconclusive’, according to Bromwich’s report. Years later, a second person was accused in the case, and his blood did match the stain. According to Bromwich, Bolding did not retest the stain but simply altered the original report to show a positive match. Bromwich also wrote that Bolding was even untruthful about his own qualifications: in a sexual assault case he testified that he had a Ph.D. in biochemistry from Texas Southern University, but he later admitted to Bromwich that he did not have a Ph.D. degree at all.

There were also DNA cases in which the analyst reported having run controls (such as reagent blanks) but the lab data showed that no controls had been run. Such cases of apparent ‘dry-labbing’ are said to be pervasive in crime labs around the United States.

The analysts consistently misreported the statistics of their cases. Just as happened in the Sutton case, the analysts would cite the frequency of the accused’s DNA profile in the population (a very low but irrelevant number), but not the likelihood that the accused’s profile would be present in a mixed evidence sample purely by chance (a much higher number). ‘It is clear that the DNA analysts in the Crime Lab, including Mr. Bolding, did not fully understand the scientific basis of calculating frequency estimates,’ wrote Bromwich.

Perhaps the overriding problem at the Houston Crime Lab was the lack of effective oversight. ‘We have found no semblance of an effective technical review program or quality assurance regime to detect and correct these problems,’ wrote Bromwich. ‘As a result, they continued unabated.’

Bolding eventually ceased cooperating with the investigation, and Kim had refused to answer Bromwich’s questions from the beginning. Although this lack of co-operation may have been motivated by a fear of self-incrimination, no Crime Lab employees have in fact been indicted in connection with the investigations.

Although many convictions have been brought into question by Bromwich’s review, so far only two persons have been released from prison. One was Sutton, and the other was a man who served more than 17 years for rape and who, like Sutton, was exonerated by new tests.

In the summer of 2006, after being shuttered for three years, the Houston Crime Lab reopened for business with official certification, a new director, new or retrained analysts, a quality-control programme and largely new facilities. A year later, Bromwich’s final report spoke approvingly of these changes, but also listed some persisting shortcomings in the techniques used to analyse and interpret DNA data.

In fact, there are considerable differences of opinion about the nature of the underlying problems and how to solve them. According to Bill Thompson, the core problem is that police crime labs are instruments of the state, so their analysts will, consciously or unconsciously, favour the prosecution. The way to solve this problem, according to Thompson, is to close the police labs and turn all work over to independent agencies. In theory at least, these labs would be more likely to conduct scientifically rigorous tests and to present the results in an impartial manner.

In addition, Thompson believes that all labs should be subject to periodic random testing in which known samples are provided for ‘blind’ analysis - that is to say, the laboratory would not know when they were being tested. Currently, if such quality-control tests are done at all, analysts are given advance notice. As a result, we may assume that they are put on their best behaviour. Thompson believes that blind testing could be used to derive some measure of a lab’s false-positive rate - the likelihood that an incriminating test result would be in error. Even if the false-positive rate were very low - say, one in a thousand - that would still make nonsense of the ‘one-in-a-billion’-type statistics that are commonly presented to juries in an effort to secure conviction.

A committee of the US National Research Council has rejected these ideas. It would be impracticable and too expensive to conduct enough blind testing to obtain reliable estimates of false-positive rates, the committee believed. And if defence lawyers believed their case was affected by pro-prosecution bias among police-lab analysts, they should be encouraged to commission independent testing to counter it.

That’s not realistic, says Thompson. The defence attorneys may not be familiar enough with DNA technology to perceive the need for a retest. There are often insufficient funds for independent testing. Or the police lab may have used up the entire evidence sample in their own testing - or they’ll say that they have.

Most importantly, criminal defence lawyers are often reluctant to commission independent tests for fear of harming their clients. After all, defence lawyers generally assume that their clients are guilty - because they usually are. So they expect that a test conducted by an independent lab will confirm the findings of the police lab. For sure, they don’t have to mention in court that they had their own testing done or what the result was, but the prosecution often finds some devious way to bring up the subject, according to Thompson. ‘The prosecutor says to the expert, “Oh, I notice there are some missing pieces of this fabric sample, where did those go to?” The defence says, “Objection!” and the prosecutor says, “Your Honour, it’s important for continuity, to explain, et cetera…” The judge says, “Objection overruled,” and the expert’s answer is “Oh, I sent them to the defence lab for testing.”’ Once the jury knows that the defence had its own testing done but didn’t present the results, they immediately assume that the results were incriminating, so any attack on the police lab’s tests loses credibility. Thus, there are several real-life factors that reduce the defence’s ability or willingness to conduct independent tests.

Thompson’s self-appointed mission is to educate defence lawyers about DNA testing so that they are in a better position to review tests conducted by police labs and spot the problems that call out for independent testing and for challenging the state’s experts. The lawyers and law students who staff the Innocence Projects are clearly hearing his message, because the existence of DNA evidence is no longer a bar to taking on a case the way it was when Josiah Sutton sought the Houston Innocence Network’s aid.

Have errors in forensic science ever led to the execution of an innocent man? Given the sheer number of executions - more than a thousand since the death penalty was re-instated in the United States in 1976 - it seems likely that they have, but identifying a specific instance has proved difficult. One case that has drawn a lot of attention is that of Cameron Todd Willingham, executed in Texas in 2004 for setting a house fire that killed his three children. Willingham asserted his innocence before, during and after his trial, and he did so again in his final statement before execution. At his trial, investigator Manuel Vasquez reported finding scientific indicators of arson, such as the presence of crazed glass, but subsequently-published forensic guidelines have rejected them as mere superstition. Hot glass is easily crazed by contact with water from fire hoses, for example. ‘Each and everyone of the ‘indicators’ listed by Mr. Vasquez means absolutely nothing,’ reported a commission of nationally-respected arson investigators in 2006. Willingham certainly hasn’t been proven innocent, but the evidence for his guilt has largely evaporated.

Josiah Sutton’s story isn’t over. When the independent lab did the DNA testing that ruled him out as a suspect, it was able to reconstruct a complete DNA profile for one of the actual rapists, as well as a partial profile for the other. The complete profile was used to search a database maintained by the Texas Department of Public Safety, but no match was found. Still, new DNA profiles are being entered into the database all the time, because in Texas all convicted criminals have to give a DNA sample. In 2005, a young black man named Donnie Lamon Young, who was serving time on a drug conviction, gave blood for testing. In May of 2006 the DPS found that Young’s DNA was a match to the complete profile from the Sutton case.

The Houston Police Department was notified, and a new sample was taken from Young, who was by then out of prison. Again there was an exact match. In June, Young was arrested and charged with aggravated sexual assault in the case for which Sutton was wrongly convicted. He was held in the county jail after he failed to post a $150,000 bail bond. The victim was unable to pick Young out of an identification line-up, but in January 2007 he pleaded guilty and was sentenced to 10 years’ imprisonment. He also named his accomplice, a man who had died in prison.

When asked by the Houston Chronicle for his reaction to Young’s arrest, Josiah Sutton expressed himself laconically. ‘Let’s just say, if he’s the one who did it, that I don’t think we would be two good people to put in a room together.’

His mother was more philosophical. ‘My son had been pardoned,’ she said, ‘but it still weighed on my heart that no one had been arrested and that some people would not believe in Josiah’s innocence until someone was. Now, justice can be done for the victim, and we can really close the book and say he did not do it.’

District Attorney Rosenthal said, ‘I still don’t know enough to know whether [the victim] was mistaken or not. I intend to look into it, but if Sutton is innocent, I will be the first to say he is.’

To which Bill Thompson commented, ‘Chuck, you’re a little late.’*