3. Beyond Reasonable Doubt

A conversation about the t.v series CSI got me thinking about the use of forensic evidence in particular DNA fingerprinting in a court of law.

. . .Upon arriving at the crime scene, a CSI finds a small piece of blood stained cloth on the ground two meters away from a murder victim. The cloth does not match the clothing the victim is wearing. The CSI picks up the cloth with a pair of forceps and places it in a bag labeled “biological evidence.” Several months later a DNA fingerprint of the blood will be presented in court.

There is no doubt that DNA fingerprinting has become a reliable source of evidence and since its introduction has been used not only a way of securing convictions but also to exclude suspects who might otherwise be falsely charged with and convicted of serious crimes. However, the results of DNA fingerprinting are not 100% certain and like most knowledge claims fall along a true-false continuum. Before we can consider how much we can trust this type of evidence we first need to consider some of the issues surrounding its reliability.

The term DNA fingerprint implies that, like a fingerprint, the pattern for a given person is completely unique to that person. Actually, a DNA fingerprint just presents a probability that the person in question is indeed the person to whom the pattern belongs. Typically, evidence in court is presented by way of a ‘match probability’, the probability that the defendant’s DNA profile matches that of the crime sample. Two of the methods of DNA fingerprinting are RFLP and PCR. The RFLP method gives a much higher probability of a match but analysis of a sample can take a couple of months, a larger amount of sample (thousands of cells) is needed and the cells have to be fresh. The match with PCR is less probable but requires a smaller sample of about 50 cells, analysis is can be done in a couple of days and the sample does not have to be fresh, it could be decades old. Both PCR and RFLP can only analyze nuclear DNA found in the nucleus of a cell. However bone, hair and teeth do not have cells with nuclei so another process involving the analysis of mitochondrial DNA (mtDNA) is used. In the investigation of murder cases that have gone unsolved for many years, where bone hair and teeth may be all that remain mtDNA will be used.

The actual process of creating a DNA fingerprint is time consuming and requires meticulous laboratory work and correct protocols to be followed otherwise the sample can become contaminated. Therefore the quality of the lab doing the analysis, its techniques and hygiene standards can affect the accuracy in the results. This is especially problematic in countries where there are no nationwide standards for the testing of DNA.

Crime scene evidence which has been altered, tampered with or destroyed is not admissible to court so the method with which DNA evidence is collected and transported is important. When crime scene investigators collect evidence they have a duty to make sure that the evidence is not compromised. The quality of the sample depends on the competence of these investigators because if the DNA sample is not in perfect condition it can affect the accuracy of the analysis. Extreme heat and other physical conditions at a crime site can also damage the sample making it hard to analyze.

So how much can we rely on the results of a DNA fingerprinting presented as evidence in court? Can it be relied upon beyond reasonable doubt? Thinking about this has lead me in to asking the following questions.

1. One definition of knowledge is a true belief based on strong evidence. Can DNA evidence be considered strong enough justification to convince a jury beyond reasonable doubt?
2. A key factor that distinguishes acceptable from unacceptable justification is reliability. How can the reliability of DNA fingerprinting be established?
3. Evidence in criminal law is any item (e.g. weapon, document, clothing, artifact, DNA fingerprint) or testimony (e.g. oral or written statement) that assists in the proof of a prosecution or defense. Which type of evidence would be considered the most trustworthy in court and why?
4. Because DNA fingerprinting is a product of ‘science’ might there be a tendency for members of a jury to consider the results more reliable than other types of evidence. Could the prior beliefs of a jury be altered in light of results presented by DNA fingerprinting?
5. In court expert witnesses are called upon to provide an expert opinion like the interpretation of the results provided by DNA fingerprinting. How does the reliability of the evidence presented by a forensic scientist compare to that of eye witness testimony. What are the strengths and weaknesses of both forms of evidence?
6. DNA fingerprinting provides a probability that the person in question is the person to whom the pattern belongs. Consider the probability of two individuals having a match is 1 in 1000 with PCR and 1 in 100000 with RFLP. Would the PCR method be reliable enough to convince a jury beyond reasonable doubt? What about the results from RFLP? Which probabilities would be acceptable alone to prove guilt?

What do you think?