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Within the legal system, criminal prosecution can be described using two dichotomies; first there is the inquisitorial approach and second there is the adversarial approach (Kyprianou, 2009). The nature of the inquisitorial system is such that the judge takes an active role and is primarily responsible for supervising the gathering of evidence. In this system, the judge actively directs questioning and arrives at the "truth" through their own assertions (Law Library - American Law & Legal Information, 2010). The inquisitorial system finds its origins in the French Code d'Instruction Criminelle of 1808, which created the ministère public or the French Public Prosecution Service (Kyprianou, 2009). In today's society, France is still one of the main European countries employing an inquisitorial prosecution system (Kyprianou, 2009). The adversarial prosecution system dates back to early Common Law and is most commonly found in the United States and England and Wales. The nature of the adversarial approach is such that two or more opposing parties present their evidence and arguments to a judge or jury (Law Library, 2010). However, only a small percentage of criminal cases are actually tried before a jury, accounting for only 12% of charges in England and Wales (Thomas, 2010). Despite these two dichotomous systems, not one criminal prosecution system is purely inquisitorial or purely adversarial, but rather all are found somewhere along this dimension (van Koppen & Pendrod, 2003).
At the heart of the adversarial system lies the concept of a "fair trial" in which un-biased laypersons, i.e. the jury, hear evidence and render a verdict (School of Psychology, Module 7: Psychology & the Courts, 2010). Within England and Wales and the United States, having a case put before a jury is recognised as a fundamental civil liberty of the defendant (Module 7, 2010). Juries are used in criminal trials where the offence is indictable or triable either way, i.e. can be tried before a magistrate or at crown court, and in these instances the defendant is usually able to elect to have a jury (Module 7, 2010). Within the UK, in order to be eligible for jury service one must be registered to vote, be aged 18-69 and be a UK resident for at least five years since the age of 13 (HM Courts Service, 2010). Those summoned for jury service may be excluded under certain circumstances such as mental illness, being unable to be absent from work for an extended period of time or having previously done jury service within the last two years to mention a few (HM Courts Service, 2010).
Empirical research into jury decision making has been conducted since the mid-1950's and has focused on areas such as group dynamics, evidence evaluation, pre-trial publicity effects, individual juror differences and memory (Module 7, 2010). Methods of research have included case simulation with real juries at Crown Courts (e.g. Thomas, 2010), mock jury research (e.g. Skolnick & Shaw, 2001) and post-trial interviews with jurors (e.g. Seltzer, Venuti & Lopes, 1991). As with all research, the methodologies employed in jury research are fraught with problems. For example, when using retrospective analysis of actual cases, it is not possible to know absolutely if the right verdict was chosen or if the juror's consciously rejected the legally correct verdict (Module 7, 2010). Research into jury decision making has led to the formation of models such as the "story model" formulated by Pennington and Hastie (1981). In this model, the jury organise the evidence from the trial into a "story" which they then use to reach a verdict decision. From Pennington and Hastie's (1981) Ideal Juror Model, the jury account for facts by inferring casual links between pieces of evidence. Formulation of a narrative allows the jury to organise the large amount of information they are exposed to, thereby giving them a framework in which they can organise and interrelate facts (Module 7, 2010).
In order to construct their "story," jurors need "story ingredients" such as the evidence, prior general knowledge and pre-trial publicity (Pennington & Hastie, 1981). However, as different jurors may interpret the evidence differently, several "stories" will develop as they invariably come to different perspectives. Inference and subjectivity are integral to the story evaluation process for each juror and they are not only asked to judge the actus reus of a crime (what happened) but also the mens rea (mental state) of the defendant (Module 7, 2010). Within psychology, this is based upon attribution theory which states that when cause and intention are attributed, culpability can then follow (Alicke, 2000).
During a criminal trial, a jury can be presented with associative evidence or identification evidence. Associative evidence establishes a link between crime scene evidence and the defendant and identification evidence determines the identity of a person, again most likely the defendant. When assessing the evidence, jurors must consider the probative value of each piece. For example, in the case of physical evidence, is it mobile and could have therefore been placed at a crime from elsewhere, and is the evidence relevant to the crime in question. With advances in forensic science techniques, forensic evidence is now more commonly seen in the courtroom. Forensic evidence can take many forms, for example Deoxyribonucleic acid (DNA) analysis, fingerprints, toxicology etc.
DNA can be extracted from a range of crime scene deposits such as blood, semen, hair, skin cells and saliva. With advances in forensic analysis technology very tiny samples can now be tested for DNA and then used in comparative analysis. In the case of R. v Loveridge (2001) it was stated that within England and Wales DNA samples must be accompanied by evidence as to the source of the sample and the procedure by which it was obtained and profiled. As forensic science has grown and the use of forensic evidence in the court room has increased, so has psychological research into the impact of forensic evidence on jury decision making.
After a DNA sample has been matched to another DNA profile, the next step is to determine the probability that that the match could have occurred by coincidence. This is achieved by consulting databases to determine the frequency of matching alleles within a specified population. This is known as the random match probability (RMP) and is usually presented to a jury alongside DNA evidence (Thompson & Cole, 2007). Within the courtroom jurors may be told that 1 person in 1 million would have the specified DNA profile, or in other words that the RMP is 1 in 1 million (Koehler, Chia, & Lindsey, 1995). Within the United States these frequencies may be converted to into a likelihood ratio and reported to the jury that the DNA sample found is, for example, 1 million times more likely if the sample came from the defendant than from an unrelated individual (Butler, 2005). A line of psychological research has examined how presenting statistics on the RMP of DNA evidence effects the probative value juries will place on a forensic match and consequently the verdict decision they will report as a result (Goodman, 1992; Smith, Penrod, Otto & Park, 1996).
Goodman (1992) conducted a study to assess whether jury decision making is sensitive to variations in the probative value of probabilistic DNA evidence by varying the RMPs and examining the conviction-acquittal rates. Goodman (1992) hypothesised that jurors would attribute more weight to DNA evidence when the likelihood of a random match was rare, compared to when the likelihood was greater. Using a mock-jury of 233 undergraduate students, Goodman (1992) exposed the subjects to one of four versions of a homicide narrative with a blood sample being the only evidence to link the defendant to the crime scene. In the scenarios the defendant's blood type was said to match the sample from the crime scene. However, Goodman (1992) varied the frequency of the blood type in a population of 100,000 to 0.1%, 1%, 5% or 10% in each scenario. In simple terms, the suspect pool from which the defendant was drawn ranged from 100, 500, 1000 to 10,000 people. A fifth scenario was used containing no frequency probability evidence in order to obtain a baseline conviction rate in the absence of incriminating forensic evidence (Goodman, 1992). After reading their scenario, jurors were asked to render a verdict of guilt, an estimate of the defendant's culpability and their perceived reliability of the evidence presented.
Results from Goodman's (1992) study show that 77% of the jurors rendered a "not-guilty" verdict. However, analysis did not reveal any significant differences between all four groups in the mean weight attributed to the fact that the defendant's bloody type matched the crime scene sample (Goodman, 1992). This demonstrates that the jurors recognised that the blood evidence was incriminating, but that they failed to make distinctions between probability estimates that were mildly incriminating, moderately incriminating and strongly incriminating. Indeed Goodman (1992) commented that there was very little difference in the culpability estimates and conviction rates between groups one, two and three where the likelihood of a random match ranged from 1/1000, 1/100 to 1/20 respectively.