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Forensic psychology
Forensic psychology

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2.4 Variables affecting reliability of testimony

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Figure 5 Variables affecting reliability of testimony

Wrongful convictions occur in a variety of circumstances and across many different types of crime, which the Innocence Project illustrates.

In determining what can affect the reliability of eyewitness evidence, the nature of the crime is itself significant: witnessing someone stealing from a shop is a different experience from witnessing someone physically assaulting another person; and being a victim of a handbag snatch is very different from being a rape victim. While the findings from research in one particular setting are not necessarily generalisable to all crime scenarios, they do provide useful information to the legal system. Research findings have also identified ways of enhancing the reliability of witness testimony, both in the police station and in the courtroom.

In general, when researching variables that might affect eyewitness evidence, psychological research distinguishes between factors based on whether or not they are under the control of the criminal justice system. This distinction is important as it determines how the results of the research can be applied.

System variables

System variables are those that might affect eyewitness evidence and that are under the control of the criminal justice system. System variables include the way in which the police question a witness and the procedures for asking a witness to identify a perpetrator in an identification parade. Research that investigates system variables can have important implications for policing policy and practice (Wells, 1978). For example, if one set of procedures is found to be more effective in eliciting accurate evidence, then, arguably, it should be adopted as common practice. Thus, research on system variables can be applied by altering the way that investigations and trials are conducted.

Estimator variables

Estimator variables are those that might affect eyewitness evidence and that are not under the control of the criminal justice system. This includes such things as whether the perpetrator was wearing a disguise or positioned too far away from the witness to allow for accurate identification. These are obviously factors that cannot be affected by the police or courts.

Although research on estimator variables cannot be used to alter the processes used in investigations and trials, nevertheless the findings can help in determining how likely it is that the witness is able to provide reliable evidence. For example, a witness who saw the perpetrator from a great distance is unlikely to be able to identify the perpetrator accurately. Research on estimator variables can also be very important when the case reaches court, as it is important for the jury to know whether there were factors that might have had an impact on the accuracy of the evidence being provided by the eyewitness.

Comparative law

Another factor to consider when exploring psychology and the law is that the law and different systems of justice vary from one country to another.

For example the legal system in the UK, and in other countries modelled on the English system of common law, is described as adversarial, or accusatorial. Spencer and Flin (1993, p.75) summarise such systems:

In an accusatorial system each side presents a case before a court the function of which is limited to deciding who has won. The judges have nothing to do with the preliminary investigations, give no help to either side in presenting its case, and take no active steps to discover the truth, which emerges – or so the theory goes – from the clash of conflicting accounts.

By contrast, the inquisitorial system found in many European countries and other parts of the world is described as:

The court is viewed as a public agency appointed to get to the bottom of the disputed matter. The court takes the initiative in gathering information as soon as it has notice of the dispute, builds up a file on the matter by questioning all those it thinks may have useful information to offer – including, in a criminal case, the defendant – and then applies its reasoning powers to the material it has collected in order to determine where the truth lies.

(Spencer and Flin, 1993, p. 75)

The research and police investigations described in this course are firmly located in the accusatorial system of justice that is used in the UK and the USA. This is partly due to the accusatorial system posing more problems for witnesses and the reception of their testimony (e.g. placing what may seem to be undue emphasis on oral evidence live in court on the day of the trial), but also because most of the psychological research in this area stems from the USA and the UK.