Pennington and Hastie (1988) – Explanation-based decision making: effects on memory structure on judgement

Pennington, N. and Hastie, R., (1988) – ‘Explanation-based decision making: effects on memory structure on judgement’, Journal of Experimental Psychology, Learning and Memory and Cognition 14 (3), 521-33.

Background

This is the first study we will be looking at from Reaching Verdict and  Persuading a Jury, as part of your OCR A2 Forensic Psychology course. It is further categorised into ‘order-effects/story order/witness order.’

Reaching a verdict and Persuading a Jury consider the legal system. This study specifically covers court hearings.

In the United Kingdom the final verdict in criminal trials is made by a jury of 12 citizens randomly selected from the voting register,  which upon turning 18 all UK citizens are added to. Prisoners and people diagnosed with mental illnesses are not allowed to serve on juries.

 

In the United Kingdom there are 9 stages to court hearings.

  1. Indictment
  2. Defendant’s plea
  3. Prosecution opening statement
  4. Defence’s opening statement
  5. Evidence from witnesses
  6. Prosecution closing statment
  7. Defence’s closing statement
  8. Judge’s instructions to the jury on procedures and verdicts.
  9. Jury retire to reach a verdict, of which 10 out of the 12 must agree.

One of the problems with studying the decision making process of juries is that in the United Kingdom it is illegal to ask juries how they arrived at their decision or to record the process. This means that psychologists must use alternate methods to study this process. There are two main ways which they will achieve this. Firstly, there will use 12 people to make a shadow jury that watches the case from the public gallery and then makes attempts to reach a verdict. Secondly, psychologists can attempt to create a mock trial and then record how the mock jury reaches their decision. Thirdly, they can use recordings of available trials, which is what Pennington and Hastie (1988) did.

Video on Primacy Effects:



Video on Recency Effects:


Aim

To investigate whether or not story evidence summaries are true causes of the final verdict decisions and the extent to which story order affects confidence in those decisions.

Method and Design

Laboratory experiment, the second of two (the first was in Pennington & Hastie 1986).

Participants

130 college students from Northwestern University and Chiacago University who were paid for their participation in the hour long experiment. They were allocated to one of four conditions in roughly equal numbers.

Procedure

Participants listened to a tape recording of a stimulus trial (Commonwealth of Massachusetts vs Caldwell) and then responded to written questions. This was a 1 hour long audio recording of a real trial.

The participants were told to reach either a guilty or a not guilty verdict against a murder charge on the defendant. Once they had reached their verdict, they were asked to rate their confidence in their decision on a 5-point scale. The participants reached their verdict independently and did not confer with any of the other participants, they were separated by a partition.

In the audio recording there was 39 pieces of evidence that the defendant was guilty and 39 pieces that the defendant was innocent.

The evidence was either presented in story order, that is the chronological order of the actual events, or witness order, that is the order in which the witnesses in the original trial presented the information.

There were four possible conditions:

  • Prosecution items in story order and defence in witness order.
  • Prosecution items in story order and defence in story order.
  • Prosecution items in witness order and defence in story order.
  • Prosecution items in witness order and and defence in witness order.

In all the cases the stimulus trial began with the indictment and followed normal trial procedure ending with the judge’s instructions

Findings

Percentage of participants choosing a verdict of guilty of murder by prosecution and defence order conditions:

  • Prosecution items in story order and defence in witness order. 78%
  • Prosecution items in story order and defence in story order. 59%
  • Prosecution items in witness order and defence in story order. 31%
  • Prosecution items in witness order and and defence in witness order. 63%

Mean percentages of guilty verdict:

  • Prosecution items in story order: 69%
  • Prosecution items in witness order: 47%
  • Defence in story order: 45%
  • Defence in witness order: 70%

These show that story order persuaded more jurors of Caldwell’s guilt in the prosecution case. If the defence presented it’s evidence in witness order then even more jurors would find a guilty verdict and if the positions were reversed and the defence had the benefit of the story order the guilty rate then drops to 31%

Conclusions

As predicted, the greatest confidence in their verdict was expressed by those who heard the defence or prosecution in story order. Least confidence was expressed by those who heard the two witness order conditions.

Pennington and Hastie (1988) Evaluation

+ The use of quantitive data makes it easy to analyse and therefore establish cause and effect.

+ The results are similar to the the 80% guilty verdict rate of American courts in this condition: Prosecution items in story order and defence in witness order: 78%, which suggests that the results from the alternative orders may predict real court results from using such orders.

– Ecological validity – the trial only lasted an hour and was only an audio recording. Furthermore, the participants reached their decision independently, which is unlike juries in real trials. Therefore we can argue that the results may not be generalisable to real life.

+ Reliability – both the large sample and the methodology makes this experiment highly reliable.

– Demand Characteristics – As the participants were paid for their participation, we can argue that some of the results may be due to participants trying to act in a way desirable to the experimenters.

– Ethics, we could argue, due to the nature of the case, that some of the participants may have been negatively affected by the content of the case.

+ Validity – as there was an equal amount of guilty and not guilty pieces of evidence, we can argue that the results were valid and the independent variable caused the change.

– Determinism – arguably this case highlights the determinism of the legal process: the results show that it does not matter how many pieces of evidence are for or against a guilty verdict, but more the order in which the information was presented.

 Audio Podcast

 



References

Pennington, N. and Hastie, R., (1988) – ‘Explanation-based decision making: effects on memory structure on judgement’, Journal of Experimental Psychology, Learning and Memory and Cognition 14 (3), 521-33.

 

Further Reading

OCR A2 Psychology Student Unit Guide: Unit G543: Forensic Psychology (Student Unit Guides)

 

Summary
Article Name
Pennington and Hastie (1988) - Explanation-based decision making: effects on memory structure on judgement
Description
This is the first study we will be looking at from Reaching Verdict and Persuading a Jury, as part of your OCR A2 Forensic Psychology course. It is further categorised into 'order-effects/story order/witness order.'
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