You are a crime scene officer.

You need to determine what happened in order to charge a suspect. Both time and resources are tight.

You’ve been called to the scene of a suspected murder in a residential property. Police were called to the scene by a neighbour who saw the lifeless body through an open door.

You are a crime scene officer.

Hamid Victim's friend

Wayne Owner of property

Each suspect was seen individually in the stairwell of the property by neighbours who recognised them. None of the witnesses are sure about the time they were there.

Read their statements below

Two suspects are being held in custody.

You can’t hold both suspects in custody for long. Your job is to assess the evidence and decide which pieces to analyse in order to work out what happened. You will then be able to either charge one suspect and release the other, or release both if the evidence suggests they’re both innocent.

Your job is to gather enough evidence to support a murder charge.

The Officer who was first on the scene talks you through the evidence that she’s collected...

You arrive at the crime scene...

The body is lying in a pool of blood in the flat’s open plan kitchen/living room area. There is a deep, jagged gash to the victim’s neck, and several other cuts to his hands and face. The flat itself is a mess: there is an overturned coffee table, several empty bottles of alcohol and glasses on the floor, some of which have been broken, and two knives were found near the victim’s body. There is no sign of forced entry to the flat, suggesting that the perpetrator either had a key, was let in, or found the door open.

A pool of blood...

Take a closer look at the evidence

The death was self-inflicted

An unidentified perpetrator

Wayne Property owner

Hamid The friend

Who do you think killed the victim?

From what you’ve seen of the suspects and the evidence so far, what’s your working hypothesis on the case?

Your working hypothesis...

Return to crime scene

Select your tests

You need to decide which tests to carry out in order to either charge a suspect or release both suspects. In order to charge a suspect with murder, you need to provide evidence that links them to the murder weapon. In an ideal world, you would carry out tests on every piece of potential evidence at the scene in order to put together a clear case. In reality, these tests cost money and take time, so you need to decide which are the most appropriate tests to conduct based on your analysis of the crime scene. Which combination do you think would be within time and budget, and provide enough evidence to charge a suspect?

Which tests will you select?

• Autopsy on body • DNA test on wine bottle • Fingerprints on wine bottle • DNA test on bloody knife • Fingerprints on bloody knife • DNA test on pint glass • Fingerprints on pint glass • Shoeprint analysis • Hair analysis on sleeve • Fibre analysis on chest

• Prints on wine bottle • Prints on pint glass • Prints on bloody knife • Prints on steak knife

• Autopsy on the body • DNA test on wine bottle • Fingerprints on wine bottle • DNA test on bloody knife • Fingerprints on bloody knife • DNA test on pint glass • Fingerprints on pint glass

• Shoeprint analysis • Hair analysis on sleeve • Fibre analysis on chest • Prints on bloody knife • Prints on wine bottle • Prints on pint glass

• Autopsy on the body • DNA test on wine bottle • Fingerprints on wine bottle

Pint glass

Wine bottle

Knife

Autopsy

Take a look at what the forensic evidence shows and see how it fits with the suspect’s information. Does it support or contradict your working hypothesis?

Your results are back from the lab.

Wayne Owner of property

Hamid Victim's friend

Neither

Now you have the forensic evidence, would you charge either of the suspects?

Who will you charge?

A good prosecutor will always present the evidence in a logical narrative to help the jury understand and remember it. The prosecutor will begin by introducing the crime explaining how the body was found and the police alerted. Then they would present the forensic evidence.

This is how the prosecutor is likely to present their case against Wayne:

At Wayne’s murder trial, the prosecutor’s job is to combine the forensic evidence with a plausible motive and timeline. The aim is to paint a picture that convinces the jury that Wayne is guilty, beyond reasonable doubt.

The case presented to the jury

The autopsy: This is presented as the first piece of forensic evidence to establish the time and cause of death and includes the medical examiner’s opinion about whether any of the potential weapons found at the crime scene could have caused it. In this case the fragment of glass found inside the wound is important as it suggests that the wound was caused by a glass weapon, rather than another item such as a knife. The medical examiner’s opinion would likely include ruling out the broken pint glass, as the injury was too deep to have been caused by it, but not being able to rule out the broken bottle, as the injury was consistent with a glass shard of that size. The Forensic Analyst’s report: This is presented...

next to show that Tommy’s blood was on the bottle, further suggesting that it was the cause of the fatal injury. This is likely to be done through the DNA match to Tommy, rather than the blood type match, because DNA is better for identifying individuals. The prosecution might also present the results of the analysis of the broken pint glass, showing that the blood on it was not Tommy’s, so it could not have been the murder weapon. Linking the evidence to the suspect: Having provided evidence that the broken bottle was the murder weapon, the prosecution would then try to link it to Wayne. The fingerprints on the bottle came back as a match to Wayne, and there were no other prints found on it, which suggests that Wayne handled the bottle.

In an adversarial legal system, such as in the UK, the defence does not have to prove that the defendant is innocent. The defence only has to raise reasonable doubt about the prosecution’s case in the minds of the jury. Sometimes this involves telling an alternative ‘story’ to the jury about what happened, while other times it can involve ‘picking holes’ in the prosecution’s ‘story’, or a combination of both. Without knowing what Wayne told the police...

This is how the defence might try to raise reasonable doubt in the prosecution case:

or his defence team, about what happened on the day of the murder, we can only guess what his defence might be:

• It is possible that Wayne picked up the wine bottle when he went into the flat and dropped it again before leaving (as he claimed he did) in panic.
• Wayne’s prints on the bottle do not prove that he stabbed Tommy with it – at most they show that Wayne at some point held the bottle.
• It might even be that the wine bottle is not the murder weapon after all – perhaps the defence might argue that Tommy bled onto the wine bottle after being injured by another weapon that was not found at the scene
• It is also possible that Wayne has an alibi for the time of death, which the defence...

The jury's desicion

• would enter into evidence and the prosecution would try to falsify.
• Another possibility is that Wayne would claim to have been attacked by Tommy and injured him in self-defence – although then the prosecution might question why he didn’t explain this to the police at the time.

An important feature of juries is that they are made up of members of the general public, selected randomly as a, hopefully, representative sample of society. This means that jurors are not experts in either law or forensic science (or, indeed, the psychology of jury decision-making!).

A jury might interpret the evidence in Wayne’s case as follows.

Given that the jury isn’t trained in the science of forensics, there is a risk that they interpret the evidence inaccurately.

The jury’s job is to decide whether Wayne is guilty or not guilty.

they bring a common-sense understanding, aided by experts during the trial. For example, the medical examiner might attend court and explain the autopsy results, and attempt to make sense of the story that the evidence tells. Psychological research has found that juries have a high degree of trust in DNA evidence, considering it the ‘highest’ form of evidence there could be. However, DNA evidence is not simple, as it can vary in quality depending on:

• The nature of the sample

• The type of testing
• The probabilities associated with an inclusionary (e.g. ‘this matches the suspect’) versus an exclusionary (e.g. ‘this does not match the suspect’) result

Jurors may or may not be able to accurately weigh up strong and weak DNA evidence, and there is some research showing that their ability to do so can depend on which side of the case presents the evidence. In our case, the DNA evidence served to identify that blood on a potential murder weapon (the broken bottle) came from the victim. It also linked blood on the pint glass to one of the suspects, although the autopsy report and the absence of the victim’s blood on that item ruled it out as the murder weapon. In this case, the DNA was not used to identify a suspect. However, when DNA is used to identify a suspect, who then becomes the defendant in court, jurors are very likely to infer that the defendant is guilty, even if the DNA evidence has limitations.

Jurors are also strongly influenced by other physical evidence. This includes weapon identification, as well as non-individuating physical traces at the crime scene, such as shoeprints. In some studies, physical evidence has been found to be even more persuasive than eyewitness evidence. For example in one mock-jury study, researchers found that their jurors convicted in 84.1% of cases with physical evidence compared to 67.4% of cases with eyewitness evidence – and weak physical evidence was almost as persuasive as strong eyewitness evidence (38.6% compared to 43.5% convictions, respectively). You will have noticed that our case relied quite heavily on fingerprint evidence, especially the matching of the prints on the murder weapon to the suspect Wayne.

Before DNA analysis was developed, fingerprints were seen as the ‘gold standard’ of forensic evidence, and had been since the early 20th century. However, you may be surprised to know that fingerprint matches are not 100% reliable. They are dependent on the skills of the analyst, and are prone to human error. Worryingly, psychologists have found that a fingerprint expert's decision about whether two prints match or not can be influenced by whether they believe the print belongs to a suspect or not before even looking at it. Experts may also change their mind about a match / no match decision when asked for a second opinion – even based on the exact same prints. Despite this, jurors are likely to treat fingerprint evidence as being very persuasive if it is presented in the case they are serving on.

Some psychological factors in jury decision-making The increasing influence of forensic evidence over the last few decades has given rise to interest in what is known as the ‘CSI Effect’ in legal and psychological circles. Jurors are generally not experts in forensic science. Most people’s formal science education does not go beyond

That should not be a major problem if a fingerprint match is just one piece of evidence, supported by lots of other evidence to build a case against, or exonerate, the defendant. However, in our case, it is the only piece of forensic evidence connecting Wayne directly to Tommy’s murder – and he had a legitimate reason to have been at the crime scene both on the day of the murder and previously.

what is taught at school, and their expectations of what forensic science can do are heavily influenced by representations in both factual and fictional media. Put simply, people might expect that real forensic science is like what they see on TV – which, in the case of a documentary such as 'Forensics: The Real CSI' is accurate but in the case of a drama such as ‘CSI’ is not. Quite how the ‘CSI Effect’ might affect juries is, at the moment, still very much an open question. It might make jurors think that forensic evidence is stronger than it really is (because in the programmes the evidence is always extremely strong) and therefore have a bias towards conviction. Alternatively, it might make juries expect lots of extremely strong evidence and therefore be disappointed when the evidence in a real case is not so water-tight and therefore have a bias towards acquittal.

what is taught at school, and their expectations of what forensic science can do are heavily influenced by representations in both factual and fictional media. Put simply, people might expect that real forensic science is like what they see on TV – which, in the case of a documentary such as 'Forensics: The Real CSI' is accurate but in the case of a drama such as ‘CSI’ is not. Quite how the ‘CSI Effect’ might affect juries is, at the moment, still very much an open question. It might make jurors think that forensic evidence is stronger than it really is (because in the programmes the evidence is always extremely strong) and therefore have a bias towards conviction. Alternatively, it might make juries expect lots of extremely strong evidence and therefore be disappointed when the evidence in a real case is not so water-tight and therefore have a bias towards acquittal.

For example, in our case of Tommy’s murder, there was no DNA from the defendant, Wayne, so perhaps a jury would find the prosecution’s evidence lacking. Finally, we need to consider the cognitive and social processes that are involved in jury decisions. As an example of a cognitive process, remember that you were asked early in the interactive to give a ‘working hypothesis’? Jurors are also likely to have a similar idea of what they think probably happened when hearing evidence in a case. Psychologists have found that this can bias the perception of evidence, with jurors putting more emphasis on evidence that confirms their working hypothesis and less emphasis on evidence that goes against it. This is an example of a normal human cognitive process known as ‘confirmation bias’.

Summary

As an example of a social process, remember that a jury is a group of people, who have to discuss the evidence to reach a collective decision. This sort of collective decision-making can be influenced by a number of factors, including social status and persuasiveness, as well as demographic and socio-economic factors.

Crime Scene Officers have a lot of responsibility and a lot of potential reward. If they successfully gather incriminatory evidence, they could prevent a dangerous criminal from committing further crimes. As you’ve seen through this activity, it involves a careful balance of forensic processes and the psychology of decision making. Successful Crime Scene Officers need to navigate:

Take it further

If you would like to follow up some of the research referred to in this interactive, and have access to an academic library, you may find these papers useful.

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Useful reading

Carlson, K.A. & Russo, J.E. (2001). Biased interpretation of evidence by mock jurors. Journal of Experimental Psychology: Applied, vol. 7, iss. 2, pp. 91-103.Dror, I.E., Charlton, D. & Peron, A.E., (2000). Contextual information renders experts vulnerable to making erroneous identifications. Forensic Science International, vol. 156, iss. 1, pp. 74-78.Maeder, E.M., Ewanation, L.A. & Monnink, J. (2017). Jurors’ perceptions of evidence: The relative influence of DNA and eyewitness testimony when presented by opposing parties. Journal of Police and Criminal Psychology, vol. 32, pp. 33-42.Mancini, D.E. (2013). The “CSI Effect” in an actual juror sample: Why crime show genre may matter. North American Journal of Psychology, vol. 15, iss. 3, pp. 543-564. Pennington, N. & Hastie, R. (1992). Explaining the evidence: Tests of the Story Model for juror decision making. Journal of Personality and Social Psychology, vol. 62, iss. 2, pp. 189-206.

Rodriguez. L., Agtarap, S., Boals, A., Kearns, N.T. & Bedford, L. (2018). Making a biased jury decision: Using the Steven Avery murder case to investigate potential influences in jury decision-making. Psychology of popular Media Culture, Advance online publicationhttp://dx.doi.org/10.1037/ppm0000192 Sarapin, S. H. (2012). The effects of crime drama TV viewing on mock juror self-efficacy, adherence to judicial admonitions, verdicts, and verdict certainty in a murder trial. (Unpublished doctoral dissertation). Purdue University, Indiana. Skolnick, P. & Shaw J.I. (2001). A comparison of eyewitness and physical evidence on mock-juror decision making. Criminal Justice and Behavior, vol. 28, iss. 5, pp. 614-630. Stinson, V., Patry, M. W. & Smith, S. M. (2007). The CSI Effect: reflections from police and forensic investigators. The Canadian Journal of Police and Security Services, vol. 5, iss. 3, pp. 1-9.

The autopsy

Time of death: Approximately 16.00. Neighbours called Police to the scene at 16.22. Cause of death: Severed cartoid artery in the neck. The wound was 15cm long and 5-6 cm deep. Likely weapon: A sharp, jagged, irregular object. There was a small fragment of glass in the wound, so the weapon is likely to be a piece of glass of at least 6cm in length.

The wine bottle

Fingerprints: All fingerprints on the wine bottle matched Wayne. Blood type: Type A. The victim has Type A blood. DNA: The blood DNA matches the victim.

1 test is available

Print matching could help us identify a shoe - and therefore potentially a person - that was at the scene. It won’t tell us whether that person committed the murder.”

Shoe print

“There’s a fairly clear bloody shoe print on the lino floor here. We’ve taken photographs and measurements.

3 tests are available

large piece is theneck of a wine bottle with a sharp protruding shard, about 7cm long, of the bottle's body still attached, and has blood

covering a lot of it. We’ve measured the glass, swabbed the blood and dusted the clean part for fingerprints. The blood and fingerprints could be linked to people or exclude people. Absence of blood or fingerprints does not mean that no-one else has handled the glass.”

Broken wine bottle

“There’s a fair amount of broken glass at the scene, this

Both suspects have shallow cuts to their arms and hands. They have had fingerprints, DNA and blood samples taken and the police have taken possession of the clothes they were wearing as potential evidence.

In budget? Yes Potential to charge a suspect? No

This is a good selection of tests, but it does not explain the crime scene entirely. It does not explain the other potential murder weapons, leaving lots of unanswered questions. Please reconsider what other tests you may need.Hint: Testing more pieces of evidence would give you a broader understanding of the scene.

In budget? Yes Potential to charge a suspect? No

These tests would provide a lot of information, but not enough to charge a suspect. At most, you would be able to say whether one or both of the suspects were present at the crime scene and in close contact with the victim, but that’s not enough to incriminate them. In fact, both of them already admit to being there. Hint: An autopsy would help you understand the cause of death and likely weapon.

1 test is available

Autoposy on the body

“There’s a large gash in the victim’s neck. An autopsy would help us understand the cause of death, information about the weapon used (if any),and the time of death. However, it will not provide direct evidence of who might have killed the victim, if he was indeed murdered.”

Continue

Thank you, we'll revisit your hypothesis later...

Interested in the psychology involved in criminal cases? Or perhaps the science behind the analysis? OpenLearn provides free courses where you can expand your knowledge. Look out for the links at the end of this interactive.

In budget? No Potential to charge a suspect? Yes

You have done all the tests needed to work out what was going on in the crime scene, but you are over-budget. Hint: Have a look to see if there is any tests which aren’t necessary to reach the same conclusions yet come in on budget.

1 test is available

Fibers on chest

“We found some fibres on the chest area of the victim’s top. They could be polyester, but they’d need testing.This will tell us if fibres from a particular source have come into contact with where they were found. They don’t tell us how they got there, but could be linked to clothing belonging to a suspect, for example.”

Hamid Victim's friend

“I went round to see Tommy to see if he could pay me back some money I lent him. I finished work early and went over. When I got there and knocked there was no answer but the door wasn’t locked. I went in and I saw him dead on the floor!I freaked out and ran - I didn’t want to be in that place with his body, and I was worried whoever did it might still be in the house. I know I should have called the police, but I had some blow on me and I wanted to get rid of it before talking to you. Then you showed up at my flat before I got a chance to call you.”

In budget? Yes Potential to charge a suspect? No

It’s sensible to assess all the weapons but without knowing which caused the fatal injury, the prints won’t help you identify the murderer. At this early stage, you’ll need to focus on identifying the weapon and putting it in someone’s hands. Hint: An autopsy would help you understand the cause of death and likely weapon.

1 test is available

Hair on sleeve

“This looks like hair - it was found on the sleeve of the victim’s top. It’s difficult to tellfrom sight whetherit’s his hair orsomeone else's.Testing the hair will tell us whether it’s human hair and, if so, whose it is if their DNA is in the national database.”

3 tests are available

Broken pint glass

“This is a broken pint glass with a sharp protruding shard about 4cm long. It has quite a lot of blood on it. We’ve measured it, swabbed the blood and dusted the clear part for fingerprints.The blood and fingerprints could be linked to people or exclude people. Absence of blood or fingerprints does not mean that no-one else has handled the glass.”

The pint glass

Fingerprints: All fingerprints on the pint glass matched Hamid Blood type: Type O. Both suspects have Type O blood. DNA: The blood DNA matches Hamid.

3 tests are available

There’s no visible sign of blood on the knife.

We expect there will be prints on the handle, so we’ve dusted for those. The fingerprint tests will show who handled the knife, not necessarily whether that knife was the murder weapon.”

A knife with no blood...

“This steak knife was found on the floor near the victim.

Both suspects have shallow cuts to their arms and hands. They have had fingerprints, DNA and blood samples taken and the police have taken possession of the clothes they were wearing as potential evidence.

Wayne Owner of property

''I normally rent out this flat, but it’s been vacant for a couple of months. One of the neighbours called to tell me she suspected squatters were in there, so I came down to see what was going on. When I opened the door, I saw the body.If I’m honest, I panicked! I ran back out of the flat and back to my car. I wasn’t sure what I should do, so I went home to calm down first. Of course I was going to call the police, but then you called me before I called you.”

The knife

Fingerprints: All fingerprints on the knife matched the victim. Blood type: Type O. Both suspects have Type O blood. The victim has Type A blood. DNA: No result. The blood DNA could not be identified as it was from more than one person.

3 tests are available

Bloody knife

“This is a domestic knife with blood on the blade. It was found on thefloor near the victim.We’ve swabbed the blood on the knife anddusted the handle for prints.Tests on the blood could show whose blood is on the knife. DNA will identify them if their blood is in the DNA database, or blood type testing can be used to exclude people. For instance, if the victim is a type A blood and the blood found is type O, the blood cannot be from the victim. Fingerprint matching will show who handled the knife, not necessarily who committed the murder.”

There’s no evidence at this stage to incriminate Hamid.

The fingerprints on the most likely murder weapon - the wine bottle - do not belong to Hamid.Hamid can not be held without charge for very long. As there is no evidence against him, he will be released without charge. Try another option.

You're not 100% sure that either suspect committed murder.

This is true; there’s more testing to be done to rule out other options, but you do have significant incriminatory evidence against Wayne. His fingerprints are on the wine bottle, which the autopsy identified as the most likely murder weapon, which is enough to charge him with murder.As the owner of the property the victim was squatting in, he also has a plausible motive for aggression. Once Wayne has been charged, you will have more time and further budget to carry out more tests and investigation. Criminal prosecutors will then tie the evidence together into a narrative to present to a jury in Wayne’s murder trial.

Fast forward to trail

Spot on; the forensic analysis provides incriminatory evidence against Wayne.

His fingerprints are on the wine bottle, which the autopsy suggests is the most likely murder weapon. This is enough to charge him with murder.As the owner of the property the victim was squatting in, he also has a plausible motive for aggression. Once Wayne has been charged, you will have more time and further budget to carry out more tests and investigation. Criminal prosecutors will then tie the evidence together into a narrative to present to a jury in Wayne’s murder trial.

Fast forward to trail

In budget? Yes Potential to charge a suspect? Yes

Good decision. Although you have done more tests than you strictly need to identify the murder weapon, the additional tests have explained further evidence at the crime scene, which means you have plausible explanations as well as a suspect. Money is tight, but these tests just about come in on budget.

Budget

Forensic analysis is expensive. In a case of murder, the resources are usually sufficient for multiple analyses, however there are constraints on forensic science budgets, and tests take time to complete, so in reality there does need to be some prioritising of these resources. In lower-priority cases, such as burglary or car theft, the budget and staffing may simply not be available to process every potential piece of evidence.In all cases, though, the CSIs and detectives need to make careful decisions and try to set aside their pre-conceptions, biases and ideas about who might have done what, in order to follow the evidence.

Time

Often, time is against the police, who are under pressure to be able to charge or release a suspect or to identify an unknown perpetrator.When the police have a suspect in custody, they can hold them for only a limited time before charging them; if there is insufficient evidence, they must be released. If there is no suspect in custody, then identifying the perpetrator quickly is important so they don’t abscond, destroy evidence or escape justice.

Jury

If a case gets to court, a jury will need to follow the evidence in a similar way to the Crime Scene Office - but without the same scientific or investigative training. This can lead juries to think that the evidence is more or less persuasive than it actually is, so when giving evidence in court, the Crime Scene Officers and detectives need to clearly explain both the limitations and the strengths of the evidence.

• Discover how limitations of the human brain have lead to miscarriages of justice in our FREE course 'Forensic Psychology'.

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