The collection and processing of forensic evidence (Biological)

Introduction to Forensic Evidence

When we think of forensic science, we often imagine high-tech labs and 100% certainty. However, the collection and processing of forensic evidence is done by humans, and humans can be influenced by their surroundings. In this chapter, we explore the biological and psychological factors that can lead to bias in forensic work, focusing specifically on how emotions and motivations might accidentally change the way an expert "sees" a fingerprint.

Don't worry if this seems a bit technical at first! We are essentially looking at one big question: Can a fingerprint expert stay objective even when they know the crime was particularly "nasty"?

1. Background: Motivating Factors and Bias

Forensic experts are highly trained professionals, but they are still subject to cognitive biases. These are mental shortcuts or "glitches" that can happen when our brains process information.

What is Bias in Forensics?

Bias happens when an expert's judgment is influenced by things outside the actual evidence. There are two main types to know:

• Expectancy Bias: This is when an expert expects to find a match because they have been told other evidence points to the suspect. They "see" what they expect to see.
• Emotional Context: This is when the details of the crime (e.g., a very violent murder versus a minor theft) create an emotional state in the expert that might push them toward making a decision.

Motivating Factors

Experts are often motivated by a desire to "solve" the crime or provide justice for a victim. While this sounds like a good thing, it can create a "need-satisfaction" bias. The expert wants to be helpful to the police, which might unconsciously lead them to be less cautious when looking at a blurry or poor-quality fingerprint.

Quick Review: Bias isn't about being "bad" at the job; it's about how the human brain naturally reacts to pressure and emotion.

2. Key Research: Hall and Player (2008)

This is the essential study you need to know for this chapter. Hall and Player wanted to see if the emotional context of a crime would actually bias the decisions of fingerprint experts.

The Aim

To test whether fingerprint experts were affected by the emotional context of a case and whether this context would make them more likely to say a poor-quality print matched a suspect.

The Methodology

• Participants: 70 fingerprint experts working for the Metropolitan Police. Their experience ranged from less than three months to over 30 years.
• The Task: They were given a latent fingerprint (one that is hidden or poor quality) on a \(£50\) note. They were also given a "tenprint" card (clear prints from a suspect) to compare it to.
• The Manipulation: The researchers made the print intentionally "ambiguous" (blurry/poor) so that it wasn't an obvious match.

Two Conditions (Independent Measures Design)

1. Low-Emotional Context: Experts were told the print was from a suspect in a forgery case (passing a fake note). This is a relatively minor, non-violent crime.
2. High-Emotional Context: Experts were told the print was from a suspect in a murder case, where a gunman had shot a victim and escaped.

The Results

• Emotional Impact: 52% of the experts in the high-context group said they felt affected by the information they read, compared to only 6% in the low-context group.
• The Final Decision: Surprisingly, despite feeling more emotional, the experts in the high-context group were no more likely to identify the print as a match than those in the low-context group.
• The Consensus: There was no significant difference in the final "match" or "no match" decisions between the two groups.

Conclusion

Hall and Player concluded that fingerprint experts are emotionally resilient. Even though they feel the pressure of a high-stakes murder case, their professional training allows them to remain objective when making the final identification. In short: the emotional context didn't actually lead to more mistakes.

Key Takeaway: Professionalism and experience can act as a shield against the bias caused by high-emotion crime stories.

3. Application: Strategies for Reducing Bias

Even though Hall and Player found that experts performed well, the legal system still wants to reduce the risk of bias as much as possible. Here are strategies used to keep forensic evidence "clean":

Strategy A: "Blind" Testing

The simplest way to stop an expert from being biased by the "story" of a crime is to not tell them the story! In blind testing, the expert is given the prints without any case notes. They don't know if it's a murder or a shoplifting case, so their brain can't be influenced by emotional context.

Strategy B: Filler Control (The "Lineup" Method)

Normally, an expert is given one suspect's print and one crime scene print. This creates a \(50/50\) chance of a match and puts pressure on them. A better strategy is to give the expert six different prints (one suspect and five "fillers" who look similar). This forces the expert to be much more precise and reduces the chance of a "lazy" match.

Strategy C: Linear Sequential Unmasking (LSU)

This is a fancy term for a simple process: the expert must analyze the crime scene print first and write down everything they see before they ever look at the suspect's print. This stops them from accidentally "finding" details in the crime scene print just because they saw them on the suspect's clear print.

4. Summary Table for Revision

Use this table to quickly check your knowledge before an exam:

Common Mistake to Avoid: Don't assume Hall and Player found that experts are biased. Their study actually showed the opposite—that the experts were very professional and didn't let the "murder" story change their technical results.

Final Quick Check

• Can you define expectancy bias?
• What was the difference between the "Low Context" and "High Context" groups in Hall and Player’s study?
• Why is it better to analyze the evidence before seeing the suspect?

If you can answer these, you're well on your way to mastering this chapter of Criminal Psychology!