Crime-Topic 2 (collection+processing of forensic evidence)

• Crime Scene Analyst


  To decide whether ridge details on a suspect's fingers match those on fingermarks collected from crime scene
• Motivating Factors Bias


  • Conformity Effect
  • Need-determination Perception
  • Overconfidence Bias
  • Expectancy Bias
• Conformity Effect


  Expect won't challenge previous decision if asked to agree/disagree
• Need-determination Perception


  If there is strong desire to solve crime, experts may say they've found match for FP even if more accurate outcome is that they can't solve it
• Overconfidence Bias


  Experts who are overconfident may believe they're always right even if others disagree
• Expectancy Bias


  Experts anticipate a certain outcome
• Example Research


  • Expectancy bias - Dror et al.
  • FP experts (av 17yrs experience) were studied for 12 months as part of everyday work
  • Unfamiliar with Mayfield case so were less likely to show decision change
• Dror et al. study procedure

  1. Experts asked to examine pair of FP they had seen previously 5 years ago on a real case + recorded as a definite match

  2. Experts told they were wrongly identified + expectation was no match

  3. 419 experts changed decision that FP were matched

  4. 3 claimed definite mismatch (complete opposite of earlier decision)
• Intra-Observer Consistency


  Same experts make different decisions at different points in time
• Hall & Player study

  Aim was to investigate if written report of the crime would emotionally influence FP experts and if emotional context would bias judgement
• Hall & Player study sample

  • 70 volunteer FP experts
  • Met Police
  • Experience (mean 11yrs)
• Hall & Player study procedure


  1. Experts given routine crime scene, examiners report (optional to read)

  2. No time limit, could leave whenever but not discuss

  3. Half told FP belonged to someone who tried to pay for goods with £50 (forged)

  4. Other half told FP belonged to someone suspected of firing gunshots at a victim
• Hall & Player study results

  • 57/70 read crime scene
  • 52% of 30 in high emotional group said context affected their decision compared to 6% in low emotional group
  • No significant difference between 2 groups when came to final decisions
• Some experts chose not to read crime scene as it was not needed to establish a match, suggesting experts are capable of analysing FP in a detached objective manner
• Working in isolation - examiner should be unaware of crime scene details like confessions, meeting victims. Verifier should be blind to conclusions of examiner. This should prevent Conformity Effect, Expectancy Bias, Emotional Determination