Motivating factors and bias in evidence collection

• Name types of forensic evidence

  • Fingerprints
  • Blood splatters
  • Hair samples
  • Skin
  • DNA
  • Shoe prints
  • Ballistics
  • Bones
• What are patent fingerprints?

  • Visible to the naked eye
  • Often made in blood and paint
  • Easy collection as you can just take a photo
  • Less bias

  
• What are latent fingerprints?

  • Impressions of skins transferred to another surface through bodies natural oils and sweat
  • Mainly not visible so require use of specialist fingerprint powder, chemical reagents or alternative light sources
  • More biased as analysis is needed

  
• What is bottom up processing and how does it affect bias of fingerprint experts?


  • Data driven
  • What enters the eyes is translated by cognition
  • What you see is what you get
  • Fingerprint experts need to use bottom up by looking at specific ridges you can see
  • More objective and less open to bias
  • More on patent prints
• What is top down processing and how does it affect bias of fingerprint experts?


  • What enters through the eyes is translated first by our cognition and prior experience
  • If fingerprint experts do this it becomes subjective and open to bias because they have to look at the contextual information e.g. Emotional state of the expert
  • More on latent prints
• Brandon Mayfield - inaccuracy of fingerprints:


  • US citizen recently converted to Islam
  • Falsely accused of Madrid train bombing
  • FBI investigation
  • Latent fingerprint lifted from bag believed to belong to bomber
  • Analysed by multiple experts and matched Mayfield & was arrested
  • Fingerprint also matched to Algerian national
  • Mayfield hadn’t left US according to passport & was released and given full public apology from FBI
• What evidence shows forensic evidence can be inaccurate?

  Kassin:

  • Consider Mayfield case and other cases prove forensic science is not infallible

  Hampikian:

  • Called “the genetics of innocence” found several types of forensic testimony found to have convicted innocent individuals

  Found:

  • 38% incorrect blood analysis
  • 22% incorrect hair comparisons
  • 3% incorrect bite mark identification
  • 2% incorrect fingerprint identification
• What are examples of motivating factors?

  • Sense of rewards experts feel
  • Public interest
  • Child victim
  • Elderly / vulnerable victims
  • Emotional motivation
  • Cognitive closure
  • Desire for consensus (agreement)
• What is emotional motivation?


  • A strong desire to find offender e.g. child victim and if fingerprint expert also has a child
• EMOTIONAL MOTIVATION EVIDENCE: Charlton (emotion)


  • Investigated whether emotions in fingerprint examination influenced analysis
  • Qual approach - semi-structured interview on 13 experienced fingerprint experts & asked "How did you feel about succeeding in matching prints?"
  • Main themes associated:
  • Reward of job satisfaction including pride using their skill
  • Satisfaction associated with catching criminals
  • F.print experts emotionally motivated to achieve results for selves but also shows police & wider society influenced - erroneous conclusion depending on context & strong enough motivation
• EMOTIONAL MOTIVATION EVIDENCE - Dror (crime type)


  • If some crimes more motivating for fingerprint experts bc of associated emotional context - ambiguous more matches
  • Exp.
  • 27 volunteer uni students
  • IV - low emotional context (bike theft) & high (murder)
  • Photos & stories reinforce context
  • Ppts match 96 prints either unambiguous (bottom-up) or ambiguous (top-down) & more realistic to crime scene print
  • Desc. crime given on computer w/ photos before see matched print - pressed same or match
  • Unambiguous - unaffected by emotional context
  • Ambiguous - influenced by contextual info - top-down
• What is cognitive closure?

  • Need for definite conclusion
  • Fingerprint experts would look to make firm identification so their part of the investigation is complete
• COGNITIVE CLOSURE: Kruglanski


  • Found that when there is a higher need for cognitive closure quicker judgements are made with more confidence
  • When need is low a large number of possibilities are considered and there is better decision making
  • Suggests in high profile cases where there is strong motivation for cognitive closure use of contextual evidence such as case reports would create potential for faulty yet confident fingerprint identification - unfounded confidence paradox
  • This is shown in the Brandon Mayfield case
• BIASES AND FORENSIC EVIDENCE: Kahneman


  • Human decision making not nearly as rational as we would like to believe & can make systematic and unintentional errors in judgement
  • This is a cognitive bias and present in Mayfield case
• What is a contextual bias?

  • When irrelevant contextual information about event or the way information is presented influences reasoning
  • Fingerprint experts affected by this as given details of crime and background information about the suspect
  • Poor quality fingerprints means expert is more likely to use evidence if context of serious crime
  • Prior knowledge of suspect can also lead to focus on suspect
• What is a confirmation bias?

  • When people interpret information or look for new evidence in a way that confirms assumptions or pre-existing beliefs
  • Most requests for analysis are from police or prosecution services to solve case & secure prosecutions - leads to powerful cognitive effect on decisions
  • Requires verification process & verifier will have prior knowledge of original examiners findings and may expose experts to a risk of confirmation bias and a conformity bias