Medium
Crime & Forensics
Evidence & Investigation
What forensic techniques can establish, and what they cannot.
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Forensic science is best defined as what?
What a Crime Scene Reveals
Physical evidence is powerful and narrower than television suggests. Several long-trusted techniques have not survived serious review.
About this topic
Crime scene work rests on a simple idea, usually credited to Edmond Locard: two things that touch exchange material. Fibres transfer, soil travels on shoes, glass fragments lodge in clothing. Everything else — the sealing of a scene, the single controlled entry route, the elimination samples taken from officers — follows from taking that idea seriously in both directions, because investigators contaminate scenes as readily as offenders do. The harder question is what a trace proves. A technique can be genuinely useful for narrowing a field and useless for identifying one person, and forensic science spent much of the past two decades sorting its methods into those categories. Reviews by national scientific bodies have been blunt about which comparison disciplines had real statistical foundations and which had mostly examiner confidence, and several of these questions turn on that distinction.
Questions & answers
What a Crime Scene Reveals: all 10 questions and answers
Every question in this quiz is listed below with its correct answer and the reasoning behind it. Play first if you would rather not see the answers — or read through as a study sheet.
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1Locard's exchange principle states what?
- A A confession must always be corroborated
- B Every contact between two things leaves a trace in both directions Correct
- C No two people share the same fingerprint
- D Evidence degrades at a predictable fixed rate
Answer: B. Every contact between two things leaves a trace in both directions
Why: Edmond Locard set up one of the first police laboratories, in Lyon around 1910. It is better treated as a working assumption than a law, since plenty of contacts leave nothing that can be detected.
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2Luminol is used at a scene to do what?
- A Reveal traces of blood that have been cleaned away Correct
- B Lift latent fingerprints from glass
- C Detect accelerants in fire debris
- D Preserve soft tissue for transport
Answer: A. Reveal traces of blood that have been cleaned away
Why: It glows on contact with the iron in haemoglobin. It also reacts with bleach and certain metals, so a glow has to be confirmed by a more specific test.
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3Forensic entomology estimates time since death using what?
- A Body temperature alone
- B The contents of the stomach
- C The acidity of the surrounding soil
- D The insects colonising the body and the stage they have reached Correct
Answer: D. The insects colonising the body and the stage they have reached
Why: Species arrive in a fairly predictable succession. Because insect development tracks temperature, local weather records usually form part of the calculation.
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4Rigor mortis is what?
- A The settling of blood at the lowest points
- B Discolouration of skin caused by bruising
- C The temporary stiffening of the muscles after death Correct
- D The gradual cooling of the body
Answer: C. The temporary stiffening of the muscles after death
Why: It develops over hours and then passes off again. Warmth speeds the whole cycle, which is why it gives a rough window rather than a time.
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5Bite-mark comparison is now regarded how within forensic science?
- A Legally required in all assault cases
- B Largely discredited as a way of identifying a specific individual Correct
- C The most reliable identification method available
- D Reliable only when the victim survived
Answer: B. Largely discredited as a way of identifying a specific individual
Why: A US National Academy of Sciences review found no established basis for matching a mark to one person's teeth. A number of convictions that rested on it have since been overturned.
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6Forensic anthropology examines skeletal remains chiefly to establish what?
- A A biological profile such as age, sex, stature and signs of injury Correct
- B The person's occupation and income
- C The timing of their last meal
- D Whether the person was literate
Answer: A. A biological profile such as age, sex, stature and signs of injury
Why: Bone records healed fractures and long-term physical stress. Those marks sometimes match surviving medical records once soft tissue is gone.
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7A questioned document examiner compares what?
- A Bank statements for signs of fraud
- B Court transcripts against recordings
- C Metadata inside digital files
- D Handwriting, ink, paper and printing to test a document's origin Correct
Answer: D. Handwriting, ink, paper and printing to test a document's origin
Why: Machine-produced documents can be examined too. Printers and typewriters accumulate characteristic defects, and those defects change over time in traceable ways.
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8Why do investigators establish a single controlled route in and out of a scene?
- A To give the press a fixed vantage point
- B Because it costs less to secure
- C To limit contamination and keep a record of who entered Correct
- D To speed up the photography
Answer: C. To limit contamination and keep a record of who entered
Why: Everyone crossing a scene adds fibres, hairs and shoe marks of their own. Elimination samples are routinely taken from attending officers for the same reason.
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9Blood pattern analysis is used to infer what?
- A Whether the blood is human or animal
- B The likely positions and movements of people and objects when blood was shed Correct
- C The blood group of the person injured
- D How long ago the blood dried, to the minute
Answer: B. The likely positions and movements of people and objects when blood was shed
Why: Its narrower conclusions about direction and impact angle rest on straightforward physics. Its more sweeping courtroom claims have drawn sustained criticism from expert reviews.
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10Casting at a scene preserves which kind of evidence?
- A Three-dimensional impressions such as tyre tracks and shoe marks in soil Correct
- B Dried blood on fabric
- C Latent fingerprints on paper
- D Residue from a discharged firearm
Answer: A. Three-dimensional impressions such as tyre tracks and shoe marks in soil
Why: Dental stone poured into the impression captures wear and accidental damage. Those individual features are what separate one worn shoe from another of the same make and size.
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