The Nature of
Evidence
What is Evidence?
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Evidence is information in the form of:
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Personal testimony (testimonial evidence)
Text of documents
Material objects
Used to make a fact or proposition more or
less likely
“Trier-of-fact” is whoever determines guilt
or innocence (jury or judge)
Forensics as Historical Science
Forensics
Archaeology
Geology
Time Frame
Hours, days,
months
Hundreds to
thousands of
years
Millions of years
Activity Level
Personal;
individual
Social;
populations
Global
Proxy Data
Mass-produced
Hand-made
Natural
Earliest record of physical evidence
Archimedes (287-212
BC) shows that a
crown give to the king
was made of a goldsilver allow rather than
pure gold
Common types of physical evidence
(name some)
drugs, toxic substances
paints
gunshot or explosive residues
firearms and ammunition
shoeprints
petroleum products
alcohols
rubber material
resins, plastics
serial numbers
documents
tool marks
fibers, hair
soil
glass
blood
bite marks
fingerprints
body tissue
pollen
wood
feathers
bones
body fluids
Real vs demonstrative evidence
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Real evidence is generated as part of the
crime; recovered from the victim, suspect,
or place the crime was committed
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Hair, blood, paint, footprints, bullets
Demonstrative evidence was not generated
directly but was created later to explain the
significance of real evidence
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Charts, computer simulations, DNA testing
methods
Varieties of evidence (see handout)
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Circumstantial
Conclusive
Conflicting
Corroborating
Derivative
Exculpatory
Foundational
Hearsay
Incriminating
Presumptive
Prima facie
Probative
Rebuttal
Tainted
Why do we say that forensic science is
an historical science?
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Events that occurred
are already in the past
We only see proxy
data (remnants of the
event)
Similar to other
historical sciences
Transfer and Persistence
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A central guiding theory of forensic science
Developed by Edmund Locard (early 20th
century)
An exchange of information occurs even if
it is too small to be found
All evidence is transfer evidence
Locard’s Principle
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Every contact produces some level of
exchange
Any transfer that occurs after the crime is
considered contamination
What are some of the conditions
that affect transfer?
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Pressure
Number of contacts
Ease of transfer
Form of evidence
How much transferred
Direct vs indirect (you will understand this
intuitively)
Persistence: how long evidence
remains
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Depends on:
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What it is
Location
Environment
Time
Activity around site of
collection
www.uiowa.edu/.../drybones/images/2-9.jpg
Identification
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Chemical and physical properties of the
object or material
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Is this white powder cocaine or talc?
Determining that a colored chip is automotive
paint
The hair comes from a cocker spaniel
Identification allows the object to be
classified – class evidence
Two objects sharing a class identity
may come from a common source
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Fibers on a body and
fibers in the suspect’s
car
Paint flecks on the
victim(s) and paint
used by suspect in his
work (Green River
killer)
Read article about the
case
The best evidence is individual evidence
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Anything that can be
linked to a unique
source, i.e. a class
with one member
Includes fingerprints,
DNA patterns,
handwriting
Can be physical
matches (toenail in the
1st CSI episode)
The forensic scientists main goal is to find
a unique source of the evidence
Or, common feature in unique combination
Unique fingerprint
Relationship and Context
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Relationships between people and places
are critical to determining the relevance of
evidence
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Example: Co-habitation creates unavoidable
transfers of some types of evidence
Coincidental association – stranger-on-stranger
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Studies show this is very rare because of huge
variety of goods and consumer choices
Direct vs Indirect evidence
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Prima facie evidence is direct evidence – “I
saw him steal the tomato.”
Most evidence does not prove a fact and is
called indirect evidence
Some physical evidence proves something
(person has possession of a drug or an
elevated blood alcohol level) but may not
necessarily prove guilt or innocence
Circumstantial evidence implies a fact or
event without proving it
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The more circumstantial evidence there is
the greater weight it carries
Probability and statistics are important
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Example: blond hair in dark-haired victim’s hand,
footprint of size 11 running shoe near body
Suspect is blond and wears size 11
The case against suspect is stronger than if only
one or the other piece of evidence existed
Probative means supplying proof
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Finding many different types of class
evidence linking suspect to crime or victim
increases the probative value of the
evidence
Probative evidence need not always be
individual evidence
Probative evidence can prove or disprove a
hypothesis
Things you should be able to do
1.
2.
3.
4.
State Locard’s Principle
Describe what is meant by physical
evidence and give examples
Tell individual evidence apart from class
evidence
Determine the significance of class
evidence