Chapter 4
The Psychology of Forensic Identification:
DNA, Fingerprints, and Physical Trace Evidence
LEARNING OBJECTIVES
After reading this chapter you should:
• understand that forensic identification is a comparative and probabilistic process.
• be able to explain the difference between inclusion and individuation.
• know the four ways in which forensic experts communicate information to juries.
• understand the difference between reliability and validity.
• understand the fundamental way DNA is constructed.
• understand the basis for random match probabilities.
• know that there are no absolute standards for matches using forensic evidence.
• understand that human biases are present in evaluating forensic evidence.
• know the basic patterns used by fingerprint experts to identify matches.
• understand the process of blind identification.
SUMMARY
Madrid Subway bomb case is a good example of how physical
evidence are not immune to the biases that shape the
psychological processes of perception, judgment, and decisionmaking.
Forensic Science is defined as the collection, analysis, and
interpretation of physical evidence. Forensic Identification is the
process of linking a piece of physical trace evidence to an
individual, usually a criminal suspect.
The 1st first technique of identification that was considered
scientific was called anthropometry, devised by Alphonse
Bertillon. He developed something called Bertillon measurements
and pioneered a field called biometrics. It is the id of an individual
based on measureable anatomical traits.
Identification rests on a comparison between two samples. 1st
may include trace evidence LEFT at the scene or TRANSPORTED
from the crime scene and the 2nd are samples taken from a
suspect or tool USED by the suspect. Scientists may say that they
are a “match” or have an “inclusion”. A stronger term,
INDIVIDUATION is sometimes used to express the conclusion hat
a trace found at the crime scene came from this source to the
exclusion of all other sources in the world.
Scientists look for differences between Class Characteristics and
Individual Characteristics. Class characteristics are features that
are common to a general class or category of objects (tread on a
particular pair of Nike running shoes), whereas Individual
characteristics are unique to a particular object (wear patterns on
the tread of the shoes).
Different types of forensic data are used for forensic
identification, primarily fingerprint and DNA evidence. Two
samples are compared--samples taken from a suspect and trace
evidence left at a crime. If they match, a source attribution can
be made. Examiners determine inclusion of an individual as a
suspect based on sample similarity. If all other sources can be
excluded, individuation has been achieved.
There are four ways in which an expert might communicate these
results to a jury:
Using a qualitative statement (how strong is the match);
using a simple match statement (the evidence is consistent with
the suspect's guilt);
using a match plus statistics statement (a statement like the
above with statistics to back it up);
and using an individuation statement (the match perfectly
identifies the suspect).
Guidelines allow that an expert giving testimony may only
provide one of three conclusions: inconclusive results, exclusion
of the suspect, or individuation. This unfortunately leads jurors to
believe that fingerprint evidence is absolute and gives no
information about possible false positive rates.
To be considered scientifically meaningful, a measure must
possess both reliability and validity. Reliability is the consistency
of repeatability of a measure or observation. Validity refers to
whether or not a technique measures what it is supposed to.
Interrater reliability is the degree to which two or more observers
or analysts independently arrive at the same measurement.
For evidence to be scientifically meaningful, it should be reliable
(consistent) and valid (measure what it says). DNA evidence, if it
is available at the crime scene, can be used to identify an
individual by comparing the genetic profile found at the scene to
that of a suspect. Sometimes there is a clear match, but when
the DNA is incomplete, poorly collected, or poorly stored the
results can be ambiguous. Thus, this evidence is usually
accompanied by a probability statement (a random match
probability) regarding how likely it is that the match occurred by
chance.
A different, less objective, type of evidence is fingerprints whose
patterns are described as a series of loops, whorls, and arches.
Despite popular culture claiming no two fingerprints are alike, no
evidence exists to support this claim. In addition, the processes
used to compare fingerprints from the crime scene (latent prints)
and the suspect are not standardized; individual bias may enter
the matching process if the examiner believes the prints are from
a guilty suspect. This subjectivity also comes into play when bite
marks or bullet striations are being compared. All of these
comparative analyses require that there be a known, captured,
suspect or weapon for the comparisons to take place.
DNA identification is the most objective form of identification
evidence. In 1988, DNA was introduced as evidence in US Courts.
The largest, most frequently used DNA database in the world is
the Combined DNA Index Sytem maintained by the FBI.
Know pages 83-84 and the extra material on DNA
Sir Francis Galton was the early pioneer in the field of
psychometrics, which led to fingerprinting. He was one o fthe first
to advocate the idea that individual abilities and psychological
traints can be objectively measured and the results of the
measures can be quantified.
Know the difference types of fingerprints patterns, page 85. Is it
true to say that NO two fingerprints are identical.
Confirmation bias is still a problem. It is the tendency to search
out evidence that confirm our beliefs and ignore evidence that
contradicts them. Measures have been suggested for eliminating
some of the bias inherent in the matching process. These include
proficiency testing and certification. During testing, analysts are
given a sample and asked to make a judgment about a match.
They are blind to the source of the sample and cannot be biased
by knowing who the “suspect” is. Since the samples are obtained
from known individuals, their answers can easily be determined
to be accurate or inaccurate. When jurors are presented with DNA
evidence they tend to undervalue the accompanying probability,
which gives them more faith in the evidence than is warranted.
The Psychology of Forensic Identification
Chapter 4 Questions
1. How often are forensic dentists correct in identification?
2. How do we reduce error and bias?
3. What is the difference between single blind and double
blind?
4. Is DNA a better form of forensic identification evidence than
fingerprints? Why or Why not?
5. Fingerprint evidence is admissible in court. Should it be?
Why?
6. How can we make forensic identification evidence more
useful to the jurors who much evaluate it?
7. Why was the fingerprint of Brandon Mayfield mistakenly
identified as the fingerprint of the Madrid bomber? How
might this mistaken identification have been avoided or
detected earlier?
8. What is meant by the “scientific foundation” of trace
evidence?
9. What is Individuation testimony?
10.
What type of education do most Forensic Scientists
have, according to your textbook?
USE THESE VOCABULARY WORDS IN A SENTENCE:
1. STRIATIONS
2. CSI Effect
3. Psychometrics
4. Source Attribution
5. Interrater reliability
6. Alleles
7. Anthropometry
8. Minutiae
9. Trace evidence
10. Qualitative statements