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b.
Inference and Association
The weak and limited definitions of association found in the forensic litera-
ture reveal the mistaken notion that the step from individualization to an
inference of contact is inexorably easy and obvious. The reasoning behind
this belief stems from our experience with strong personal individualizing
evidence (fingerprints and physiological fluids), where the presence of the
“mark” virtually “proves” the presence of the individual; how else could the
evidence have been deposited if not by the individual to whom it belongs?
* Note that he does not want to 
 
determine
the cause, but rather to 
 
assist the fact finder
in determining the cause.
The distinction is crucial, as we hope to make clear in the
remainder of the chapter. 
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Association and Reconstruction — Inference of Contact
167
As our example with Fred Flintstone’s fingerprint illustrates, however, this is
still an inference.
When considering other potentially individualizing evidence, such as tool-
mark or shoeprint evidence, the inference is less immediate, and requires
additional assumptions before intimating contact. At the very least, we need
to connect the person with the tool that made the mark, and place the person
with the tool at the scene at the time the crime was committed. Even in this
simple example, we have named several assumptions external to the conclusion
that the tool made the mark, necessary to associate the person with the crime.
In the phrase from Kirk quoted earlier, he links an item of evidence with
a perpetrator by suggesting that:
identification of a murder weapon 
 
may
lead to its possessor at the time of
the crime [emphasis added].
He clearly understood that individualizing a bullet to a firearm also might
 
not
lead to the perpetrator, and that other evidence would be needed to
establish this fact more convincingly. An expansion of the previous comment
from DeForest et al. (1983) seems particularly applicable to evidence limited
for whatever reason to classification:
Depending on the degree of individuality exhibited by the samples, various
conclusions can be drawn about the association between people and the phys-
ical evidence in the case… These kinds of considerations need to be kept in
mind when evaluating the value of 
 
associative
physical evidence comparisons.
Here, “various conclusions” can be construed as different explanations for
the occurrence of the physical evidence, including multiple sources or mul-
tiple possibilities for the transfer of the evidence from the source to the target.
In this situation, numerous assumptions must be accepted to sustain an
inference of common source, and even more assumptions to support an
inference of contact. Any discussion of “association” generally defaults to a
plea for better methods of individualization, completely missing the concep-
tual point. Most authors fail to detail any approach that would describe the
relationship between the evidence and the crime or criminal besides the
useless admonition that these considerations “need to be kept in mind.”
Stoney (1991) comes the closest when he says:
Association is an inference of contact between two objects, and is the only
way in which legally relevant evidence can be generated by a scientist. Mere
source determination is not legally relevant, 
 
per se
.
The field would benefit from further development of this important thesis.
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Principles and Practice of Criminalistics