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—Anonymous
 
A.
In the Beginning …
The cornerstone of forensic science since the early 1920s has been a maxim
attributed to Edmund Locard. It appears in two or three permutations in his
writings, but the most comprehensive statement translates as follows:
No one can commit a crime with the intensity that the criminal act requires
without leaving numerous signs of it: either the offender has left signs at the
scene of the crime, or on the other hand, has taken away with him — on his
person or clothes — indications of where he has been or what he has done.*
Locard himself never proffered this as a principle; his students and colleagues
were the ones who transformed this simple 
 
raison d’être
into a foundational
principle of forensic science. In the process, Locard’s musings were trans-
formed into the definitive, “Every contact leaves a trace.” In the same way
that Quetelet’s “Nature exhibits an infinite variety of forms” was adulterated
to “Nature never repeats herself ” (Thornton, 1986) subtle but important
differences exist between Locard’s original quote (1920) and the modern
redux of it. Among other distinctions, the redacted version retains no men-
tion of a crime; the reader is left with the impression (whether correct or
not) that transfer is equally likely and equally important under any circum-
stance. Further, Locard implies that the criminal is acting under stress and
with anxiety (“the intensity that the criminal act requires”). This leaves no
room for the psychopathic criminal who feels no emotion whatsoever in the
commission of a criminal act, and so does not experience the type of stress
and anxiety implied by Locard. Nor does it allow for the serial criminal, who
may perfect the crime’s 
 
modus operandi
with each new commission of it,
reducing the chances of leaving traces behind. Neither Locard’s original
* Translation courtesy of Sharon Kruzik. Alternate translations of some of the words appear
later in this chapter.
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The Origin of Evidence — Divisible Matter and Transfer
85
writings nor current interpretations explicitly address the possibility of transfer
in both directions, although one might argue that Locard implies it. Our
expectations with regard to cross-transfer impact on both the search for
evidence and the interpretation of that which is found. For example, an
expectation that two-way transfer should occur might weaken an association
for which traces of contact are not found in both directions.
After reviewing Locard’s writings (1920; 1923; 1928; 1930), it seems to
us far more likely that, rather than intentionally articulating a global princi-
ple, he was merely reflecting on the reasons a careful scrutiny of the crime
scene, including victims, suspects, and witnesses, was worth the effort. Fre-
quently (or perhaps, in Locard’s mind, inevitably) contact between two
objects will be indicated by small traces of each left on the other. Find the
traces, and contact is established.
Arthur Conan Doyle had Sherlock Holmes deduce the cigarette smoked
by the type of ash that it left, discern the part of London a person was from
by the mud on his jacket, and even the part of the world where a tattoo was
acquired by the particular delicate shade of pink (Doyle, 1891). It is difficult
to know if such a simplistic view of physical evidence could be justified in
the time of Locard’s Lyon. However, it is reasonably certain a person’s sur-
roundings contained more individual character than they would today. The
reasons for greater homogeneity in today’s world, than in even the recent
past, derive from the mass production of items and the worldwide distribution
of these goods. Additionally, people have acquired increased mobility, hence
exposure, to all parts of the globe. The disintegration of physical boundaries
containing both people and things means that any individual will perennially
bathe in a wide variety of materials that are ubiquitous in the world, such as
white cotton, sugar, salt, and glass. Consequently, traces found on a person
or in her environment may not provide any useful information to differentiate
this person from the rest of the world. To take this a step farther, merely
finding traces that “match” a reference material does not necessarily establish
contact. A conclusion of contact is an inference, not a fact, and that inference
is stronger or weaker depending on how much must be assumed about a
variety of hard-to-test premises, including transferability, persistence, detect-
ability, and the frequency of the evidence in the world.
We conclude from the foregoing that Locard hit upon an essential, but
not all-inclusive, precept for the forensic scientist. For example, transfer fails
to explain either impression or physical match evidence. He wrote volumes
describing the kinds of traces that might be detected as a result of contact,
and described the kinds of techniques that utilized the unique nature of the
material to identify and classify it (Locard, 1931–1940). He therefore knew
that understanding the nature of the evidence was just as important as
explaining its presence through contact. It follows that understanding the
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86
Principles and Practice of Criminalistics
nature of evidence and the mechanisms that allow it to be transferred are
also useful concepts for the forensic scientist. It seems clear to us that Locard
understood that the nature of the evidence comprised the central part of his
work, and that it was so obvious he did not need to state it. We inch farther
out on the limb and suggest that, to Locard, transfer was merely the mech-
anism for finding the evidence before doing the important work.
We propose that understanding the origin of evidence does not begin with
transfer; it must begin with understanding the nature of the evidence and the
mechanisms that make it available for transfer. In this chapter, we develop the
idea that matter must first divide before it can be transferred, and that the
combination of division of matter and transfer after division accounts for the
origin of physical evidence in connection with criminal events.