A. In the Beginning ... 84 B. The Nature of Matter ... 86 C. Divisible Matter — A New Fundamental Principle in Forensic
Science ... 87 1. Properties Useful for Source Determination... 88 2. Properties Confounding Source Determination ... 89 3. More on the Nature of Matter: Thermodynamics and
Entropy... 91 4. An Example of Divisibility ... 92 5. Impression Evidence — Does Divisible Matter Apply?... 93 D. Transfer Theory (Locard)... 93 1. Physical Transfer... 94
a. Trace Evidence — Transfer of Very Small Physical
Entities ... 95 b. Macroscopic Evidence — Transfer of Larger Physical
Entities ... 95 c. Factors Affecting Transfer and Detection... 95
4
Figure 4.1 The continents. When matter divides, some characteristics retained by the smaller pieces are unique to the division process. These traits are created at the boundary of the fracture. Boundary roughness is a natural consequence of breaking one surface into two, and these new surfaces are closely, but not com- pletely, complementary. At the moment of division the separated fragments commence to change and become different both from each other and from the original object. At some point, it might become impossible to associate the two fragments by physical matching of their complementary edges. Do Africa and South America share a common origin? The figure allows you to make your own judgment.
84 Principles and Practice of Criminalistics
2. Spatial Trait Transfer ... 96 a. Impression Evidence ... 96 b. Three- to Two- to One-Dimensional Transformations... 96 E. Summary ... 98 References ... 99
Every contact leaves a trace
—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.
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
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.
B.
The Nature of Matter
The examination of evidence relies on an understanding of the fundamental nature of matter. When matter becomes evidence by virtue of its connection with a criminal act, this modulates to an understanding of the nature of the evidence. Matter (and hence physical evidence) is the “stuff” around us that is composed of the fundamental building blocks of the universe, ordered according to well-articulated chemical and physical principles. We need not concern our- selves with subatomic particles when discussing physical evidence, but rather atoms and how they are arranged to compose our physical world. Because they can only be arranged in specific ways allowed by the laws of chemistry and physics, their fundamental properties remain constant and reproducible each time we examine or analyze them. Forensic scientists take advantage of these properties of matter in forensic science by applying tests designed to reveal the underlying nature of the material so that we might draw some useful conclusions about what it is, and what the potential source of the evidence might be.
A forensic scientist is thus well grounded in the fundamentals of chem- istry and physics; here is the starting place for understanding what property of an item of evidence might be examined to answer a useful legal question. Analysts must come to appreciate how the material is generated, whether by nature or by human hands; they must ascertain the characteristics that define the material, including which of those characteristics are shared with other materials, and which are unique to this material. From the synthesis of this knowledge comes the ability to determine what traits may be used to cate- gorize or classify the material, what properties might lead to source deter- mination, and whether that source might be unique.
For bloodstain patterns and ballistics, an understanding of the substance, combined with a grasp of its dynamic interaction with the environment, is 8127/frame/ch04 Page 86 Friday, July 21, 2000 11:48 AM
integral to interpreting the evidence. For this kind of “dynamic” evidence, we are as interested in the process as the end result. In particular, we look to this type of evidence to assist in the final reconstruction of the event, in other words, the movement of matter through space and time. We will address this aspect in more detail in Chapter 7.
Once an appreciation is gained for the fundamental nature of matter, and consequently for physical evidence, it is necessary to describe those principles and processes that form the unique aspects of the enterprise we call forensic science.