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e.
Explainable Differences
It can be tempting to consider any and all perturbations to a sample as
originating from negligent causes such as contamination or analyst incom-
petence. The fact remains that all samples are unique (see Chapter 6), even
samples that originate from a common source. They have accumulated dif-
ferences as a result of their separate local environments in space and time
and will never exactly resemble another in all respects. The analyst must use
her knowledge, education, and experience to discern which elements differ-
entiate two samples as having different origins and which are merely explain-
able differences. This, more than any other single attribute, differentiates the
criminalist from the technician.
some ants. For some unknown reason, the entomologist never thought to identify the type of ants
present on the body (a photograph was available). Therefore, at the scene they merely collected ants
that were present and visible. In a curious twist, they also consulted the entomologist retained by the
prosecution, Dr. Snelling. The defense expert’s only question to Snelling, however, was to identify the
ants that they had collected 
in the field
from the crime scene. The entomologist easily identified them
as ubiquitous Argentine ants.
The defense expert then carried out some of his own experiments. He discovered (to his surprise)
that ants do have ABH-like substances. In his case, the Argentine ants showed type A antigenic activity.
He further demonstrated that the ants could deposit this activity simply by walking around on a
substrate such as a piece of paper. While this expert had previously questioned the possibility that
ants could contribute ABH-like substances, for this case he was satisfied with his results showing they
could; they failed to explain the type B antigen found on the nipple of the deceased victim.
At trial, the defense attorney presented only one piece of physical evidence supporting his client’s
innocence, that of the nipple swab containing type B antigenic activity. His expert argued that amylase
was found, consistent with the presence of saliva, and further that type B antigen was present. He
argued that together, these supported the hypothesis that saliva from a type B secretor individual was
present. He further testified that both the defendant and his previously convicted accomplice were
type AB non-secretors. During direct and cross-examination, the expert presented his findings regard-
ing the ants. At one point he identified the ants that he tested as Argentine ants, and concluded that,
while they did exhibit A antigen activity, they did not exhibit B antigen activity, so they could not be
the source of the B activity found on the nipple of the victim. Thus, in his opinion, neither the ants
nor the defendant(s) could be the source of the B antigen.
On rebuttal, the prosecutor decided to put up only the entomologist from the Museum of Science
and Industry. After qualifying him, the prosecutor asked if he could identify the ants on the breast
area of the deceased. When he responded that they were fire ants, one of the jurors was reportedly
seen to clasp his forehead and mutter 
sotto voce
, “They tested the wrong ants!”
The jury did convict the defendant of murdering this victim, and so could not have given the
defense theory, the only possible exculpatory evidence, much weight.
This scenario illustrates the importance of knowing as much about a case as possible to formulate
relevant hypotheses and design experiments that address the specific circumstances of the crime.
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Principles and Practice of Criminalistics