8127/frame/fm

 
Sidebar 11
 
The Hair That Cried Uncle — An 
Inference of Association
A 12-year-old girl accused her 18-year-old uncle of rape. To the detective’s questions, she related that
after the assault, she had taken a shower and dried herself with a towel. Detectives seized the towel,
and also transported the victim to the hospital for collection of physical evidence. They also learned
that the uncle lived in the same house as the little girl.
Analysts at the crime laboratory first examined the sexual assault kit; however, no semen or
other evidence of sexual assault was found. The towel, however, yielded a pubic hair. The victim was
clearly eliminated as a possible donor of the hair as she was in the early stages of puberty and still
lacked any pubic hair. Reference pubic hairs were duly collected from the suspect. After a microscopic
comparison, the laboratory was unable to eliminate him as a potential donor of the evidence hair
collected from the towel.
The district attorney in the case had little evidence besides the story of the victim, and so
requested DNA analysis of the hair. She was cautioned that any finding would have little meaning,
but she persisted, and the laboratory acquiesced to the examination. This was in the early days of
DNA typing, and only one locus, DQ
α
using the reverse dot-blot procedure, was available to test
small amounts of DNA such as might be found on a hair root. The DQ
α
test showed no differences
between the type of the suspect and of the evidence hair. Although some number of other people in
the world could have contributed the hair, probably none of them, other than the uncle, had access
to the bathroom where the towel resided.
However, the fact that the uncle couldn’t be eliminated as a donor of the hair on the towel misses
the point completely. In point of fact, the uncle’s pubic hair on the girl’s towel established precisely
nothing useful pertaining to the accusation. Many alternate explanations existed for the presence of
his hair on the towel, including transfer from anywhere in the bathroom to the victim and then to
the towel, or transfer to the towel from anywhere in the bathroom. Further, when considering the
mechanism of purported transfer, the hypothesis would have to include the hair surviving the shower
to be transferred during the drying action.
Even if we accept that the hair was from the uncle, no clear connection existed between the
presence of the hair on the towel and the crime event. The evidence was simply not useful in
establishing intimate contact between the girl and her uncle, and hence was only marginally relevant
to the case. Far too many assumptions were needed to sustain an inference of contact related to the
crime event. This is an excellent example of a piece of evidence that could not settle a fact in dispute.
In the language of law, the physical evidence was simply not relevant.
In spite of this, the defendant pled guilty!
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Good Forensic Practice — Obligations of the Analyst
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It is interesting to note in this example, that, although source determination
was the original question, answering it did not further an understanding of
the case. Too many alternatives existed for how the hair came to be associated
with the towel, with no clear way to decide among them.
It is interesting to note that the inability of science to determine 
 
when
frequently underlies the lack of utility of the evidence. This is particularly
true when the evidence is not necessarily the result of violent activity. For
example, fibers from a suspect’s clothing would be expected in his vehicle;
large bloodstains are less likely to result from normal daily activity even if
they occur in an area frequented by their donor.