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Re: Common misconceptions



Ed, David:

>> This statement is beyond my comprehension.  If a user is positively
>> identified (by eyewitnesses, DNA evidence, and his smartcard's
>> signature in a POS terminal) at a crime scene, he has little power to
>> repudiate his involvement.
>
>Thank you for the opportunity to clarify my statement -- which is a general
>statement valid in many legal systems (including common and civil law). If
>a person is positively identified  by DNA evidence of his sperm on a dress
>then the person has full liability to the extent of that identification (say,
>1:1000,000)  and to the extent that production of the sample was (most
>liked than not) voluntary and within his *power* to avoid.  Likewise, if a
>fresh sample of a person's blood is present at a crime scene then this
strongly
>implies that the person was present and has full liability for that presence
>to the extent of the time factor (fresh) and blood identification because
production
>of the sample at that time and place was (most likely than not) within his
>*power* to avoid.

Sorry, but you're both confused here.  This is something I know a lot about,
as
my wife is a forensic DNA analyst who's testified to DNA evidence on many
occasions.

A sample of DNA evidence has nothing whatsoever to do with non-repudiation.
Non-repudiation evidence is evidence of intent; DNA evidence is emphatically
**NOT** evidence of intent.  Both of Ed's examples get this wrong.

Sperm on a dress *may be* evidence of one party's sexual activity in the
presence of
a dress (not necessarily of the owner of the dress, or even the person who
most
recently wore it).  The most common defense against a charge of rape to which
DNA evidence is adduced as evidence, is consent.  In other words, the accused
party claims that the accuser consented to the act.  This defense is
successful
in strikingly many cases. The DNA evidence has no bearing on repudiation, as
there is no presumption or prior claim *by the accused* of intent or action.
In fact, in the USA, given the presumption of innocence in criminal
proceedings,
exactly the opposite is the case - the accused is presumed NOT to have
committed
the act, hence the evidence is a priori presumed not to indicate intent (hence
guilt)
unless it's so compelling that the jury finds it convincing all by itself.

The O.J. Simpson case should dispel any doubts that blood DNA evidence
creates a presumption of intent, at least in the USA.  Blood DNA indicates
that
a party's blood, with some probability, was present at the scene.  Again, in a
criminal proceeding, this evidence must overcome a presumption of innocence,
and is NOT indicative of intent, which MUST be established separately, at
least
in the USA.

Incidentally, no person is *ever* "positively identified" by DNA evidence.  If
you look
at the FBI's guidelines for presenting DNA evidence (even as they were before
the
FBI crime lab's run of bad publicity recently), this is very clear.  The FBI,
and all US
crime labs, have elaborate statistical rules for determining the probability
that

        a randomly chosen member of a population would match a given
        evidentiary sample

If this probability is low, and the suspect (or defendant) does match the
sample,
this can be taken as evidence that the suspect or defendant "contributed the
sample".
On this basis the suspect or defendant "cannot be excluded" by the sample.

I know of NO legal system whereby either

    1. A person incurs any liability as a result of contributing DNA to an
evidentiary sample.

        (persons incur liability through findings of fact, which may take DNA
as evidence)

    2. The extent of liability is contingent upon "the extent of
identification".

        (no legal system I know of judges strength of identity by inverse of
probability of
         DNA match, for example)

>Of course, the above comparison between sperm and blood samples already
>dicloses a basic difference between both in terms of degree of
non-repudiation
>which also links to power, in that the second depends on a much lesser degree
on
>that person's power to prevent it -- a simple needle will do, in  a casual
and even
>unperceived encounter (a  brief pain).  For example, a sperm stain on a dress
>can be much more devastating as evidence (as recent events showed) than a
>blood stain, which may even provide no evidence at all (seen the last episode
>of  "The Practice"? OJ?)

Sorry, but at least in the US legal system this simply isn't so.  DNA evidence
is
gathered from three kinds of samples: blood, sperm, and epithelial cells (e.g.
skin
cells).  Which type of sample generated a DNA match has as far as I'm aware
NEVER been raised as an issue either against strength of identification or
against
intent (to which DNA evidence isn't relevant anyway) in a US court case.

The only court I know of where "a sperm stain on a dress can be much more
devastating
evidence" is the court of public opinion, where the standard of evidence is so
low
that non-repudiation certainly isn't an issue.



--bob

Bob Blakley (blakley@dascom.com)
Chief Scientist, Dascom

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