The Preponderance of the Evidence: 
Reflections on Just Criminal Procedures

In Anglo-American law, in order to get a criminal conviction one
must normally prove the case against the accused "beyond a 
reasonable doubt."  In contrast, in a civil case one need merely
provide a "preponderance" of the evidence.  I think that our moral
intuitions strongly favor these rules, and perhaps incline us to
think that the standard of proof in civil cases should be increased
to that of criminal cases.  At least that was my initial moral
intuition.  After some reflection, however, I think that I erred,
and that in fact a just legal system would in fact use the weaker
"preponderance of the evidence" standard for all cases.

What is the argument against the reasonable doubt standard?  Well,
imagine that you are observing a trial for murder.  Most of the
evidence goes against the accused (who allegedly murdered a 5-year-old
girl).  The girl's father sits anxiously, awaiting the verdict;
he testifies that he witnessed the murder personally, but various
other strands of evidence exist which create some doubts.    

After witnessing the whole trial, a definite conclusion about the
evidence strikes you: the accused is probably guilty, but not guilty
beyond a reasonable doubt.  And the jury, apparently, felt the same
way: they regretfully emerge from deliberations and proclaim the
defendant "not guilty."

Suddenly, the enraged father whips out a concealed hand gun and
aims it at the smiling acquited defendant.  He shoots and kills
the defendant.  Now the question arises: what, if anything, may
be justifiably done to the father?

I assume that if the defendant were truly his daughter's murderer, the father
would have had every right to kill him.  On the other hand, if the
defendant were not the murderer, then the father's act is itself
a murder, and it is the father who must be treated as a murderer.
But how could the father ever be convicted?  Not only is he not
a murderer _beyond a reasonable doubt_; he isn't ever a murderer
according to the preponderance of the evidence!  In fact, the
preponderance of the evidence is that the defendant was a murderer
and that the father's act was justifiable.  In short, the use of
the reasonable doubt rule puts us in a position where if we were
to uphold the law and defend our hypothetical defendant, we would
probably be defending the _wrong man_.  Imagine you were the
bailiff for the trial and you saw the father draw his gun; should
you shoot and possibly kill the man who probably is _not_ a murderer,
in order to save a man who probably _is_?  Only the preponderance
rule lets us escape the bizarre conclusion that we should kill
the man who is less likely to be a murderer in order to save the
man who is more likely to be a murderer.

Of course, to many my perspective will seem bizarre, since I assume
that every man has the "right to take the law into his own hands."
But in fact, I think this is just common sense: if it is right
for the state to execute a murderer, how could it be wrong for anyone
else to do so?  Doesn't that violate the most elementary moral
principle of universalizability?  The case for procedure must therefore
be based upon convenience rather than justice; a convenience which
people have no _duty_ to accept even though we might urge them to do
so to preserve social peace.

Of course, I assume that the legal system is itself just; that is,
it only punishes individuals for acts which merit punishment.
In an imperfect world which punishes far more acts that can justifiably
be punished, insisting on the standard of reasonable doubt might be
a sensible strategy.  But the underlying theory of the reasonable
doubt standard is not merely weak, but simply incoherent, insofar
as it councils us to punish those less likely to be criminals rather
than those who are more likely to be criminals.