Attempts to derive the Born rule, either in the Many Worlds or Copenhagen
interpretation, are unsatisfactory for systems with only a finite number of
degrees of freedom. In the case of Many Worlds this is a serious problem, since
its goal is to account for apparent collapse phenomena, including the Born rule
for probabilities, assuming only unitary evolution of the wavefunction. For
finite number of degrees of freedom, observers on the vast majority of branches
would not deduce the Born rule. However, discreteness of the quantum state
space, even if extremely tiny, may restore the validity of the usual arguments.Comment: 5 pages, revtex, 1 figure. Revised version, to appear in Physics
Letters B. (Small clarifcation, references added.