  Contrary to an oft-made claim, there can be observational distinctions (say
for the expansion of the universe or the cosmological constant) between
"single-history" quantum theories and "many-worlds" quantum theories. The
distinctions occur when the number of observers is not uniquely predicted by
the theory. In single-history theories, each history is weighted simply by its
quantum-mechanical probability, but in many-worlds theories in which random
observations are considered, there should also be the weighting by the numbers
or amounts of observations occurring in each history.
