  In Everett's many worlds interpretation, where quantum measurements are seen
as decoherence events, inexact decoherence may let large worlds mangle the
memories of observers in small worlds, creating a cutoff in observable world
size. I solve a growth-drift-diffusion-absorption model of such a mangled
worlds scenario, and show that it reproduces the Born probability rule closely,
though not exactly. Thus deviations from exact decoherence can allow the Born
rule to be derived in a many worlds approach via world counting, using a finite
number of worlds and no new fundamental physics.
