Comment by marginalia_nu

If quantum probability is a wave function, and interaction introduces a phase shift, this alone is enough to lead to decoherence through the same mechanics as classical optical (de)coherence.

In the same circumstances a light beam stops producing an interference pattern in the Young experiment, quantum wave functions do as well. This is pretty easy to derive, just introduce a random phase shift term, and average across it, and the interference pattern disappears and a bell curve emerges instead.