  Overlapping resonances are shown to provide new insights into the extent of
decoherence experienced by a system superposition state in the regime of strong
system- environment coupling. As an example of this general approach, a generic
system comprising spin-half particles interacting with a thermalized oscillator
environment is considered. We find that (a) amongst the collection of
parametrized Hamiltonians, the larger the overlapping resonances contribution,
the greater the maximum possible purity, and (b) for a fixed Hamiltonian, the
larger the overlapping resonances contribution, the larger the range of
possible values of the purity as one varies the phases in the system
superposition states. Systems displaying decoherence free subspaces show that
largest overlapping resonances contribution.
