  When the interactions between cooperators (C) and defectors (D) can be
partially avoided within a population, there may be an overall enhancement of
cooperation. One example of such screening mechanism occurs in the presence of
risk-averse agents (loners, L) that are neutral towards others, i.e., both L
and its opponent, whatever its strategy, receive the same payoff. Their
presence in the Prisoner's Dilemma (PD) game sustains the coexistence of
cooperators and defectors far beyond the level attained in their absence.
Another screening mechanism is a heterogeneous landscape obtained, for example,
by site diluting the lattice. In this case, cooperation is enhanced with some
fraction of such inactive, interaction-averse sites. By considering the
interplay of both mechanisms, we show that there is an explosive increase in
the range of densities, just above the percolation threshold, where neutrality
is prevented and loners become extinct, the behavior reverting to the pure PD
game. Interestingly, this occurs despite defectors being usually abundant in
that region. This has to be compared with the corresponding loner-free region
in the undiluted case that, besides being very small, is dominatedby
cooperators.
