  Mutualistic relationships among the different species are ubiquitous in
nature. To prevent mutualism from slipping into antagonism, a host often
invokes a "carrot and stick" approach towards symbionts with a stabilizing
effect on their symbiosis. In open human societies, a mutualistic relationship
arises when a native insider population attracts outsiders with benevolent
incentives in hope that the additional labor will improve the standard of all.
A lingering question, however, is the extent to which insiders are willing to
tolerate outsiders before mutualism slips into antagonism. To test the
assertion by Karl Popper that unlimited tolerance leads to the demise of
tolerance, we model a society under a growing incursion from the outside.
Guided by their traditions of maintaining the social fabric and prizing
tolerance, the insiders reduce their benevolence toward the growing
subpopulation of outsiders but do not invoke punishment. This reduction of
benevolence intensifies as less tolerant insiders (e.g., "radicals") openly
renounce benevolence. Although more tolerant insiders maintain some level of
benevolence, they may also tacitly support radicals out of fear for the future.
If radicals and their tacit supporters achieve a critical majority, herd
behavior ensues and the relation between the insider and outsider
subpopulations turns antagonistic. To control the risk of unwanted social
dynamics, we map the parameter space within which the tolerance of insiders is
in balance with the assimilation of outsiders, the tolerant insiders maintain a
sustainable majority, and any reduction in benevolence occurs smoothly. We also
identify the circumstances that cause the relations between insiders and
outsiders to collapse or that lead to the dominance of the outsiders.
