The selection of altruistic behaviour
  Altruistic behaviour is disadvantageous for the individual while is advantageous for its group. If the target of the selection is the individual, one would expect the selection process to lead to populations formed by wholly homogeneous groups, made up of either altruistic or egoistic individuals, where the winning choice depends on the balance beetwen group advantage and individual disadvantage. We show in a simple model that populations formed by inhomogeneous groups can be stabilized in some circumstances. We argue that this condition is realized when there is a relative advantage conferred by the presence of a few altruists to all the members of the group. 
R.Donato, L. Peliti, M. Serva