  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.
