  In this paper, we examine \emph{hedonic coalition formation games} in which
each player's preferences over partitions of players depend only on the members
of his coalition. We present three main results in which restrictions on the
preferences of the players guarantee the existence of stable partitions for
various notions of stability. The preference restrictions pertain to \emph{top
responsiveness} and \emph{bottom responsiveness} which model optimistic and
pessimistic behavior of players respectively. The existence results apply to
natural subclasses of \emph{additive separable hedonic games} and \emph{hedonic
games with \B-preferences}. It is also shown that our existence results cannot
be strengthened to the case of stronger known stability concepts.
