  We show that statistics is crucial for the instability problem derived from
higher time derivatives. In fact, and contrary to previous statements, we check
that when dealing with Fermi systems, the Hamiltonian is well bounded and the
quantum states are normalizable. Although, ghost states are still present, they
do not affect unitarity under certain conditions. We first analyze a quantum
oscillator involving Grassman variables and then we generalize it to a Dirac
field. Finally, we discuss some physical implications
