  We analyze the validity of the fluctuation-dissipation theorem for slow
relaxation systems in the context of mesoscopic nonequilibrium thermodynamics.
We demonstrate that the violation arises as a natural consequence of the
elimination of fast variables in the description of a glassy system, and it is
intrinsically related to the underlying activated nature of slow relaxation. In
addition, we show that the concept of effective temperature, introduced to
characterize the magnitude of the violation, is not robust since it is
observable-dependent, can diverge, or even be negative.
