  The standard model of glasses is an ensemble of two-level systems interacting
with a thermal bath. The general origin of memory effects in this model is a
quasi-stationary but non-equilibrium state of a single two-level system, which
is realized due to a finite-rate cooling and very slow thermally activated
relaxation. We show that single particle memory effects, such as negativity of
the specific heat under reheating, vanish for a sufficiently disordered
ensemble. In contrast, a disordered ensemble displays a collective memory
effect [similar to that described by Kovacs for glassy polymers], where
non-equilibrium features of the ensemble are monitored via a macroscopic
observable. An experimental realization of the effect can be used to further
assess the consistency of the model.
