  We discuss the history dependence and memory effects which are observed in
the out-of-equilibrium conductivity of electron glasses. The experiments can be
understood by assuming that the local density of states retains a memory of the
sample history. We provide analytical arguments for the consistency of this
assumption, and discuss the saturation of the memory effect with increasing
gate voltage change. This picture is bolstered by numerical simulations at zero
temperature, which moreover demonstrate the incompressibility of the Coulomb
glass on short timescales.
