A signed digraph model, which has been used to assess strategies intended to stabilize models of energy use and air pollution, is extended in a simple manner to include the effects of time lags on stability, and on the growth rate of instabilities. It is shown that in an analogous model using differential equations, the effect of time lag, involving integration of a variable over all earlier times, can be incorporated equally simply, if the weighting factor in the integral (memory function) is chosen in a specific form. The results of this procedure are compared in some simple models with the results of using discrete time lag, and of numerically solving the integro-differential equation with a square memory function.