This is the graph of Effective Demand plotted with real GDP (aggregate output) since 1967. Effective Demand is represented by the red line above real GDP. I will draw your attention to how effective demand behaves when it is close to real GDP. Look at the years 1979, 1995-1998 and 2005-2008. Effective demand came down to meet with real GDP but wouldn't go lower. Between the years 1995-1998, effective demand flat-lined with real GDP. This behavior agrees with the principle that aggregate supply (real GDP) does not like to surpass effective demand.