Other scholars have more linked population and economics to a third variable, political change and political violence, and to show how the variables interact. In the early 1980s, Jack Goldstone linked population variables to the English Revolution of 1640–1660 and David Lempert devised a model of demographics, economics, and political change in the multi-ethnic country of Mauritius. Goldstone has since modeled other revolutions by looking at demographics and economics and Lempert has explained Stalin's purges and the Russian Revolution of 1917 in terms of demographic factors that drive political economy. Ted Robert Gurr has also modeled political violence, such as in the Palestinian territories and in Rwanda/Congo (two of the world's regions of most rapidly growing population) using similar variables in several comparative cases. These approaches suggest that political ideology follows demographic forces.