In the 1990s, evolutionary anthropologist and quantitative historian Peter Turchin noticed that the equations used to model the populations of predators and preys can also be used to describe the ontogeny of human societies. He specifically examined how social factors such as income inequality were related to political instability. He found recurring cycles of unrest in historical societies such as Ancient Egypt, China, and Russia. He specifically identified two cycles, one long and one short. The long one, what he calls the "secular cycle," lasts for approximately two to three centuries. A society starts out fairly equal. Its population grows and the cost of labor drops. A wealthy upper-class emerges while the life for the working class deteriorates. As inequality grows, a society becomes more unstable with the lower-class being miserable and the upper-class entangled in infighting. Exacerbating social turbulence eventually leads to collapse. The shorter cycle lasts for about 50 years and consists of two generations, one peaceful and one turbulent. Looking at United States history, for example, Turchin was able to identify times of serious sociopolitical instability, 1870, 1920, and 1970. He predicted that in 2020, the U.S. would witness a period of unrest at least on the same level as 1970 because the first cycle coincides with the turbulent part of the second in around 2020. He announced this prediction in 2010. He also warned that the U.S. is not the only Western nation under strain.