PULLMAN, WASHINGTON—Kyle Bocinsky and Tim Kohler of Washington State University and their colleagues at the National Center for Supercomputing Applications at the University of Illinois analyzed data from 1,000 archaeological sites in the American Southwest, and 30,000 tree-ring dates, in order to examine patterns of societal expansion and collapse. They found that periods of drought and crop failure were followed by periods of exploration and exploitation. Between A.D. 600 and 700, people stored their maize in underground chambers. That practice ended with a mild drought. The following Pueblo I period was marked by aboveground storage rooms and perhaps a more restricted exchange of food between family groups. This period, ended by drought, was followed by the large shared plazas and great houses, and hierarchal social structure, of the Pueblo II period. Some of the greatest evidence of social inequality is seen during the Pueblo III period, ended by the largest and most widespread drought. During the following Pueblo IV period, thought to be more egalitarian, people built pueblos that shared plazas and ceremonial spaces. “There’s a point where people say, ‘This isn’t working. We’re leaving,’” Kohler said in a press release. Then “there’s a new period of wealth creation, investment in architecture and culture change,” he explained. To read more about archaeology in this area, go to "Early Parrots in the Southwest."