Center for Experimental Economics in Education (CEEE), Shaanxi Normal University, Xi'an, Shaanxi, 710119, China. West China School of Public Health, Sichuan University, Chengdu, Sichuan, 610041, China. Rural Education Action Program, Freeman Spogli Institute for International Studies, Stanford University, Stanford, California, 94305, United States of America. School of Advanced Agricultural Sciences, Peking University, Beijing, 100871, China.