Using Greenfield's theory of sociocultural change and human development as a point of departure, we carried out two experimental studies exploring the implications of decades of globalised social change in Mexico for children's development of cooperation and competition. In rural San Vicente, Baja California, the baseline was 1970 and the historical comparison took place 40 years later. In Veracruz, the baseline was 1985 and the historical comparison took place 20 years later. In Veracruz, children were tested in both rural and urban settings. We hypothesized that cooperative behavior would decrease in all three settings as a result of the sociocultural transformations of the past decades in Mexico. The Madsen Marble Pull Game was used to assess cooperative and competitive behavior. As predicted by Greenfield's theory of social change and human development, the Marble Pull procedure revealed a striking decrease over time in levels of cooperative behavior, with a corresponding rise in competitive behavior, in all three settings.