Greenfield's theory of social change and human development is based on the distinction between Gemeinschaft (low-income agricultural communities with low levels of formal education and technology) and Gesellschaft (wealthier commerce-based societies with high levels of formal education and technology). Cooperation is more adaptive in a Gemeischaft environment; in contrast, competition is more adaptive in a Gesellschaft environment. As Mexican ecologies moved in the Gesellschaft direction over

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... cent decades, children's cooperative behavior declined, as predicted by the theory. The current quasi-experiment extends this finding from a two-person game, the marble pull, to a new situation, Madsen's cooperation board, a game that requires cooperation among four children. Based on a sample of 57 groups of four children each tested in 2017 and 70 groups of four children each tested in 1967, the Wilcoxon Signed Ranks Test ( p < 0.001) showed that the decline of cooperation and the rise of competition generalizes across middle-class urban, low-income urban, and rural children in Mexico and that it applies to male, female, and mixed groups. In conclusion, we provide continuing evidence that child behavior is responsive to ecological conditions and shifts over time in order to adapt to them. Given that cooperation is a fundamental human trait that binds social units together, our study also contributes to the conclusion that globalized social change in the Gesellschaft direction entails human loss as well as gain.