Résumé
Future generations have no agency in today's decisions, making their well-being a defining challenge of our time. Climate change, biodiversity loss, and resource depletion all depend on trade-offs between immediate gains and long-term sustainability. These dilemmas are often attributed to shortsightedness. We show instead that the critical obstacle lies within generations themselves: coordination failures among contemporaries can undermine sustainability even when individuals care about the future. Using a lab-in-the-field intergenerational goods game with a threshold-based regeneration rule, we compare settings with a single decision maker per generation to ones with three contemporaries deciding simultaneously without communication. When individuals act alone, resources are almost always preserved; when contemporaries must coordinate, conservation collapses. Our models explain this pattern by combining intergenerational altruism with beliefs about others' restraint: pessimistic expectations erode altruistic motives, driving overextraction. These insights have direct implications for climate governance and natural resource management, where failures in coordination today can be as detrimental as lack of concern for the future.