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Theories on the emergence and maintenance of cooperation expect expressions of gratitude to conﬁrm essential bonds of reciprocity. In large networks of cooperation and exchange, people contribute to common goods through upstream reciprocity. Scientiﬁc theories expect that expressions of gratitude are indirectly caused when people receive beneﬁts from others and communicate back to the provider their feelings and sense of obligation. What if those expressions of gratitude also move forward, inﬂuencing further thanks and extended contributions to a community? We tested this hypothesis in a ﬁeld experiment with Wikipedia communities in the Arabic, German, Polish, and Persian language editions.