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The social environment is often the most dynamicandfitness-relevant environment animals experience. Here we testedwhether plasticity arising from variation in social environments canpromote signal-preference divergence—a key prediction of recentspeciation theory but one that has proven difficult to test in natural sys-tems. Interactions in mixed social aggregations could reduce, create,or enhance signal-preference differences. In the latter case, social plas-ticity could establish or increase assortative mating. We tested this byrearing two recently diverged species ofEnchenopatreehoppers—sap-feeding insects that communicate with plant-borne vibrationalsignals—in treatments consisting of mixed-species versus own-speciesaggregations. Social experience with heterospecifics (in the mixed-species treatment) resulted in enhanced signal-preference species dif-ferences. For one of the two species, we tested but found no differencesin the plastic response between sympatric and allopatric sites, sug-gesting the absence of reinforcement in the signals and preferencesand their plastic response. Our results support the hypothesis that so-cial plasticity can create or enhance signal-preference differences andthat this might occur in the absence of long-term selection against hy-bridization on plastic responses themselves. Such social plasticity mayfacilitate rapid bursts of diversification.