AbstractAnimals can influence their social environment by preferentially associating with certain conspecifics. Such preferential association has gained increasing theoretical attention, as it may influence social evolution and population dynamics. However, relatively little empirical work has examined the occurrence of preferential association and its effects on cooperative group formation. Here, we test the factors associated with cooperative group formation inPolistes dominulusnest‐founding queen wasps.P. dominulusare a good system to study preferential association, as foundresses can nest alone or in groups and group membership is flexible. We found that both social and environmental factors were associated with partner choice. First, facial patterns were associated with cooperation. Wasps with more similar facial patterns were more likely to cooperate than wasps with less similar facial patterns. This preferential phenotypic association fits the theoretical criteria for the evolution of tag‐based cooperation. Season was also associated with cooperation; wasps on early‐season nests were more likely to cooperate than wasps on late‐season nests. High levels of aggression by nest owners during initial interactions were also correlated with lower probabilities of subsequent cooperation, suggesting that nest owners have some control over group membership. Other factors including body weight, weight similarity and nest productivity were not linked with cooperation. Overall, multiple factors influence cooperation in paper wasps, including facial pattern similarity. The occurrence of preferential phenotypic association in paper wasps is quite interesting and may influence the evolution of cooperation and population divergence in this group.