This paper experimentally tests the contribution of twodistinct aspects of social interaction to the creation ofshared symbols: behaviour alignment and concurrentpartner feedback. Pairs of participants (N= 120, or 60pairs) completed an experimental-semiotic game,similar to Pictionary, in which they tried to communicatea range of recurring meanings to a partner by drawingon a shared whiteboard (without speaking or usingnumbers of letters in their drawings). The opportunityfor sign alignment and/or concurrent partner feedbackwas manipulated in a full factorial design. Each processmade a distinct contribution to the evolution of sharedsymbols: sign alignment directly influencedcommunication success, and concurrent partnerfeedback drove sign simplification and symbolization.These complimentary processes led to the interactiveevolution of effective and efficient humancommunication systems.