This paper experimentally tests the contribution of two distinct aspects of social interaction to the creation of shared symbols: behaviour alignment and concurrent partner feedback. Pairs of participants (N= 120, or 60 pairs) completed an experimental-semiotic game, similar to Pictionary, in which they tried to communicate a range of recurring meanings to a partner by drawing on a shared whiteboard (without speaking or using numbers of letters in their drawings). The opportunity for sign alignment and/or concurrent partner feedback was manipulated in a full factorial design. Each process made a distinct contribution to the evolution of shared symbols: sign alignment directly influenced communication success, and concurrent partner feedback drove sign simplification and symbolization. These complimentary processes led to the interactive evolution of effective and efficient human communication systems.