The throughput of mobile ad hoc networks subject to disruption, loss and interference can be significantly improved with the use of network coding. However, network coding implies extra work for forwarders. Selfish forwarders may prefer to simply forward packets without coding them because of the processing overhead introduced by network coding. This is especially true in secure network coding where the coded packets are protected from pollution attacks by processor intense homomorphic signatures. To drive selfish nodes to cooperate and encode the packets, this paper introduces social norm based incentives. The social norm consists of a social strategy and a reputation system with reward and punishment connected with node behavior. Packet coding and forwarding is modeled as a repeated alternate gift-giving game. The interaction between nodes in the repeated game is formalized, the conditions for social strategy sustainability (or compliance) are identified, and a sustainable game that optimizes the social welfare is designed. For this game, the impact of packet loss rate, reputation thresholds and reputation update frequency on performance is evaluated.