For many technologies and behaviours, an agent’s benefit from adopting depends on his contacts adopting, and the benefit to his contacts of adopting depends on their contacts adopting. This paper examines how the architecture of these connections shapes the success or failure of the diffusion of innovations. We start with a standard model of diffusion with the key addition that some agents can coordinate their decisions. This captures the idea that people often talk and make decisions together with friends or family to adopt technologies. We show that insularity of connections, that is, the extent to which agents tend to concentrate their connections to a narrow set of other agents, determines contagion. However, whether insularity helps or hinders depends on the technology being diffused. For technologies that are valuable even without many contacts adopting, we find insular connections hinder adoption, but for technologies that are valuable only when many contacts adopt, insular connections facilitate adoption.

Products are increasingly made by assembling separately produced modules. Motivated by the notion that a firm’s production function drives its organization, we explore how modular production shapes a firm’s communication structure. Decisions are partitioned into modules and require closer coordination within modules than across. Each agent knows the state his decision must be adapted to. The principal decides whom each agent tells about his state, given that each communication link comes at a cost. We show that optimal communication networks follow a simple threshold rule and exhibit the threshold property. We discuss comparative statics, applications, and empirical implications.

This article develops a model to explore how favour exchange influences wealth dynamics. We identify a key obstacle to wealth accumulation: wealth crowds out favour exchange. Therefore, households must choose between growing their wealth and accessing favour exchange. We show that low-wealth households rely on favour exchange at the cost of having tightly limited long-term wealth. As a result, initial wealth disparities persist and can even grow worse. We then explore how communities and policymakers can overcome this obstacle. Using simulations, we show that community benefits and place-based policies can stimulate both saving and favour exchange, and in some cases, can even transform favour exchange into a force that accelerates wealth accumulation.