Influencing more environmentally friendly and sustainable behaviour is a current focus of many projects, ranging from government social marketing campaigns, education and tax structures to designers’ work on interactive products, services and environments. There is a wide variety of techniques and methods used—we have identified over 100 design patterns in our Design with Intent toolkit—each intended to work via a particular set of cognitive and environmental principles. These approaches make different assumptions about ‘what people are like’: how users will respond to behavioural interventions, and why, and in the process reveal some of the assumptions that designers and other stakeholders, such as clients commissioning a project, make about human nature. In this paper, we discuss three simple models of user behaviour—the Pinball, the Shortcut and the Thoughtful—which emerge from user experience designers’ statements about users while focused on designing for behaviour change. We characterise these models using systems terminology and examine the application of each model to design for sustainable behaviour via a series of examples.