In this paper we argue that unsustainable behaviors often stem from a common averaging bias when people estimate the environmental impact of a set of environmentally friendly and less friendly objects or actions. In Experiment 1, we show that people believe that the total carbon footprint of a category of items (a community of buildings in this case) is lower, rather than higher, when environmentally friendly (“green” buildings) items are added to the category, a negative footprint illusion. Experiment 2 showed  that the carbon footprint estimate assigned to a category with a mix of environmentally friendly and less friendly objects (“green” and conventional  buildings) is the average of its subsets (the “green” buildings and the  conventional buildings, respectively), an averaging bias. A similar averaging  process may underpin estimates of the environmental impact of people's own actions, explaining why people believe that environmentally friendly actions can compensate for less friendly actions.