Why do people contribute to important societal causes, such as sustainability? This study hypothesized that people are motivated to help because they anticipate a sense of warm glow from acting green. Although results reveal that 'feelgood' affect mostly drives low-rather than high-cost behaviour changes, harnessing people's intrinsic motivation to help the environment may be an underleveraged mechanism for promoting sustainability.Although the study of why people contribute-or fail to contribute-to important societal causes has captured the attention of behavioural science for many decades 1 , the motivational foundations of cooperative behaviour seem especially relevant in the face of what is perhaps the biggest social dilemma of our time: ensuring a sustainable future for our planet 2,3 . While the psychological factors that motivate sustainable behaviour have shown to be diverse 4 , one important observation is that humans are often intrinsically motivated to contribute to societal causes because people derive a sense of 'warm glow' from helping others 5 .Yet, the 'half-life' of empathy-driven prosocial behaviourwhether it concerns raising support for debilitating diseases such as amyotrophic lateral sclerosis, conserving energy or humanitarian responses to conflict-tends to be short-lived 6-8 . This is problematic when it comes to issues that require long-term support and cooperation from people, such as sustainability. Accordingly, solutions that leverage people's 'intrinsic' motivation to help the environment may sustain behaviour change longer than policy incentives that are contingent on extrinsic rewards 6-8 . Nonetheless, an important prerequisite of this assumption is that people actually experience or anticipate positive 'internal' rewards from acting green. This is a tenuous assumption for several reasons. First, compared to helping other humans in need, sustainability often includes non-human nature and so it is less clear whether people also derive a sense of warm glow from contributing to large-scale and relatively depersonalized collective action problems. To this extent, emerging research suggests that helping the environment does in fact make people feel good about themselves 9-11 . This is also evidenced by the fact that extrinsic incentives, such as financial rewards, often backfire 12 , presumably because they dilute the purity of the prosocial act 13 , which generally makes people feel less positive about their contribution to the environment 14 .Second, prior studies have mostly measured 'experienced' emotions to help the environment retrospectively, often for one particular action at a single point in time and typically using 'good intentions' as a substitute for behaviour. It is therefore unclear to what extent people act sustainably because they anticipate that engaging in morally desirable behaviours will make them feel good about themselves. In other words, to what extent does a sense of anticipated warm glow from acting sustainably actually predict future green behaviour, especiall...