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Motivating individuals to choose energy from sustainable sources over conventionally produced power constitutes one of the biggest policy challenges for societies 1,2 . Here we present the results of a randomized controlled trial in Germany that tested the impact of default rules (that is, a type of 'nudging') on voluntary purchases of 'green' energy contracts that entirely stem from renewable resources. Setting the default choice to more expensive 'green' energy (that is, where consumers have to actively opt out if they do not want it) increased purchases of such nearly tenfold. Furthermore, county-level political preference for the green party uniquely predicted behaviour in the absence of the nudge, suggesting that default setting potentially overrules motivational aspects of green energy purchases. In follow-up experiments, we provide further evidence that the e ect does not seem to be driven by unawareness. Summarizing, the present research provides an example of using behavioural science 3-9 for climate change mitigation and shows alternatives to the use of subsidies or other economic incentives.As there is a broad consensus that consumer behaviour presents a viable opportunity for mitigating climate change, a core question for scientists and policymakers alike is how to effectively promote environmentally friendly behaviour on the large scale. Especially regarding climate change mitigation, scientists have tried to augment 'green' (that is, pro-environmental) behaviours of the general public by scaling up 'behavioural principles' derived from laboratory experiments 1-9 . Being coined 'nudging' 7,10 , this method has become popular among policymakers who are now capable of subtly steering individual decisions towards goals set by them without using coercion. However, even though research on 'nudges' and, in particular, non-binding defaults 11-13 has delivered promising results in various domains of social and economic policy, no research has thus far tested its efficacy in the case of energyrelated behaviours. 'Default nudges' typically manipulate initial choice-sets without infringing liberty or autonomy but still affect a person's decision. They have been characterized as the 'choice alternative a consumer receives if he/she does not explicitly specify otherwise' (p. 592; ref. 14), a notion that is widely agreed on in the literature 15-17 . Defaults allow a pronounced change in outcomes and help to promote policy goals such as sufficient organ availability, lack of old-age poverty, or-in our case-reduction of greenhouse gases. Probably the most widely known example of nudging by default-setting stems from organ donation 11 . A simple difference can be sufficient to provide different participation rates as shown by country-level correlations between locally set default rules and outcomes. Opt-out rules lead to high participation, whereas actively opting in keeps participation at low levels in the respective countries 11 .Here, we test whether this principle can also be applied to the domain of energy choic...

Altruistic punishment refers to the phenomenon that humans invest their own resources to redress norm violations without self‐interest involved. We address the question of who will intervene in situations that allow for altruistic punishment. We suggest that individual differences in a genuine concern for justice, as reflected by the personality trait of justice sensitivity, determine the experience of moral emotions in the face of injustice, which in turn trigger altruistic punishment. Results of two studies support the proposed mediation effect for other‐regarding justice sensitivity, even though an opportunity for compensation of the victim (Study 2) was offered as an alternative to punishment (Study 1). Furthermore, the mediation effect was observed when moral outrage was measured by means of quantified open statements (Study 1) and self‐report scales using discrete emotions (Study 2). The findings help to explain the psychological mechanisms underlying engagement in costly social sanctioning of norm violations.