Finding mechanisms to promote pro-social behaviour is fundamental for the well-being of our societies and is more urgent than ever in a time of key global challenges such as resource conservation, climate change, and social inequalities1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23. Among these mechanisms, especially important, because easy and cheap to implement, are the so-called nudges, that is, mechanisms that can promote pro-social behaviour, without changing people’s action space and their economic incentives24,25.

Here we focused on moral nudges. In our experiments, participants had to decide between a pro-self and a pro-social course of action, but, before making their actual choice, they were asked what they think is or what they think others consider to be the morally right thing to do. Our data show that these moral nudges have a positive impact on altruistic (Study 1) and cooperative (Study 2) behaviour in economic games. This positive impact is not limited to the choice made immediately after the nudge but persists to a second interaction (Study 3). Moreover, a significant proportion of this effect even spills across contexts: nudging altruism in a first interaction promotes cooperation in a second, non-nudged, interaction, and nudging cooperation in a first interaction promotes altruism in a second, non-nudged, interaction (Study 4). Furthermore, this positive impact is not limited to economic games: nudging altruistic behaviour can be used to successfully increase charity donations (Study 5).

This is an important improvement over previous work. Although it has been repeatedly shown that people tend to follow what they perceive to be the norm in a given context26,27,28,29,30,31,32,33,34,35, there is still a lot of uncertainty about the effects of moral nudges, especially concerning their spillover effects across contexts. This question is particularly important in light of previous literature offering evidence of both positive46,47,48,49,50 and negative spillovers. In particular, negative spillovers are reported in the literature on moral cleansing, which shows that moral actions are often followed by compensatory immoral actions, suggesting that some moral nudges may backfire38,39,40,41,42. Our results show that this is not the case, at least for the moral nudges, the window of time, and the economic decisions considered in this paper. Not only do our moral nudges not backfire, but they even persist over time and spill across contexts.

To the best of our knowledge, only two papers have previously considered moral nudges of the type we have used in this work. Brañas-Garza74 found that telling DG dictators to “note that he relies on you” increases donations in the laboratory. We go beyond the results of this paper along several dimensions, as we explore the effect of moral nudges, both in the DG and in the PD, we look at its persistence over interactions and across contexts; and we also look at its effect on crowdfunding charity donations to humanitarian organisations. Dal Bó and Dal Bó33, instead, found that reminding the “Golden Rule” in the middle of a repeated PD with random re-matching in groups of eight people and feedback after each round increases cooperation for a few rounds before eventually vanishing. However, the fact that the interaction is repeated in small groups with feedback after each round implies that the persistence of the increase in cooperation cannot be attributed to the persistence of the nudge with certainty. For example, it could be driven by reputation, if, for example, subjects play a Tit-for-Tat strategy conditional on the strategy they encountered in the previous interaction. We thus go much beyond the results by Dal Bó and Dal Bó33, in several ways: we look at the effect of moral nudges also in the DG, at its persistence across interactions while giving no feedback about the previous interaction (and therefore ruling out the potential confound of conditional strategies), at its persistence across contexts, and at its effect on crowdfunding charity donations to humanitarian organisations.

Our results have potentially impactful applications for policy design. Several mechanisms to promote pro-social behaviour have been explored in previous work, including giving material reward, such as a t-shirt or a mug, in exchange to a pro-social action75,76, augmenting donations using matching77,78, making people’s actions observable by others79,80, informing people about the actions of others to make a social norm salient81,82, giving gifts while asking for a donation, in order to induce a reciprocal feeling of obligation83,84, soliciting people to cooperate85,86, and many others (see Kraft-Todd et al.6 for a review). Our results expand this list of mechanisms significantly. We show that moral nudges not only increase pro-social behaviour in the choice immediately after the nudge, but that their effect also persists over time and spills across contexts. Furthermore, compared to these studies, moral nudges have the crucial upside that they are extremely easy and cheap to implement, while still being very effective. For example, Landry et al.87 entered people who donated to a fund-raiser into a lottery to win a personal cash prize, and found a 47% increase in the amount of money raised relative to the control condition with no lottery. Our Study 5 shows that moral nudges produce – at least in the domain of crowdfunding donations in which we used them – essentially the same increase (44%), but are free of cost, that is, they allow to save the money for the prize of the lottery and the time needed to organise it and conduct it. This implies that our moral nudges might be a promising tool for crowdfunding charity donations to humanitarian organisations. This is an especially interesting application, in light of the emergence of crowdfunding websites such as gofundme.com, kickstarter.com, and donationcrowdfunding.com. Of course, the fact that our moral nudges work well in increasing donations for crowdfunding does not imply that they also work well in promoting everyday pro-social behaviour in other parts of life. Exploring the external validity of moral nudges is an important direction for future work.

Our findings also have some limitations. First of all, they show that moral nudges increase pro-social behaviour, persist over time, and spill across contexts, but only within the window of time and choices considered in this paper, that is, two interactions (DG and PD) within the same experiment (i.e., separated by a few minutes). Future work should thus consider interactions separated by a longer time span and a greater number of interactions to explore the boundaries of the effectiveness of moral nudges. Furthermore, it is to be expected that the effectiveness of a moral nudge will not only depend on the particular type of nudge used, but also on the underlying social interactions. Thus, more generally, classifying moral nudges in terms of their effectiveness in different social interactions is an important direction for future research, with many applications to policy design. Second, especially Studies 1–4 use very small stakes, thus it is not clear if and how our results generalise to larger stakes. However, we believe that this is not a major limitation because previous literature found that people’s behaviour in AMT experiments using small stakes is essentially equivalent to behaviour in experiments using standard lab stakes both in the DG and in the PD88,89,90 (See ref.90 for a review). Additionally, our first session of Study 5 uses larger stakes and finds results that are in line with the other studies. Here the pro-social choice is relatively expensive, because we asked participants to donate the whole participation fee (50 cents), and we do so at the very end of the survey. This adds to the monetary cost of the altruistic action, a non-monetary utility associated with the sense of deservedness of the participation fee, which decreases the attitude to donate91. Even in this “extreme” case, we find a significant effect of the moral nudge (coeff = 4.899, p = 0.018). Third, our experiments do not allow us to draw conclusions about the psychological underpinnings of our effects. Do people change their behaviour because they are motivated by a sincere desire of doing what they think is the morally right thing to do? Or, alternatively, do they change their behaviour because the experimental setting makes it salient that there is an appropriate thing to do? Indeed, the effect of the moral nudges in our experiments might have been driven by an experimenter demand effect92. This is not an issue, however, because susceptibility to experimenter demand effects can be thought of as an instance of norm-sensitivity93. Another potential explanation for our results is that they are driven by reputation: it could be that moral nudges make reputation salient, and that this ultimately has the effect of increasing pro-sociality. At this stage, our findings only allow us to conclude that moral nudges can be used effectively to increase pro-social behaviour (cf. especially Study 5). We named them “moral nudges” because they literally refer to morality, but this name is not meant to refer to the underlying psychological mechanism that makes these nudges effective. Understanding the psychological underpinnings of our effect is an interesting direction for future research. Fourth, our experiment does not control for subjects’ level of experience in similar AMT experiments. As noted in previous research70, experienced subjects may decrease effect sizes due to the fact that they are more resistant to experimental manipulations. This implies that the size of the positive effect of moral nudges reported in this work is likely to be a lower bound of its true size. Fifth, Study 5 shows that moral nudges increase charitable donations in the domain of gains, that is, in cases in which participants are asked to donate a sum of money that they have just earned. Future work should explore whether the same nudges work also in the domain of losses, that is, in situations in which participants are nudged to donate a sum of money that they already possess. Sixth, we analysed the effect on pro-sociality of only two moral nudges: a nudge meant to make the personal norm94 salient (“what do you think is the morally right thing to do?”) and a nudge meant to make the descriptive norm28 salient (“what do you think your society considers to be the morally right thing to do?”). Future research should extend this approach by investigating other ways to activate the same type of norms or even other norms, as for example, prescriptive norms (“what do you think your society thinks that you should do?”) or proscriptive norms (“what do you think your society thinks that you should not do?”).

Finally, our data also has theoretical applications. We have shown that people’s behaviour in one-shot anonymous interactions can be influenced by moral nudges. This is in contrast to the predictions of the standard models proposed by behavioural economists to explain human pro-sociality in one-shot anonymous interactions. According to these models, people have preferences for minimising social inequities95,96, or maximising social welfare97, or a combination of both97. These preferences are described in terms of the economic outcomes of the available actions and parameters representing the extent to which an individual cares about equity and/or efficiency. Therefore, these models predict that people should be insensitive to cues about the rightness or the wrongness of an action, because these cues do not change the economic outcomes of the available actions. This prediction is not satisfied by our data: cues about what is right in a given situation can significantly change people’s behaviour. Thus, our results highlight the necessity of going beyond outcome-based preferences, and speak in support of incorporating social norms into people’s preferences. This is in line with an emerging strand of research suggesting that people, in their decision-making, strive for balance between maximising their material payoff and doing what they think is the morally right thing to do34,35,93,98,99,100,101,102.

In sum, we have presented a novel technique to increase pro-social behaviour. Asking people “what’s the morally right thing to do?” before they make a choice, makes the morality of an action salient. This promotes pro-social behaviour, and this positive effect persists to a subsequent choice, even when the social context changes. Moreover, it can successfully be used to increase donations to charitable organizations in crowdfunding campaigns. These results hold within the natural domain of our experiments. Future research should explore potential generalisations, with a particular focus to larger stakes, longer time spans and a greater number of interactions.