  The conflict between pro-self and pro-social behaviour is at the core of many
key problems of our time, as, for example, the reduction of air pollution and
the redistribution of scarce resources. For the well-being of our societies, it
is thus crucial to find mechanisms to promote pro-social choices over egoistic
ones. Particularly important, because cheap and easy to implement, are those
mechanisms that can change people's behaviour without forbidding any options or
significantly changing their economic incentives, the so-called "nudges".
Previous research has found that moral nudges (e.g., making norms salient) can
promote pro-social behaviour. However, little is known about whether their
effect persists over time and spills across context. This question is key in
light of research showing that pro-social actions are often followed by selfish
actions, thus suggesting that some moral manipulations may backfire. Here we
present a class of simple moral nudges that have a great positive impact on
pro-sociality. In Studies 1-4 (total N = 1,400), we use economic games to
demonstrate that asking subjects to self-report "what they think is the morally
right thing to do" does not only increase pro-sociality in the choice
immediately after, but also in subsequent choices, and even when the social
context changes. In Study 5, we explore whether moral nudges promote charity
donations to humanitarian organisations in a large (N=1,800) crowdfunding
campaign. We find that, in this context, moral nudges increase donations by
about 44 percent.
