'Other people don't drop litter?! That means I shouldn't either!'
This sign is a nifty example of people in power using the persuasive tactics to get us to do what they want us to do. In this case, local authorities are telling us not to drop litter, while also telling us that other people don't either. But why should telling us that other people don't make us less likely to drop litter?
The persuasive tactic used here is called social consensus. The idea is that the more it appears everyone else it doing 'it', the more likely others are to join in and agree. Deutsch and Gerard (1955) propose that social consensus engages two psychological processes that promote conformity. Firstly, they suggest that social consensus provides information or social proof about what to do and think via the rule of 'if other people are doing it, it must be correct'. Secondly, social consensus provides social pressures (normative influences) to agree to or to go along with what everyone else is doing. In this case, you may still not believe that dropping litter is a bad thing, but you don't drop litter because you don't want to be the odd one out.
Croson, Handy and Shang (2009) demonstrate how social consensus can be used to influence people's giving behaviour. In their study, experimenters gave participants a scenario in which they had to pretend that they had called up a radio station to pledge $25 to charity, and were then told by the telephone handler that another person had given $10 or $50. Participants were then asked how much they thought the average listener had given that year (descriptive norms) and how much they would contribute in a subsequent year (giving). They found that people in the high social information group ($50) believed the descriptive social norm average (donations given by an average donator) to be higher than in the low social information group ($10). They also found that those in the high social information group were more likely to contribute a higher amount of money in the subsequent year than those in the low social information group, as shown in figure 1. This suggests that people are influenced by knowledge of what other people are doing, and change their behaviour to fit in. In this experiment, participants changed their giving behaviour based on how much they thought other people were giving.
Croson, J., Handy, F., & Shang, J. (2009). Keeping up with the Joneses: The relationship of perceived descriptive social norms, social information and charitable giving. Nonprofit Management and Leadership, 19, 467-489.