Social proof is the tendency to use the actions of others to guide one’s behaviour (Cialdini, 1993). People imitate the actions of others in order to satisfy their need to behave in an appropriate way (Deutsch & Gerard, 1955). Effects of social proof can occur under time pressure or when a person is distracted (Huh et al., 2014), supporting that they are based on simple mental short-cuts or heuristics. Social proof effects increase when: there is uncertainty about the correct behaviour for the situation (Baron et al., 1996), a large group is establishing the norm for behaviour (Milgram et al., 1969), and the group members are similar to the individual (Platow et al., 2005). Social proof can influence behaviours from the mundane, such as littering, to the more serious outcomes, like helping to enhance treatments the help with phobias and addictions. Savvy marketers employ social media influencers and orchestrate online reviews to increase sales.