In his book Contagious: Why Things Catch On Jonah Berger writes about what he calls 'behavioural residue'.

Behavioural residue refers to a publicly visible indication that a person has engaged with your product, organisation or cause. Seeing examples of behavioural residue creates what Berger calls 'social proof'. When we see others, especially others who we admire, are engaged in a behaviour such as wearing a badge to promote a particular cause it generates 'social proof' and makes us more likely to also support that cause or to wear a badge.

Berger gives an example of using behavioural residue to generate social proof and increase voter turnout.

It’s hard to get people to turn out to vote...Unless you actually happen to see all the people who go to the polls, you have no idea how many other people decided voting was worth the effort...But in the 1980s election officials came up with a nice way to make voting more observable: the “I Voted” sticker. Simple enough, but by creating behavioural residue, the sticker made the private act of voting much more public, even after people left the polling station. It provided a ready reminder that today is the day to vote, others are doing it, and you should too.

Behavioural residue can be generated by printed promotional products and there is a huge range of choice out there including badges, magnets, and key rings. So which should you choose and are some better than others? Berger explains:

Some of these giveaways provide better behavioural residue than others. Giving away a makeup carrying case is fine, but women usually apply makeup in the privacy of their bathrooms, so it doesn’t make the brand that observable.

Badges are an ideal way to create behavioural residue and communicate social proof as they are highly visible and public. Badges are also strongly associated with the particular person who wears it and so enables you to piggy back on that persons social influence among their peer group.