During the primary elections in the Spring, a friend of mine was walking around the neighborhood asking people to put a yard sign on their lawn in support of his favored candidate. By the end of the week, most of the neighborhood was dotted with yard signs. He was doing this 7 or 8 months before the general election. Why?
Yard signs are good advertising, of course. If a candidates name is all over the place, then it can increase people's sense of the popularity of that candidate. It is a standard concept in advertising that reach and frequency increase people's awareness of a candidate (or brand of products for that matter).
But, putting up a yard sign can have an affect on the person who puts out the yard sign as well. In particular, getting someone to put up a yard sign may solidify their commitment to a particular candidate. This process is called escalation of commitment.
Classic psychological studies have demonstrated that if you ask people to perform a simple task that they are unlikely to refuse, you can increase their chance of agreeing to a related task that requires a higher investment of time or money later. For example, asking a store owner to put a sign in a window to advertise an upcoming charity auction will increase the chances that they will later agree to donate something to that auction. Asking a patron of a museum to wear a button declaring their support for the museum will increase the likelihood that they will later give a donation to the museum.
Those people who put up a yard sign are starting on the road of an escalation of commitment. By putting up the sign, the person is showing a public commitment to a candidate. The candidate is now part of that person's ingroup. So, this person is likely to interpret this candidate's actions favorably over the course of the campaign. This person is now more likely to talk about the candidate in conversations. Elections are long processes, and people may change their minds about candidates over time. Starting with this initial commitment of a yard sign makes a person less likely to change their mind about this candidate later. This individual might even be more likely to make donations to that candidate later.
So, the power of yard signs goes beyond the impact that the sign may have on others. The act of putting a sign on your yard will increase your own commitment to the candidate or cause that you are advertising.