To influence our customers' decisions, we must understand what motivates them.
In 2004, Cialdini and Goldstein published a remarkable paper* which neatly explains three basic human goals that make us susceptible to persuasion. I'll briefly describe each below.
When making a buying decision, we need to feel confident that we're relying on accurate information and that buying is the correct thing to do.
This explains why Obedience to Authority is such a powerful persuasion technique: where better to get reliable information than from an expert?
Social Proof is also empowered by our desire for accuracy. When trying to decide how to act (especially in ambiguous circumstances) we look to see how others are acting. "If lots of people are doing it, it must be the right thing to do."
We're strongly motivated to form and maintain positive social relationships.
This is why, if you want to be persuasive, you've got to be likeable. The more we like someone, the stronger our motivation to cultivate a relationship with him or her.
The Rule of Reciprocation (our strong desire to repay favors) relies on this principle too. Reciprocation helps build trust with others, and pushes us towards equity in our relationships.
3. Favorable Self-Concept
We have a strong need to build and maintain a positive self-image. To do this, we're highly motivated to act consistently with our prior actions, statements and commitments.
This principle is especially strong where we've made our previous statement or commitment actively and publicly. See my post on Public Written Statements.
The above forces tend to be subtle, indirect and outside of our awareness... which of course makes us particularly vulnerable to them.
* Robert Cialdini and Noah J Goldstein. Social Influence: Compliance and Conformity. (2004) Annu. Rev. Psychol. 2004.55:591-621