What is the Law of Reciprocity?
The law of reciprocity is just a fancy name for a very universal human response to receiving an unsolicited gift.
If you give someone a gift with no strings attached, that person will most likely feel obligated to repay you in some way at some future point.
Naturally there is a small percentage of people in the world who don't feel any sense of obligation to anyone, and society often labels them 'takers', as opposed to givers. But they are definitely in the minority.
For the average person, their sense of balance or fair play is affected by the gift, and they feel a need to redress the balance by giving something back.
How does this apply to promotional products?
Suppose you give your customers a promotional fridge magnet with their first purchase of one of your products. You point out that there is an incentive program offered only to magnet holders (click here for more details) and you emphasize the importance of keeping it handy for future reference.
You customer is pleased for two reasons:
You gave her a free gift that is useful in its own right (for holding notes to the fridge, etc.).
You are offering additional future value to her as a magnet recipient.
Instead of just asking for her future business, you've given her something of value, with additional rewards included if she does future business with you.
Have you increased the likelihood of her doing repeat business with you? Definitely.
Not only have you come across as a nice person, but you've invoked her sense of obligation with your gift, PLUS you've answered her unconscious "What's in it for me?" question with regard to making future purchases from you.
To find out more about the law of reciprocity, read Robert Cialdini's brilliant book Influence, The Psychology of Persuasion, which includes some amazing real-life examples of how it works.