Recently I wrote about the role of reciprocity in the evolution of morality. Pretty much all social mammals understand give and take in a social environment. I also wrote about how George Lakoff identified the image of balancing the books or paying debts as essential to how we conceptualise morality. Obviously reciprocity lends itself to this debt and repayment model.
Here's the thing though. Marketing people have identified a cognitive bias that they exploit to lure you to buy stuff. If you give someone something for free, that person feels subtly indebted to you. They may not be consciously aware of the transactional nature of the exchange, but reciprocity goes very deep for social animals.
After you receive your free gift, or opportunities to taste something, your "on-sale" or "two-for-one" items, you can come away with a subtle sense of indebtedness. If you are in a shop that gives you something for free, you tend to feel an obligation to spend money there. That's why they have free tastings in supermarkets in the first place. It's not a compulsion and some people can walk away. It's more of a tiny voice in the back of your mind reminding you that fairness requires give and take.
It has become quite a common marketing trick on the internet too. I know some of my friends have been advised to take this approach, because, well it works. So I thought I'd point out the underlying psychology of the approach.
Rolf Dobelli lists reciprocity as a kind of cognitive bias in his book The Art of Thinking Clearly. Which surprised me at first. I thought of it as only as a virtue. But he points out that retaliation, revenge, feud, and vendetta are also forms of reciprocity. Thinking about stuff certainly makes life complicated. I sort of sympathise with people who just give up on it.