Why does receiving a free sample often lead a person to buy the full-size product?

Answer: Receiving a free sample makes people feel they should return the favor by buying.

Receiving a free sample makes people feel they should return the favor by buying. ✓ — Correct. A free sample creates a small sense of debt: the recipient feels a social pull to return the favor, and buying is an easy way to do that. The sample also lets people try the product, lowers the risk of a disappointing purchase, and makes the brand more familiar. These effects work together, so the cost of a small sample can be repaid by a sale.

People buy because they enjoy collecting free items, and the sample starts a habit of seeking more. — Not quite. A free item does attract people, and some do enjoy collecting samples. But collecting is not the main reason a sample leads to a purchase. Companies use the sample to start a relationship: the gift creates a pull to reciprocate, and the trial reduces doubt about the product.

The sample is better than the full product, so people buy hoping the full product will match it. — Not quite. The sample is normally the same product in a smaller amount, not a better version. It can seem better because it is free, comes in a small portion, and is tried with no cost or commitment. Buying the full product is not a way to secure special sample-only quality; the purchase is driven more by obligation, trial, and reduced risk.