The endowment effect is a phenomenon from behavioural economics that states that people tend to value things they already own more highly than similar things they do not own. The effect was first described by Richard Thaler, a well-known behavioural economist. He showed that people tend to consider things they own to be more valuable than they objectively are. This means that they would demand higher compensation to sell something they own than if they were to buy something they do not own.

The possession effect has an impact on various areas, such as the purchase and sale of goods or the valuation of investments. Companies can utilise the effect by giving customers the feeling that they already own something or that it belongs to them, for example by offering them trial versions or samples. This can lead to customers identifying more strongly with the product and being more inclined to buy or keep it.