What psychological principle makes people more likely to buy something if they see others also buying it?

The psychological principle that makes people more likely to buy something if they see others also buying it is Social Proof. Social Proof is a powerful psychological phenomenon and a type of conformity where individuals assume the actions of others in an attempt to reflect correct behavior for a given situation. This means that when people are uncertain about a product's quality, value, or the appropriateness of a purchase decision, they look to the actions and opinions of others as a guide. The more people observed buying or endorsing a product, the stronger the signal that the product is desirable, high-quality, or the correct choice. This creates a mental shortcut for the observing individual, reducing their perceived risk associated with the purchase and validating the potential decision. For instance, seeing a product labeled as a "bestseller" or noting a large number of positive customer reviews on an e-commerce site signals that many others have already made the purchase and found it satisfactory, thereby increasing the likelihood that a new customer will also buy it. This principle operates on the fundamental human tendency to trust the collective wisdom of the crowd, believing that if many people are doing something, it must be a correct or optimal action.