Which psychological principle do retail recommendation systems like Amazon's 'Customers who bought this also bought' most heavily exploit?

Modern e-commerce platforms leverage sophisticated recommendation algorithms to boost sales and enhance customer experience. These systems analyze vast amounts of data including past purchases, browsing history, and demographic information to suggest products customers might want to buy. But which psychological principle drives the effectiveness of these systems the most? Test your knowledge of the intersection between retail technology and consumer psychology!

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Social proof (the tendency to view a behavior as more appropriate when others are doing it)
The IKEA effect (valuing products more when we've invested effort in them)
Loss aversion (preferring to avoid losses over acquiring equivalent gains)
The mere exposure effect (developing preference for things simply because we're familiar with them)