The reason a return window changes the decision to keep

Extending a return period reduces returns rather than raising them. The buyer has more opportunity to send the item back and takes it less often, because during the extra weeks the item is in their house, in use, and increasingly assessed as something they own rather than something they are evaluating.

What everyone sees

A generous window is read as a concession that transfers risk to the seller, so longer periods should mean more returns. That model treats the decision as fixed and the window as a constraint on acting. The decision is not fixed; it is made repeatedly, and each day of possession shifts the terms on which it is made.

What is actually happening

Kahneman, Knetsch and Thaler documented the endowment effect experimentally: the compensation people demand to give up an item they hold exceeds what they would pay to acquire it, so possession itself changes the valuation. Reb and Connolly then separated legal ownership from the subjective feeling of possession and found the effect tracked the feeling, which accumulates with handling and time held rather than with title. A longer window therefore delivers exactly what the mechanism requires: more days in which the item is handled and integrated, after which returning it registers as giving something up instead of declining to buy.

Why it stays hidden

The inversion hides because sellers reason about the policy and buyers experience the object. Policy analysis asks how much opportunity to return has been granted; the buyer is not exercising an option but living with a thing. The extra weeks are read by the seller as risk and by the buyer as ownership, and only the second reading affects behaviour.