The endowment effect
We value what we own more than the identical thing we don't.
Richard Thaler's finding. People given a coffee mug demand roughly twice as much to sell it as people offered the same mug demand to buy it. The mug didn't change. The ownership did. The value attached to ownership is real and large.
The bias is a consequence of loss aversion. Selling feels like a loss. Buying feels like a gain. Losses are weighted more heavily than gains, so the threshold is asymmetric.
For operators, the endowment effect shows up in pricing (customers value features they have more than features they'd gain), in negotiations (sellers always seem to ask more than the buyer's max), and in personal investment portfolios (we hold positions longer than we'd buy them).
Examples in the wild
When a product takes away a feature, customer backlash is much larger than the benefit when that feature was first added. The endowment was real.
Holding losing positions long past sense partly reflects endowment. The position is yours; selling feels like losing it. Honest re-examination requires forcing yourself to imagine you don't own it.
Decluttering is hard because of endowment. Each item you'd never buy now feels like a loss to give up. The trick is asking 'would I buy this today' instead of 'should I keep this.'
The endowment effect is one of the mental models we apply through real cases inside the Pareto MBA — a part-time program for professionals who want to think clearly about business.