cognitive bias

You value your own possessions more than the market does.

In a 1990 experiment, participants demanded more money to sell a mug they were given than they were willing to pay to acquire it.

Kahneman et al., 1990

You are biased toward what you already own simply because it is yours.

Where this comes from

Study: Thaler, 1980, Journal of Economic Behavior and Organization
What the check found: The endowment effect was indeed discussed by Thaler in 1980, but the specific mug experiment cited occurred in a 1990 study by Kahneman et al.
Confidence: Solid, with limits

One of these every day, and every one names the study it came from. When a finding is contested, it says so.