What Is the Endowment Effect?
The endowment effect, demonstrated in classic experiments by Kahneman, Knetsch, and Thaler (1990), shows that people demand roughly 2 to 3 times more to give up an object they own than they would pay to acquire the same object. In the original mug experiment, owners valued their mugs at about $7 on average, while buyers were willing to pay only about $3. This gap persists even for financial assets, where ownership should carry no sentimental premium.
Impact on Investment Portfolios
The endowment effect causes investors to overvalue stocks they already hold, making them reluctant to sell even when better opportunities exist. A study of Finnish investors found that stockholders required a 15-20% premium above market price before they would consider selling inherited shares. This bias contributes to under-diversification: employees who receive company stock through compensation plans hold an average of 40% of their retirement portfolio in that single stock, far exceeding prudent concentration limits.
Key Considerations
A useful mental exercise is the 'clean slate' test: if you did not already own this asset, would you buy it today at the current price? If the answer is no, the endowment effect is likely inflating your attachment. Systematic rebalancing on a fixed schedule (quarterly or annually) forces portfolio adjustments regardless of emotional attachment to individual holdings. For inherited or gifted securities, treat them as cash and evaluate whether they belong in your target allocation.