In a recent interview with Barron’s, behavioral economist and Nobel Laureate Richard Thaler said that although investors should sell their losing stocks at year-end, they resist doing so.

Thaler said, “The most common mistake at the end of the year has an interesting behavioral twist, which is the failure to sell losers. We all know that if you have a choice between selling a winner and a loser, it’s a good idea to sell a loser,” adding that it’s better from a tax perspective since investors can use losses to offset capital gains.

But he explains that investors are reluctant to take losses, since it requires them to admit to the loss. He says, “when you sell a stock that’s gone down, you have to declare it both on your tax return—but that should be a plus—and you have to declare it to yourself, maybe to your spouse.” Instead, he says, when investors hold onto losing stocks, they can ignore the loss.