The market’s resilience over the past year might be due to an investor perception that they are “playing with house money,” according to a Barron’s article.

The article explains that this human tendency is rooted in behavioral finance research findings showing how the human brain suffers more from loss than it feels pleasure from the same amount of gain. Nicholas Colas, co-founder of firm DataTrek, says this runs counter to much of classical economic thought. In the current market environment, he says, it seems that investors are more willing to take risks with what they view as “house money”—the big gains they’ve accumulated over the long bull run.

“This pertains more to retail investors, but it clearly seeps over to professionals as well,” the article argues. “Faith-based action is a lot easier to take when you are already sitting on a lot of gains.” According to Colas, it’s less scary to buy market dips when an investor thinks he’s using house money, but contends, “Once the money is in your pocket, it is yours, and all future decisions should resemble those made with your capital. But humans are funny that way.”

Colas says that the “house money” mentality will persist throughout 2018 for U.S. stocks, but will end at some point—”trees don’t grow to the sky. But you don’t want to bet against the ‘house’ yet.”