Sunk costs do have a small effect—decision makers are biased in favor of their previous investments—but three other factors are more powerful. One is anticipated regret: will I be sorry that I didn’t give this another chance? The second is project completion: if I keep investing, I can finish the project. But the single most powerful factor is ego threat: if I don’t keep investing, I’ll look and feel like a fool. In response to ego threat, people invest more, hoping to turn the project into a success so they can prove to others—and themselves—that they were right all along.
