You'll be very sorry.

• Author: Robinson, David
• Position: Economically Speaking

I met a man waiting for chemotherapy who had made a choice he regretted. He had been planning and saving for a trip with his family for many years. His doctor told him he should begin treatment immediately, but he felt that if he didn't make that trip he would regret it for the rest of his life. He made the trip, and it was everything he had hoped. When he returned, the doctor told him that his disease had progressed to the point that a cure was unlikely.

Avoiding regret plays a big role in many of our decisions, and it should. We look ahead and worry about what we will feel when we look back.

Economists once thought of regret as a vague emotional problem rather than a part of normal decision-making. As a result, our economic models couldn't explain what real people were doing in many situations. Then in 1982, a couple of economists, Graham Loomes and Robert Sugden, came up with what they called an "alternative theory of rational choice." It was really just a trick that let them calculate how much regret people might feel. Trick or not, the result seemed to explain a lot of behaviour that economists had previously found inexplicable.

Economists had missed the way regret affects us, but popular culture hadn't. Country and western songs ooze regret. Kris Kristofferson sings, "I'd rather be sorry for something I've done than for something that I didn't do." Marlon Brando's Academy Award for the 1954 movie On the Waterfront turns on the line, "I coulda been a contender." In Casablanca, Rick says, "If that plane leaves the ground and you're not with him, you'll regret it. Maybe not today. Maybe not tomorrow. But soon, and for the rest of your life."

Time travel movies make regrets the theme of high art. Think about Marty McFly in Back to the Future, the highest-grossing film of 1985. Or Terminator (1984), Peggy Sue Got Married (a movie nominated for three Academy Awards in 1986). Or Groundhog Day (1993), Twelve Monkeys (1995), Source Code (2011), Men in Black 3 (2012), X-Men: Days of Future Past (2014), or Edge of Tomorrow (2014). These are all about undoing decisions that someone regrets.

There are websites that describe the seven regrets that people have at the end of life. It's interesting that the regrets are all pretty...