(REUTERS/ Christian Hartmann)
"I found this lesson from the experts to be surprising."

"What do you regret when you look back on your life?"

That's what Karl Pillemer, professor of human development at Cornell University, founder and director of the Cornell Institute for Translational Research on Aging, and author of "30 Lessons for Living: Tried and True Advice from the Wisest Americans" and "30 Lessons for Loving: Advice from the Wisest Americans on Love, Relationships, and Marriage," asked hundreds of older people as part of Cornell University's Legacy Project.

As he writes on Quora, he was unprepared for the answer he so often received: "I wish I hadn't spent so much of my life worrying."

Several years ago, when Pillemer, a world-renowned gerontologist (someone who studies older people), met June Driscoll, a particularly spirited 90-year old woman in a nursing home, she told him, "It's my responsibility to be as happy as I can, right here, today."

That interaction inspired Pillemer to find out how a generation that's experienced the most loss, troubling historical events, and illness could possibly be the happiest and to pass this knowledge down to younger generations.

Pillemer launched the Legacy Project in 2004 and asked more than 1,500 Americans over 65 years of age about the most important lessons they learned over the course of their lives. In"30 Lessons for Living" he refers to his subjects as "the experts" because they hold more tried-and-true wisdom than any self-help book or pundit could possibly offer.

Pillemer writes on Quora that he had expected "big-ticket items" like affairs, bad business deals, or addiction as his experts' biggest regrets.

But over and over again he heard versions of "I would have spent less time worrying" and "I regret that I worried so much about everything."

"I found this lesson from the experts to be surprising," Pillemer writes in "30 Lessons for Living." "Given that they had lived through difficult historical periods and great personal tragedies, I thought they might endorse a certain level of worry."

Instead, Pillemer explains that the experts view time as one of our most precious resources, and worrying about events that may not occur or that we have no control over is an inexcusable waste of this resource.

"The key characteristic of worry, according to scientists who study it, is that it takes place in the absence of actual stressors; that is, we worry when there is actually nothing concrete to worry about," he writes on Quora. "This kind of worry — ruminating about possible bad things that may happen to us or our loved ones — is entirely different from concrete problem solving."