I have it straight from the doctor's mouth: No more eighth-place medals for kids.

We talk about grit all the time here at Spartan Race but Dr. Angela Lee Duckworth is the woman who literally wrote the book about it (or at least she will be when Grit: The Power of Passion and Perseverance is published May 3 by Scribner). You probably know her from articles in The New York Times and her TED Talk. She is THE expert on the subject of grit.

I think Angela was testing me from the start: she said to meet her at her office to conduct the interview, but she didn't say when. So I camped there for 40 hours until she showed up. We're not beginners at this grit thing.

It was amazing to talk to the woman whose work has inspired me so much. Angela is a professor of psychology at the University of Pennsylvania, the founder of Duckworth Labs and a MacArthur Fellow (sometimes called the "genius grant"). You'd think with all of these credentials, Angela would put a lot of emphasis on smarts. Not so.

Angela says that the thing that predicts success far more than intelligence or a privileged upbringing is GRIT, which she defines as sustaining interest, passion and persistence for a goal over the long term. She's been studying this for more than a decade and doesn't think she'll ever be done--there is so much more to study and she feels she's only brushed the surface. Talk about grit.

At Spartan, we believe grit can be developed or taught, but I asked Angela if she found this to be true in her work--just to make sure. "Like any other human characteristic there's a genetic component, but it is also environmentally determined," she said. In other words, yes, you can learn grit.

I think I'm a naturally gritty guy, but some people see that as stubbornness. I wondered about people who keep plugging on even after they've lost all hope. Is that foolishness or grit? Angela said, "It may look on the outside that the person has lost all hope, but on the inside they've never lost that spark. That's what makes them gritty."

I got pretty excited listening to Angela because I have a lot of the characteristics she was describing. But I was a little surprised by what she had to say about being able to abstain from instant gratification.

"Grit and self-control are related, but they're not the same thing," Angela said.

You might have heard of the self-control test that Angela has given kids she's studied. She gives them a choice between a small pile of their favorite treat, which they can have immediately, or a huge pile they can have if they wait 10 minutes. Kids almost always say they want to wait for the big pile, but that's when reality kicks in--can they really wait?

I know the jokes about "not trying this at home," but I tried it with my son, Jack, who was 6 at the time. I gave him a scoop of ice cream (something I almost never give him) and told him if he waited five minutes he could have two scoops. Jack looked at me and thought a moment. Then he asked, "How long do I have to wait to get 15 scoops?"

Isn't that the greatest answer ever?

Angela was impressed, but she pointed out that although people who are good at overcoming temptation tend to be grittier, it's wrong to think high achievers have great self-control. "What's true of the most eminent individuals in society is that they have the capacity for zest and sustained hard labor," she said.

For Angela, it's not self-control that makes gritty people so awesome. It's their ability to persevere and maintain hope in spite of setbacks, invisible progress and even their own poor judgment. How are they able to do this? One word: optimism.

Gritty people can look at a situation, ignore 90 percent of things they can't change and instead focus on the 10 percent they can. It's like a booster shot against depression (which is the opposite mindset). Everyone thinks I'm crazy because I focus on that 10 percent, but Angela said that's because they're looking at the situation from the outside in.

So how do we become gritty if we're not? Teaching optimism is a good start, but just as important is a growth mindset. Stanford psychologist Carol Dweck defined a growth mindset as the belief that your abilities are changeable. Too many people think they can't climb a rope or scale a wall because they've never been athletic, but believing you can learn and grow builds new skills, and grit. And not just in childhood--adults can learn new abilities, too. Angela said science proves this over and over again.

So what tools do we need to build a growth mindset and optimism? Angela said one of the best tools is a culture of grit.

That's what we teach at Spartan. At your first race, you learn that you can do things you didn't think you could do. (And you really learn it by watching someone's grandmother or a veteran who is missing limbs complete the obstacles.) So you push yourself, and when you get to the finish line, you get that rush of endorphins--you've done it! And you're willing to push your ability a little more the next time because your brain is looking forward to that reward and you're surrounded by people who are applauding your success. This is equally true in life and in business.

The culture of grit is another reason we have Spartan Kids races. Sure, we want to show them how much fun it is to get off the couch, but we also want to teach them that getting to the finish line, as Angela put it, isn't "graceful and effortless." We need kids to see that they can learn and conquer. We do them no favors when they don't see the thousands of hours of hard work that goes into the things they love. They need to know it's OK to work hard. It's OK to fail. It's OK to get dirty. It's OK to do it over and over again as they try to get a little bit better--and a little bit grittier--every time.