Angela Duckworth, PhD, is a 2013 MacArthur Fellow and professor of psychology at the University of Pennsylvania. She is the co-founder of the Character Lab, a nonprofit whose mission is to advance the science and practice of character development in children. She is the author of a new book, GRIT: The Power of Passion and Perseverance (Scribner). The opinions expressed in this commentary are solely those of the author.

(CNN) "What's wrong with millennials?" This is a question many older Americans are asking. Why do they keep changing their minds about what they want to do with their lives? Why does even a hint of critical feedback send them into a tailspin of self-doubt?

In a word, why don't they have more grit?

This last question is particularly important to me because I am a psychologist who studies grit. I define grit as passion and perseverance for long-term goals. It's what keeps us going when everything else makes it seem easier to give up. In my research, I find that how you score on my Grit Scale —a short survey of your current level of passion and perseverance—predicts achievement.

Grittier students are more likely to earn their diplomas, grittier teachers are more effective in the classroom, grittier soldiers are more likely to complete their training, and grittier salespeople are more likely to keep their jobs. The more challenging the domain, the more grit seems to matter.

I now have Grit Scale scores from thousands of American adults. My data provide a snapshot of grit across adulthood. And I've discovered a strikingly consistent pattern: grit and age go hand in hand. Sixty-somethings tend to be grittier, on average, than fifty-somethings, who are in turn grittier than forty-somethings, and so on.

So, why are millennials at the bottom of the heap in grit? There are two possible explanations. That's because the sixty-somethings I've surveyed differ from the twenty-somethings in two ways. One difference is that they grew up in the "Mad Men" era rather than the new millennium. But it's also true that they have more than twice as much life experience.

Do millennials lack grit because our culture devalues a work ethic?

Let's consider the first possibility and assume that older adults are grittier than their younger counterparts because in their formative years, they were shaped by different cultural forces. Back in the day, the story goes , you were expected to grow up to do one thing for a living and then retire. You were exhorted to work hard, and you were told that nothing in life comes easy. These cultural norms validated a solid work ethic and a single lifelong career.

If you're a baby boomer, chances are you agree with this explanation. I've lost count of the business leaders who've told me that their young employees are far less gritty than they themselves were at the same age. Millennials, they complain, sigh loudly when work needs to be taken home over the weekend, spend less time at the office than older colleagues with children at home, and are both puzzled and indignant when, after a few months on the job, they haven't been promoted.

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There is ample evidence that certain cultural attitudes have changed. We know this because there are surveys that have been given to young adults again and again over the decades. For instance, millennials are comparatively more likely to support same-sex marriage and the legalization of marijuana. They have less faith in institutions like our elected government.

Scientific evidence, however, fails to confirm what older business leaders believe about their younger employees. When it comes to differences in psychological attributes like grit, the evidence of generational difference is much less robust than you might think. Because I created the Grit Scale only about a decade ago, I can't directly test whether young adults growing up in earlier epochs would score higher than young adults today.

Photos: Millennials describe the 2016 election in 1 word Photos: Millennials describe the 2016 election in 1 word CNN's #campaigncamper traveled across the country to gauge the pulse of college students. The words they used to describe this election ranged from excitement to concern. Hide Caption 1 of 11 Photos: Millennials describe the 2016 election in 1 word Janelle Owusu from Georgia Tech in Atlanta describes the election as "ill-focused." She is personally concerned about health care. Hide Caption 2 of 11 Photos: Millennials describe the 2016 election in 1 word This is Gina from Vanderbilt University in Nashville, Tennessee. Her one word for the election? "Fierce." She's mostly concerned with the rising cost of education and "the burden it puts on us entering the workforce." Hide Caption 3 of 11 Photos: Millennials describe the 2016 election in 1 word This student from Georgia Tech isn't feeling great about the 2016 prospects: "Help." Hide Caption 4 of 11 Photos: Millennials describe the 2016 election in 1 word This is Brooke, she's a freshman at the University of Florida in Gainesville. She says she's afraid for this country when "non-whiteness is criminalized." Hide Caption 5 of 11 Photos: Millennials describe the 2016 election in 1 word Hunter Clary from the University of Florida notes this election is "important." Specifically, the national debt is a big concern for him. Hide Caption 6 of 11 Photos: Millennials describe the 2016 election in 1 word Hugh Wood III from the College of Charleston in South Carolina can't get enough of this election cycle. He describes it as "captivating." Hide Caption 7 of 11 Photos: Millennials describe the 2016 election in 1 word College of Charleston student Clay Koch is feeling "confident" about 2016. Hide Caption 8 of 11 Photos: Millennials describe the 2016 election in 1 word Sanjana is an International Affairs major from Georgia Tech. She describes the elections as being similar to a reality show such as "Keeping Up with the Kardashians." Hide Caption 9 of 11 Photos: Millennials describe the 2016 election in 1 word How does Aradhana from Georgia Tech describe this election cycle? "Turnt." Hide Caption 10 of 11 Photos: Millennials describe the 2016 election in 1 word Georgia Tech's Robert Bland just asks for one thing in this election season, "I just want some honesty!" Hide Caption 11 of 11

Be that as it may, plenty of archival data has been collected on psychological traits highly correlated with grit, like conscientiousness. Some of these studies have found absolutely no differences in conscientiousness. Others, such as one study of Dutch college students, suggests that those entering university 30 years ago were, if anything, slightly less conscientious than those entering just a decade ago.

Or do baby boomers have more grit because they have more life experience?

Now let's test our second hypothesis for why millennials might have less grit: the possibility that we develop passion and perseverance with life experience. This makes good intuitive sense. After all, as the years go by, we get to know ourselves better. We figure out what we're interested in. With practice, we learn that trying to do something hard, and failing at it, isn't the end of the world. As a friend who lost a job once told me, there's nothing like getting fired to make you realize how bumpy the road of life is, and how surprisingly capable we are of getting up after falling down.

In longitudinal studies conducted by different scientists on different people over varying swathes of the lifespan, the consistent pattern is that positive psychological traits get better over time. As our knees and hips and eyesight deteriorate, we become more dependable, less impulsive, kinder, and less moody. Psychologists call this the maturity principle

My own life experience fits this principle to a T. I spent my twenties skipping around from career to career. I worked very briefly in the White House, writing speeches. I spent a year doing management consulting. Another year, I helped run a nonprofit website for parents. I taught math in urban public schools. At one point, I thought seriously about starting a charter school. Betwixt and between, I earned degrees in neurobiology and neuroscience.

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By the time I turned 30, I realized that not having a clear direction in life was tremendously unsatisfying. After much soul searching (our living room couch nearly turned crusty with the salt of my tears) I turned to my husband and declared I was going to graduate school.

I wanted to become a psychologist so that I could understand, finally, why some people end up successful in life and why others in identical circumstances do not. And that's exactly what I did. When I look back on that decision and the work I have done since then, I realize that in an important sense, knowing with certainty why older adults are grittier than their younger counterparts is less important than simply acknowledging that they are.

This knowledge could revolutionize how we think about older workers in what Anne-Marie Slaughter calls "phase three." Slightly slower-moving silverbacks may have more passion and perseverance, not to mention perspective expertise and emotional equanimity , than younger whippersnappers. If the quality and quantity of continuous effort toward goals matters as much as I think it does, we may actually get more productive, not less, as we get older—even if we can't pull all-nighters like we used to.

In the end, grit and growth go hand in hand

Even more encouraging—whether because of life experience or cultural changes—age trends in grit affirm that our character is never entirely fixed. Who we become in life is not entirely determined by our genes. Sure, the DNA we inherited from our parents inclines us toward more grit or less, just like it influences everything else about us from our waistline to our risk for skin cancer. But like those traits, grit is also influenced by what happens to us, and what we make happen.

I am almost exactly the same age as the writer Pamela Druckerman, who observed shortly before her forty-fourth birthday that "the biggest transition of the 40s is realizing that we've actually, improbably, managed to learn and grow a bit." As an example, she points out that "soul mate" is a title earned, not discovered. I'd say the same of career callings.

So what's wrong with millennials? Nothing. They just haven't grown up. Yet.