If it's possible for personality traits to be in fashion, grit is having a bit of a moment. Thanks to the work of University of Pennsylvania psychologist Angela Duckworth -- and her extremely popular TED talk -- the idea is in vogue that a healthy chunk of our success in life isn't about smarts or luck, but is instead about perseverance in the face of challenges.

Advocates of the idea are particularly excited about the findings because they believe that grit can be taught. And if mental toughness is both a big leg-up in life and readily learnable, that's great news for parents and educators who hope to give kids the best chances of success.

But, of course, as is the case with every trend, there comes a time when the hype gets a little too loud, motivating someone to take a closer, more critical look at whatever has gotten everyone so excited. It seems that moment has come for the boosters of grit.

What's the impact of grit on grades?

A group of psychologists based in London recently studied thousands of pairs of twins (some fraternal, some identical) to tease out the role grit plays in exam outcomes, publishing their results in the Journal of Personality and Social Psychology. Essentially, the scientists wanted to know how important grit is in getting good grades and whether the trait was mostly down to our experiences or our genes.

Here's the British Psychological Society's Research Digest blog summing up the results: "The main finding is that the participants' overall personality scores were related to about 6 percent of the variation seen in their exam performance. Grit specifically was related to just 0.5 percent of the differences seen in exam performance. Given the small size of this relationship, the researchers said, 'We believe that these results should warrant concern with the educational policy directives in the United States and the United Kingdom.'"

Grit, it seems, really doesn't play such a big role in success at school, and devoting too many resources to teaching it, the research team concluded, might be wasteful. Their skepticism about the value of teaching grit was reinforced by another finding, too. "How much grit the participants had was to a large extent inherited (about a third of the difference in grit scores were explained by genetic influences), and none of the difference in grit was explained by environmental factors that twin pairs shared, such as the way they were raised by their parents and the type of schooling they had," reports BPS.

In short, whatever differences in grit the team observed between siblings seemed to be caused by individual experience, not the school or household where they were raised. The current vogue for teaching grit in classrooms, then, is likely to have little impact.

But the team wasn't entirely dismissive of the concept of grit. It could theoretically be possible to develop ways to strengthen this trait in children, they allow; we just don't know what those are yet, so approach any individual or group that claims to know the magic formula for increasing kids' mental toughness with a healthy dose of skepticism.