If you're looking for traditional gift ideas for your employees, my Inc.com colleagues have you covered with lots of great suggestions. But if you're looking for an out-of-the-box idea that won't cost you a penny (and might even nudge your team to work harder in the new year), Adam Grant has the perfect suggestion for you.

In the current edition of his monthly newsletter Granted (hat tip to Quartz for the pointer), the star Wharton professor and bestselling author tells the story of how he first came up with this unique alternative to a traditional gift:

In the fall of 2003, I started grad school. One of the more daunting assignments was to contact 15-20 friends, family, and colleagues who knew me well, and ask them to share a story about a time when I was at my best. My task was to create a portrait of my strengths based on the patterns. It was a powerful learning experience, but it felt unbalanced--what about my weaknesses? I started asking people to share times when I was at my worst. Vomit... but it was just as valuable.

As the end of the year approached, I began to feel like something else was missing. I had received a lot of feedback but I hadn't given any. So over break, I spent a week writing emails to the 100 people who mattered most in my life, telling them what I appreciated most about them. It's one of the most meaningful things I've ever done... Looking back, 100 in one week was a little intense. This holiday season, what if we each picked ten people and told them what we appreciate about them?

It's a wildly simple idea to show appreciation to your team this holiday season - why who, just, you know, tell them straight up why you appreciate them? - but as Quartz's Leah Fessler points out, it has some serious scientific backing.

"Ample research shows that feeling valued and supported by one's supervisor is the top determinant of employee satisfaction and trust," she writes. "In one study led by Grant and Francesca Gino, a behavioral scientist and professor at Harvard Business School, when a single manager visited a call center to thank employees for their effort, the number of calls these employees made over the next week spiked more than 50 percent."

But expressing attitude isn't just likely to be good for your team's happiness and performance, it's also likely to be good for yours as well. A whole other line of research shows that expressing gratitude towards others is one of the biggest happiness boosters out there, while neuroscience shows that gratitude actually rewires your brain, helping you be more positive and solutions oriented.

So just to recap: personally expressing your appreciation to ten team members who have positively impacted you this holiday season is an absolutely free and scientifically validated way to boost happiness and on-the-job performance all around.

Why wouldn't you give it a try?

Do you have a holiday gratitude practice? What is it?