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Write the letter, but not when you think. Photo from Unsplash/Aaron Burden.

Research shows that people routinely underestimate the value of expressing gratitude, and overestimate how harshly the literal elements of their thank-you notes will be judged.

We may also misjudge when to send thank you notes.

If you’ve just interviewed for a job, attended a party, or received a generous gift, you should express gratitude the next day, or soon after. Forgetting to do so could seem rude, or cost you the job opportunity. But according to organizational psychologist Adam Grant, author of Give and Take: Why Helping Others Drives Our Success, when people enhance your life via non-material gifts and informal interactions—mentorship, career advice, networking, informational meetings—it’s more powerful to express gratitude weeks, or even months, later.

As a professor at Wharton, Grant mentors many of his students, as he genuinely believes that sharing wisdom, and giving in general, benefits not only the giver and the receiver, but also the organizations and social structures in which they operate. (It’s a phenomenon he’s studied extensively.)

“Per my own research, I’ve found that the impact of help, like mentorship, is often hard to see in the moment,” Grant tells Quartz At Work. “It only unfolds over time.” For this reason, he says, the most meaningful thank-you notes he’s ever received from mentees are the ones that have come months or even years later. ”It makes me feel that the time I’ve spent with them mattered,” Grant says. “One of my favorites came from a student who intensely disagreed with my career advice at the time, and years later sent me a note about how I had changed his mind.”

Specificity is key here. “The best notes highlight how your life is different as a result of the advice you receive,” he says.

It’s a strategy I recently tested with one of my own mentors, and I can see why it works.

First, a bit of backstory: I jumped around career-wise a bit after college—first working at a hedge fund, then spending nearly a year recovering from surgeries, then freelancing my way into journalism. While I was on bedrest, my operated-upon foot elevated above my head, I’d write articles, send them to a bunch of editors, and routinely get ignored if not flat-out rejected. My self-confidence, already shot from inactivity, began to plummet.

But one day, wheeling myself around Macy’s with my mom, my phone buzzed with an email from an editor at Quartz. She wanted to publish my essay. Weeks later, it went viral, and I was on a plane to speak at one of the most prestigious ideas festivals in the nation. Months later, I’d been published in The Washington Post and The Atlantic. And less than a half-year later, I was hired at Quartz.

In the days after that essay first published, I emailed the editor, Sarah Todd, thanking her for giving me the opportunity. I’m sure she appreciated that. But far more meaningful was when we sat down last week, nearly two years since the story went live, and I told her, face-to-face, that her believing in my potential, and taking a risk on my writing, entirely changed my life and career.

She had no idea that other editors had said no to the essay, and revealed that she only read it because I’d pitched Quartz so many times that her boss thought I should get a chance (persistence!). Serendipity worked in both of our favors, but she was touched to know that beyond elevating my writing, she’d permanently influenced my confidence, happiness, and professional potential. Without the passage of time—she’s since become a close friend and mentor—my message of gratitude wouldn’t have held the same weight it does now.

Research from Columbia Business School shows that “givers”—people who are inherently disposed toward giving more than they take—are especially attuned to the impact of the help they dole out. In two different studies, organizational behavior professors Francis Flynn (now at Stanford) and Joel Brockner found that while receivers of help judge their relationships with givers based on the treatment they receive while being assisted (i.e. being treated with dignity, or engaging in open, honest communication), the givers’ commitment to their relationships with receivers is more associated with their judgments of the outcomes associated with the favor they perform.

Essentially, the people who help you most—the “givers” in your life, be they your mentors, friends, teachers, or colleagues—are invested in hearing about the impact of their help on your life. The more positive their impact is, the more committed they are to your relationship.

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