Write a thank you letter this holiday season. It could bring ‘tears of joy’ — to you and the recipient.

Penn undergraduates get an unusual assignment in classes taught by psychology professor Angela Duckworth: Write a letter to someone whom you have not properly thanked.

Duckworth, winner of a MacArthur “genius grant,” learned about the exercise, called the Gratitude Letter, from her graduate school advisor, Martin Seligman, who is credited as being the father of the positive psychology movement.

» READ MORE: Gratitude isn’t just polite. A Philly-born psychology movement shows it is a path to happiness.

The full exercise also includes visiting the recipient of the letter, without telling why, and reading the letter aloud.

Seligman, and others, found that this exercise can increase the happiness of the letter writer for up to a month later.

“Generally, what happens is that everyone starts crying,” Duckworth said. “But they are tears of joy.”

For those worried about reading a heartfelt letter aloud to the recipient, Duckworth says that people underestimate how important the letter will be.

“They’re not judging you. They’re not worried about grammar. They are not thinking, ‘Oh, this essay could have been better written,’” she said. “They’re crying!”

Duckworth is also the co-founder of the Character Lab, a nonprofit that brings positive psychology to educators to help kids thrive. The organization’s website includes a “gratitude playbook,” full of tips including the Gratitude Letter.

» READ MORE: Want to get into Penn? Show a little gratitude - on an essay prompt

When writing the letter, the Character Lab recommends considering what you remember most the recipient saying or doing. How would your life be different without that? What makes this person important to your life?

Duckworth said the Gratitude Letter can become a holiday tradition. Once a year, sit with family to write letters or come prepared with a letter to read to a member.

“You will not run out of people,” Duckworth said.