Hoping to get an avalanche of Christmas cards and holiday letters this year? There’s just one rule: send out a pile of your own. That’s what a BYU sociology professor named Phillip Kunz did back in the ’70s, and his address stayed on others’ Christmas lists for nearly 15 years, reports NPR affiliate KERA news. The surprise? All those cards he sent, some 600, went to people he didn’t know.

Kunz’s little experiment involved sending about 300 personal holiday cards (hand-written!) and about 300 cards that just featured a family photo, but they all exerted a subtle peer pressure to reciprocate. He got long letters back from some of the strangers, and matching-sweater-photo-cards from others—some 200 responses in all. Robert Cialdini, a well-known emeritus social psychologist and marketing professor at Arizona State University, explains to journalist Alix Spiegel that the response reflects just how well the golden rule is drilled into us as kids: “We are obligated to give back to others, the form of behavior that they have first given to us,” he says. “Essentially thou shall not take without giving in return.”

Cialdini goes on to cite the implied social rules of give and take in common practices from tipping to using those pre-printed address labels charities send out. More nefariously, this is also what’s behind so much of the quid pro quo spotted in politics and when doctors prescribe medications because of the perks pharmaceutical companies have sent their way:

This doesn’t mean that the rule of reciprocation affects all of us all of the time…. But it is powerful. One of those invisible powerful things that can subtly shape how we behave even years after someone has given us something.

And that, of course, is how we end up guiltily eying that stack of Christmas cards every year. We have to write back, don’t we?