We’re talking about “thank you.” And according to a study published in Emotion, it could be the key to a stronger, lasting relationship. Researchers from the University of North Carolina at Chapel Hill and the University of California, Santa Barbara, examined to what extent a person’s gratitude influenced the other person in a relationship. They asked members of 77 heterosexual couples—who had been together anywhere from six months to 35 years—to choose something their partner did for them recently and thank them for it while their partner rated the integrity of their reaction during the conversation. Six months later, researchers followed up with a questionnaire to see how the relationships were faring.  As it turns out, gratitude is all about the response. When the recipient of the gratitude was responsive, the more positive the psychological impact was on him or her and the relationship. And that warm feeling of pride and appreciation after being thanked isn’t fleeting, either: Researchers found the effect of giving a little gratitude kept couples happy over the course of six months. In fact, hindsight showed that a sincere response at the time of the study could nearly predict just how satisfied the couple would be six months down the road.  Two words, six months of happiness. But the best part? Gratitude works both ways. While this study illustrates the positive effect being thanked can have on someone you love, being the thanker has some benefits, too. Besides ensuring a strong, more satisfying relationship, a study from Kent State University found sitting down to pen a thank-you note the old fashioned way can boost your happiness by 20%. But with all forms of gratitude, make it sincere! “It has to be a heartfelt sentiment showing significant appreciation,” says Steven Toepfer, PhD, a researcher from the Kent State study.   More from Prevention: 2-Minutes To A Happier You