Robert Emmons, perhaps the world’s leading scientific expert on gratitude, argues that gratitude has two key components, which he describes in a Greater Good essay, “Why Gratitude Is Good.”

“First,” he writes, “it’s an affirmation of goodness. We affirm that there are good things in the world, gifts and benefits we’ve received.”

In the second part of gratitude, he explains, “we recognize that the sources of this goodness are outside of ourselves. … We acknowledge that other people—or even higher powers, if you’re of a spiritual mindset—gave us many gifts, big and small, to help us achieve the goodness in our lives.”

Emmons and other researchers see the social dimension as being especially important to gratitude. “I see it as a relationship-strengthening emotion,“ writes Emmons, “because it requires us to see how we’ve been supported and affirmed by other people.”

Because gratitude encourages us not only to appreciate gifts but to repay them (or pay them forward), the sociologist Georg Simmel called it “the moral memory of mankind.” This is how gratitude may have evolved: by strengthening bonds between members of the same species who mutually helped each other out.