The thing about people who study gratitude for a living is: They're really nice. They're also prolific thankers.

"Grateful for your calling attention to this important virtue without which we would not be fully human!!!!!" wrote one professor in the closing of an email exchange.

The social science on gratitude is pretty resolute: Feeling thankful is good for you. "There’s something called a grateful personality that some psychologists have studied," said Jo-Ann Tsang, a psychologist at Baylor University. "They find that if you’re greater in the grateful personality, you tend to have increased life satisfaction, happiness, optimism, hope, positive emotion, and ... less anxiety and depression."

Other studies suggest that diaries, daily reminders, and intentional reflection on what you're thankful for can boost happiness, positive emotions, and a sense of meaning in life, Tsang said. Physical benefits may include fewer symptoms of illness and better sleep. These activities "can even help people with moderate body-image issues, and also people with moderate anxiety issues," she added.

Given all this, it makes sense that gratitude researchers would drop a few extra "thank you's" into their everyday correspondence; benefitting from the side effects of gratitude is like a professional perk.

Most humans are not social-science robots, calculating every act with an eye to algorithmic improvement of personal well-being. Spend a long day at work, or clean your little cousin's weirdly yellow vomit off your shirt for the umpteenth time, and gratefulness is not necessarily the foremost emotion you'll feel.

But gratitude isn't just an emotion: It's also a value. In most cultures, but especially in America around Thanksgiving time, being grateful is seen as a virtue; the entire country stops working and gathers together, because being thankful is something we should do.

But why, really?

In the American context, thankfulness is a genuine puzzle of cultural inputs. It's not political; #blessed is in neither the Constitution nor The Federalist Papers. But it is nationalistic, in some sense; Thanksgiving was created by Abraham Lincoln in 1863 in the midst of civil war, a bid to restore "the full enjoyment of peace, harmony, tranquillity and Union" in the United States.

Even so, when Lincoln announced the new holiday to be held annually on the last Thursday of November, he used the language of Christianity to explain the logic of this national ritual (emphasis added).

The year that is drawing towards its close, has been filled with the blessings of fruitful fields and healthful skies. To these bounties, which are so constantly enjoyed that we are prone to forget the source from which they come, others have been added, which are of so extraordinary a nature, that they cannot fail to penetrate and soften even the heart which is habitually insensible to the ever watchful providence of Almighty God ...

While offering up the ascriptions justly due to Him for such singular deliverances and blessings ... also, with humble penitence for our national perverseness and disobedience, commend to His tender care all those who have become widows, orphans, mourners or sufferers in the lamentable civil strife in which we are unavoidably engaged, and fervently implore the interposition of the Almighty Hand to heal the wounds of the nation.

America is, and always has been, a nation of Christianity: Faith is in the country's founding, political philosophy, and history. Religion is undeniably part of the American public sphere.

But in 1863, America was a different kind of Christian nation than it is now. The country is far more religiously diverse and culturally secular than it was when Thanksgiving was founded. A strong majority of Americans consider themselves religious, but for many others, religious faith doesn't play much of a role in their everyday lives. And although roughly 90 percent of people in the U.S. believe in "God or a universal spirit," faith doesn't have much bearing on the way Thanksgiving is talked about in public life, from Butterball commercials to the Macy's Parade. Gratitude is the animus of these secular rituals, but the object of the gratitude is unclear. If people aren't thanking God, who are they thanking? You can thank your grandma for making delicious pie, but who do you thank for the general circumstances of your life?

This is why secular, Thanksgiving-flavored gratitude seems so fuzzy. Religions from Christianity to Hinduism to Wicca all emphasize the importance of thankfulness, especially as a form of prayer. This is because they rely on the premise of an other, some sort of non-human being that has some sort of control or influence in the world who you can thank for the world and the good things in it.

"One of the things that’s really interesting about the human mind is that we seem to want to see agency in the world, almost intuitively," said Michael McCullough, a psychologist at the University of Miami. "The mind really craves an explanation for the good and the bad, in terms of agency."

By "agency," McCullough means something along the lines of "a force that can act in the world and cause events to happen." In crude sociological terms, people give thanks to the forces that act in the universe—God, or god, or gods—as a bid for cosmic benevolence, whether that means making it rain or preserving a loved one's health or bringing a baby into the world. But these thanks are also an implicit metaphysical claim: Humans owe their existence, their longevity, and perhaps even their daily fortunes to a being beyond ourselves.

But if you take all of that away—either because you don't believe it, personally, or  perhaps because metaphysics isn't really something you can talk about at the Thanksgiving dinner table—what does gratitude actually mean?

"Let me explain something about gratitude," wrote Robert Emmons, a psychologist at the University of California, Davis, in an email.

We all begin life dependent on others, and most of us end life dependent on others. If we are lucky, in between we have roughly 60 years or so of unacknowledged dependency. The human condition is such that throughout life, not just at the beginning and end, we are profoundly dependent on other people. ...

Gratitude is the truest approach to life. We did not create or fashion ourselves. We did not birth ourselves. Life is about giving, receiving, and repaying. We are receptive beings, dependent on the help of others, on their gifts and their kindness.

"You see—none of this have I framed in a religious context or using religious/spiritual language," he concluded.

But commenting on the human condition and the nature of life is at least philosophical, if not spiritual, and it's certainly normative. Since humans are born, survive off the generosity of others, and then die, he's saying, gratitude naturally is and should be the organizing principle of life.

But this isn't obvious, necessarily. In the Genealogy of Morals, for example, Friedrich Nietzsche condemns the "slave morality" that underpins values like humility—this way of thinking undermines individuals' recognition of their power, their sense of independence, he says. Human civilizations—and individuals—don't necessarily have to organize themselves around gratitude and interdependence.

They just do, rather frequently. In America, that's probably caused in part by the ambient influence of Christian values.

McCullough thinks there's another reason for the ubiquity of gratitude: It's an evolutionarily beneficial trait, hardwired into the human brain.

Gratitude is amorphously Christian in a way that's distinctively American.

"Even things that are culturally constructed have to have a home somewhere up in the mind to come out in our thoughts and our behavior," he said. "Like all emotions, [gratitude] was plausibly designed by natural selection. There's some tissue up in the head whose job it is to produce gratitude."

The evolutionary explanation for this, he said, is probably that gratitude helps people initiate friendships and alliances—which then help people survive.

"I don’t think you’re going to find research that points to the topology of the brain that says, 'Ah! This is the gratitude center,'" he said. But "I have my bets on there being an evolved circuitry because of what it seems to be good at doing: It’s pretty good at getting you to notice unexpected favors."

His research suggests that when people do nice things for others unexpectedly, that produces gratitude—and increases the likelihood that people will do something "in kind" ("a really rich phrase, when you think about it," he added). Although scientists can't know the exact neurological nature of gratitude, they look at behaviors like these as a proxy for understanding why people feel certain emotions, like thankfulness.

This doesn't make gratitude less Christian, or less American, or less amorphously Christian in a way that's distinctively American. But it does suggest, McCullough seems to be saying, that it's somewhat universal.

"Nietzsche was a hard act to follow," McCullough said. But perhaps even Friedrich experienced his own kind of gratitude in the context of his own worldview. And maybe that's what's happening on Thanksgiving, when a country that's deeply divided over values and philosophical principles agrees to be thankful, together.