Photo: Paul Guzzo/Getty Images

Sure, everything is full of twinkle and joy right now, but lurking beneath it all is the uncomfortable knowledge that sooner or later, all of the good things about this time of year must end. The tree begins to brown, the discarded wrapping paper is crumpled up and thrown away, weather that was once considered festive is now just cold. And looming largest of all is that whole Santa thing — a race between kids, who are growing each year into more rational, skeptical humans, and parents, who are trying to stay far enough ahead of them to keep the magic alive.

The belief in Santa is a curious combination of things. On the one hand, it’s always temporary — sooner or later, even the most die-hard devotees will wise up. On the other hand, though, it’s more powerful and long-lasting than other beliefs of its kind. Our willingness to lose ourselves in fantasy generally lessens as we age, but one 1978 study found that certain fictional characters are, at least for a time, immune to that particular hallmark of development: A kid’s belief in Santa, the Easter Bunny, and the Tooth Fairy was unrelated to whether they had imaginary friendships or how often they played games of make-believe.

For most people, even those characters will cease to seem real sometime before adolescence. In 2011, an AP survey of U.S. adults found that 84 percent had believed in Santa at some point during their childhoods, and the average respondent stopped believing at 8.8 years old. Age, though, isn’t necessarily the best indicator of how far along in the process a kid will be. More important, as psychologist Jacqueline D. Woolley argued earlier this week on the Conversation, is how hard adults work to smooth out any incongruencies between real life and this particular fantasy.

When it comes to evaluating information — and separating the real from the bullshit — kids aren’t that different from adults, Woolley wrote. Over several studies, she and her colleagues illustrated the similarities between us and our younger counterparts: When learning something new, kids, like adults, take context into account; they measure the information against what they already know; and they consider the source, evaluating its trustworthiness and expertise, before deciding how much to believe.

The question, then: If children are just as capable of seeing through nonsense, how come we adults have figured out that Santa doesn’t exist, while kids still happily wait on line to sit on a jolly guy’s lap and throw their energy into composing letters to the North Pole?

The answer: Because we tell them it’s worth their while, and work hard to make sure they agree. “Parents and others go to great lengths to support the Santa myth,” Woolley wrote. “Children are not unthinkingly credulous and do not believe everything we tell them. So, we adults must overwhelm them with evidence — the bells on the roof, the live Santas at the mall, the half-eaten carrot on Christmas morning.” The more kids interact with live Santas, the more likely they are to believe that he’s real.

For a while, at least. Even the fiercest of unsustainable beliefs is still, in the end, unsustainable. And research has shown that the more kids learn about the workings of the physical world, the more quickly the myth begins to break down. Santa, after all, violates pretty much everything we know about time and space; eventually, kids start to question how a chubby man can squeeze through so many narrow chimneys without getting stuck, how he stays airborne on a rickety wooden sleigh loaded up with so many gifts, and how he can visit every single house in the world in the span of a single evening (to get the job done, he’d have to travel more than six million miles per hour).

But the eye-opening doesn’t happen all at once — typically, it’s a gentle process, rather than an earth-shattering one. Earlier this year, Science of Us spoke with psychologist Thalia Goldstein, a professor at Pace University who studies how children understand fictional characters (and has collaborated with Woolley on Santa research). The belief in Santa, she explained, tends to build and then fade in five distinct stages. In the beginning, there’s only one Santa — the one who comes to visit on Christmas Eve is also the one at the mall and on TV, a man who’s somehow capable of being in multiple places at the same time. Sooner or later, that last part begins to break down: There’s still one true Santa, but he can’t be everywhere at once. Instead, he dispatches his lookalike minions to take care of the grunt work in the days leading up to Christmas Eve, receiving dispatches from these lesser Santas — mall and otherwise — through some mysterious, magical channel. In the third stage, these minions lose their magic, if not their powers of communication; they may be just regular people in costumes and beards, but they still know how to reach the big guy.

In the later phases, the lines of communication close, too. Stage four is when the guy in a red suit ceases to be anything but a guy in a red suit; there’s still a Santa out there, far away in his Arctic workshop, but he doesn’t have any earthly connection to the humans who assume his identity. And finally, in stage five, even that idea – the core belief that props up the whole enterprise – disappears, as the evidence against Santa becomes too overwhelming to ignore.

“Kids trust their parents to tell them about all sorts of things that they can’t see. Kids can’t see with their own eyes that there are eight planets, or that the sun is the same thing as the other stars, just a lot closer, or that there are germs that can get you sick,” Goldstein said. “So when parents present Santa Claus in the same way as they present [the existence of] germs, there’s no reason for the kids to have any ability to distinguish between the two different types of information.” Eventually, though, the world wins out.