Xmas time is here again. Unlike many people I have no particular aversion to the holiday season. I don’t have too many emotional scars from Christmases past. Getting presents was always fun, I liked the lights on our tree, even stringing popcorn, and these days I try to keep awkward gatherings to a minimum. For me it breaks up the endless round of quotidian drudgery in a relatively pleasant way. But this year I can’t help viewing the whole spectacle through the eyes of human evolution and psychology. No matter how you might feel about the holiday season personally, it’s interesting to think about what may be behind all the crazy traditions: a little bit of the science behind the season. 
As you’re buying presents, for example, remember that how much you’ll spend on a gift often depends on how closely related you are to a person genetically. Gift-giving follows the rules of kin-selection—you’ll usually give better gifts to your sister than to your second cousin—unless, of course there’s a special romantic partner you’re trying to get closer to. You’ll buy them the most extravagant gift of all, as a part of courtship behavior. Like it or not, that scarf you’re buying for your new boyfriend is based on a biological bet that you’ll be more likely to bond and have children. Choose carefully.

For many of us, family gatherings are the most stressful part of the season, when we're trapped in conversations with racist Uncle Dan and loud-mouthed Cousin Carol. Despite being surrounded by people, you might still feel the effects of the modern condition known as social isolation, wherein your brain responds most strongly to human conflict and least to things that might make you happy, leaving you feeling lonely and depressed. However, even the Grinchiest of us benefits from participating in holiday rituals with the family, as these actions can help cement the bonds between us year after year. As you gather around the fire with your relatives, you are participating in one of the most ancient rituals of your human family. How's that for tradition? 

Maybe your only refuge during the holidays is found in food. No matter how boring the company, how difficult the office mates, how annoying the politics/religion/obsessions of your relatives, perhaps a delicious meal of holiday turkey offers you a pleasant reprieve. And the sharing of food may be among the hoariest of human behaviors. Humans in every culture worldwide practice sharing food with other members of their group, especially high-value foods like meat. If you were a hunter-gatherer, you’d be partaking of a strip of buffalo meat instead of a slice of lemon meringue pie, but the urge is the same. Research shows that even Homo ergaster—your grandma from two million years ago—regularly gathered to share food (i.e. have a feast), so rest assured that your brain is programmed to find this enjoyable on some level.

As you’re scooping up a ladle-full of spiked fruit punch, you might note that our attraction to alcohol may have an evolutionary basis in seeking fruit, which in the tropics often starts to ferment quite quickly. It’s possible that we got a taste for booze because our diet was so dependent on fruit, and our ancient ancestors were drawn to the wafting scent of fermentation. Just consider that most symbolic of holiday foods: a cake of preserved fruit that has been doused in brandy. Remember not to imbibe too much at the office party, though, or you might find yourself in the supply closet with a coworker, particularly if you're one of the individuals whose brain seems wired for uncommitted sex. If you slip up, you can console yourself with the knowledge that both men and women find the idea of their partners cheating with coworkers less threatening than many other possibilities.

It’s rare these days that a group of carolers will actually assemble outside your house and regale you with song, but you are certainly going to be hammered relentlessly with recorded Christmas music virtually every moment you’re out in public. At smaller parties, actual group singing may even occur. If you’re trapped in such a situation, you can find consolation in the fact that Christmas carols were actually banned in England and America for two hundred years (blame the Victorians for bringing them back). There’s always the possibility that we could return to the good ole days when people were fined for singing them. There’s a big debate about whether the human love of music is an evolutionary byproduct or is itself adaptive—for example, by making us sexier. Either way, singing makes people more relaxed and energized, so don’t hold back.

As you’re driving past all the houses decorated with lights, nativity scenes, Santas, and reindeer, remember that people use their decorations to signal their neighbors that they are friendly and open to community connections. Christmas trees, wreaths, poinsettias, mistletoe, and other decorative plants each have a ritual purpose, but generally they also could be an extension of our desire to bring plants into interior spaces. We evolved out in the wild, and bringing plants indoors can make you feel better.

Postal mail seems to be going the way of the dodo, but around this time of year you'd never guess. When you're sending out holiday greeting cards to your loved ones, you might note that the social networks to whom we send cards ends up being around 150 people (taking into account couples and families), which is predicted by the size of our primate neocortex. We are also more likely to respond to Christmas cards from people we think are high-status rather than those we think are low-status. 

he season of giving also inspires many of us to devote our time and money to charity, which seems contrary to popular conceptions of "survival of the fittest." But researchers recently found that a rat will go out of its way to rescue another rat that is trapped. Even when chocolate was present, the rat would first free its trapped cohort, and then actually share the chocolate with it! This would suggest that empathy or altruism runs much deeper in mammalian lineage than previously thought. While you might know some humans with less compassion than a rat, even Scrooge was (eventually) moved by the holiday season to help out his poorer fellows in need. Christmas alms-giving is hard-wired to give us pleasure and may even help us to find a mate.

No matter how meaningless the stockings, songs, and symbols might seem at first glance, all the strange customs of the holidays spring from some deep and ancient place within us. If you're a Grinch, you can console yourself in knowing it's all just a manifestation of our monkey brains, and if you're irrepressibly merry, then you can find validation for even the most exuberant joy. And if, at the end of it all, you find yourself pining for the more-perfect Yules of yore, remember that nostalgia is not only an natural anti-depressant, but that it doesn’t depend on the memories being accurate or even real. So go ahead and remember some really awesome ghosts of Christmas past that never actually happened. You’ll feel better.