What makes for a great Christmas song?

Every year at this time, some of the greatest songs in the world pop up in our playlists.Most of them are decades old, or even centuries.

If anyone knows what makes a Christmas song immortal, it's Berklee School of Music forensic musicologist Joe Bennett.He's analyzed the holiday pop charts to learn exactly what makes popular Christmas songs popular – "to try and figure out what are the musical characteristics that listeners keep coming back to again and again over the years," he said.In fact, Bennett found that most commercial Christmas hits are about one of only six things.

Those thematic characteristics include "home for the holidays" (the most common one), weather-themed songs (like "Let It Snow"), the myth of Santa Claus, parties, being in love, and being out of love.Which brings us to the #1 Christmas hit of recent years: from 1994, Mariah Carey's "All I Want for Christmas Is You." Bennett says the song is remarkable because it hits most of the Christmas-themed tropes: "We get Christmas presents, and the tree, and Santa Claus, and the snow.That lyric is so well-loved as a holiday celebration song, 'cause it's literally got it all."A few years back, "Sunday Morning" visited Walter Afanasieff, who wrote that song with Carey.

"Hey, I have a #1 hit every year, but it's the same song every year!" he laughed."We just kind of, like, go bouncing off of each other, like at a jam session," he said."No one in a million years would've thought, 'Yeah, we're gonna release this and it's gonna be a big hit.'" Musical elements And what does Maria Carey's hit have in common with most other Christmas hits? Sleigh bells! "We found that more than 65% of the songs in the corpus had sleigh bells playing eight to the bar," said Bennett.

He said the inspiration came from galloping horses: "In the mid-1800s, the horses would actually wear sleigh bells to warn people that they were coming.So, you can imagine the sort of leather across the horse's mane there, and ...