There's nothing more stressful than figuring out what you want to be for Halloween. Do you spend $50 on a costume you're only going to wear once? Do you dress up as something in the pop cultural zeitgeist? Or do you go the sexy route?

Americans will spend an average of $77.52 each on this Halloween, a slight increase from last year, according to the National Retail Federation's 2014 Halloween Consumer Spending Survey. In total, it's projected that Americans will spend $7.4 billion on Halloween this year. So clearly, people are very interested in topping what they wore last year.

But how did we get here? Surely, people a millennium ago weren't dressing up as "slutty police officers," were they? Here's why Halloween costumes were originally created and how they evolved into one of our favorite holiday traditions.

The first Halloween costumes

Halloween has its origins in an ancient Celtic festival of the dead called Samhain, meaning "summer's end," and it was probably at its height between the 9th and 12th centuries. During this time, people thought that the souls of the deceased were out and about, in addition to fairies, witches and demons. People left food and drinks out to appease these creatures, which was followed by people dressing up like them to receive treats, too.

Beginning in the 9th century, Nov. 1 became known as All Saint's Day or All Hallows in Britain, and it was a church feast day. In case you were wondering, the night before, the eve of All Hallows, was called All Hallows Even, which eventually evolved into Halloween. This was a time that kicked off a winter season where performances, tricks and processions were given treats or money in return. In County Cork, Ireland, people went begging door-to-door as they followed a man dressed as a white mare covered with a sheet and carrying a horse head. In Wales, boys and girls dressed as the opposite sex as they went door-to-door.

The tradition comes to North America

Scottish and Irish immigrants in the U.S. during the 18th and 19th centuries brought their Halloween folklore and traditions with them, which mixed in with the folklore, traditions and superstitions of other immigrants from Germany, Haiti and the Netherlands. Different regions of North America celebrated Halloween in different ways, with children in Kingston, Ontario, which had a heavy Scottish immigrant population, going out "guising" and the taverns in Philadelphia hosting costume parties. However, the idea that Halloween was a time when the spirits were out was universal, and that meant dressing and behaving differently than you would in your everyday life.

Halloween really takes off in the 19th and early 20th centuries

Halloween and the tradition of dressing up for the holiday really takes off during the Victorian era as new discoveries, such as Charles Darwin's theory of evolution, and the Industrial Revolution encouraged people to seek solace in simpler times of ancient folklore and become more connected to nature, which inspired many Halloween costumes at that time. Masquerade parties were popular in the early 20th century, and many private social clubs threw them for their members since Halloween was the first holiday after people returned from their summer homes. Halloween costumes at this time also reflected people's interest in other cultures and the exotic, such as the popular Egyptian-inspired costumes. Halloween costumes were solely homemade at this point, too.

Costumes become big business

As Halloween became a national celebration in the U.S., companies began cashing in on its popularity. One of the first was Collegeville Flag and Manufacturing Company, which started making Halloween costumes in the 1920s. Around the same time, H. Halpern Company was one of the first to license the images of fictional characters for its costumes, such as Popeye, Olive Oyl and Wimpy. The company Ben Cooper popularized the idea of dressing up like your favorite pop culture icon after World War II when Halloween costume manufacturing really took off, as well as the idea of an American pop culture, aided by the rise of TV in the 1950s. By the 1960s, Ben Cooper owned 70 to 80 percent of the Halloween costume market.

Halloween costumes get sexy

If you're a lady shopping for a Halloween costume in stores today, you can't take a step without seeing your options as "Slutty This" and "Sexy That." If you've gotten to this point of the story, you can see that Halloween costumes weren't always like this. It really started in the 1970s when gay communities across the U.S. adopted Halloween as a time for wearing revealing and outrageous outfits during parades. Soon, retailers capitalized on these skimpy costumes and by the early 2000s, they were everywhere, reflecting how much more open and less conservative America has become through the years.

These are the most popular costumes today

Every year, the National Retail Federation releases a list of the top Halloween costumes for adults, children and pets. This year, adults are dressing up as witches, animals and Batman characters, the latter of which may have been influenced by the popularity of Fox's new show Gotham. The top three children's costumes are a princess, animal and Spider-Man. Dressing up as a character from Frozen came in fourth because we will never be able to escape that mega-successful movie that hit theaters last November. And pet-lovers are transforming their animals into pumpkins, hot dogs and devils.

The first Halloween costumes may have been worn 1,000 years ago, but aside from some manufacturing and a few Hollywood characters as inspiration, our costumes really haven't changed all that much.