1. In the original Star Trek TV series, wearing a uniform with a red shirt meant that someone was in operations, which included the security department (gold or green meant command, and blue was worn by the science department). It made sense to take an extra security person on an expedition to a new planet. The landing party would consist of the show's stars, plus one unknown in a red shirt. This unknown security guard is often killed immediately after beaming down to the planet of the week, which conveniently displays how dangerous the residents of the planet are, and what strange powers they have. This device was used so often that the term "Red Shirt" came to signify an obviously expendable character who will die early in the show.
2. Did red shirts died more often than characters wearing other colors, or did it just seem that way? After all, Captain Kirk had a reputation for finding a girl in every episode, but an analysis of episodes show only seven romances in 79 episodes. Matt Bailey compiled statistics on deaths among the Enterprise crew in Star Trek: TOS. He found that 73% of those who died were wearing red shirts, compared to 10% for yellow shirts and 8% for blue shirts (the rest were wearing other uniforms). The majority of red shirt deaths happened on an alien planet. The trope was not our imagination, however the "red" part may have escaped the notice of many who didn't have color TVs in the mid-'60s.
3. Mythbusters once dressed their crash test dummy Buster in a red shirt when they tested the Gorn Cannon myth, which was based on a Star Trek episode. You can see the destruction in this video.
4. The episode of Star Trek: TOS that had the most red shirt deaths was Where no Man has Gone Before, in which twelve red shirts kicked the bucket. In second place was The Changeling, which saw six red shirt deaths.
5. Forget the debate about whether Star Trek or Star Wars is a better science fiction universe- the real conundrum is among the minor characters. You know red shirts are always killed. You also know that Storm Troopers shoot and shoot and can't hit anything. If the two groups were to meet, we would have The Redshirt v. Stormtrooper Paradox. The question feeds many forum threads to this day.