Its hard to define an urban myth but generally I would say its a story which has gained some credibility amongst a fairly large group of people and/or over a long period of time. Many of these stories have a message of some sort (especially when a disadvantaged person or underdog beats a more powerful person, especially a government), or make fun of an unpopular organisation, or are based on an extreme event of some sort, or might be really embarrassing or freakish in some way.
Most urban myths aren't true. In fact if they are true they probably wouldn't really be an urban myth, although the real definition of the word myth doesn't preclude something from being true (see note 1). Some are based on a true event but have been distorted beyond much resemblance to reality. Others seem crazy or unlikely, but are really essentially true (but as I said above, these probably wouldn't be known as a myth in this context).
The Internet has made distributing these stories so much easier. Anyone can start one which will spread through email across the Internet in a matter of hours. A little bit of backup on a web site (even if its untrue) helps. I receive many of these, and most are just innocent fun (even though they are untrue) so I don't really object to them. I do check them out and reply to the sender showing the evidence that they are fake though. I admit I get a sort of perverse pleasure in pointing out how most of the motivational, and religiously inspirational ones are fake!
You can visit a site like Snopes to get an extensive list of fake stories, but here are a few of the better ones I have received recently...
A commonly believed myth, which has been circulating for many years, is that humans only use 10% of their brain. There is absolutely no good reason to believe this is true, in fact scans show the entire brain is active in fully functioning people. Removal or damage to sections of the brain usually results in lack of a specific function. One reason this myth might have been encouraged is that it gives people who believe in various pseudo-sciences such as ESP "scientific" support. If 90% of the brain isn't used by the average person that might provide the extra abilities they claim they have. This is all nonsense, of course.
This one really annoys me. Its the story of a brilliant young scientist (who is usually Albert Einstein) outwitting an atheist professor in a lecture. Sometimes there are variations on this story, including a young woman or a marine taking on the professor in various ways. The professor has been saying there is no god and, of course, he is humiliated by the actions of Einstein (or the other character). There is no evidence an event of this type happened, although its possible that someone, somewhere had some sort of debate with some professor at some time. The fact that there are so many variations on the basic story, and the theme ridiculing the smart atheists are two classic signs this is a hoax.
Many people believe that water will drain from a sink or toilet in one direction in the northern hemisphere and the opposite in the southern. They are told this is a result of the Coriolis effect (see note 2). Its true that this effect exists and is responsible for the rotation of air masses which cause weather effects, but it only has a measurable effect on very large objects. The average (or any realistic) sink or toilet is far too small for this to be noticeable. Everybody must have seen evidence that this is a myth. Water will drain in different directions from different sinks, etc in any house no matter which hemisphere they are in. The rotation is caused by slight distortions in the shape of the container, existing movement in the water, etc. But no one seems to notice. If a myth is described in realistic scientific words most people will just believe it without thinking.
The aircraft at the top of this page was supposed to be the latest US fighter (known as the F/A-37), with awesome abilities (see note 3), and in the original email distributing it there was a close-up of the female pilot test flying it. Unfortunately, its actually a fake aircraft created for a movie, "Stealth". Not only is the story fake, but the plot of the movie sounds pretty silly as well (see note 4).
I receive a lot of emails with various claims and I have developed a fairly good instinct for detecting fakes. Occasionally I am wrong and identify a true story as fake. I also rarely identify a fake story as true. I can generally confirm a story in about 2 or 3 minutes using the Internet, so it annoys be a bit that so many people send these messages out without doing any checking first. But in most cases this is a fairly harmless form of misinformation, and is often quite entertaining, so I suppose in some ways I should welcome this as an interesting social phenomenon.
Many urban myths are totally fictitious, others are based on truth but have had major facts changed, others are really true. Its actually fun to check on these and find one that's real. Therefore I give them a moderate score on the crap-ometer!
1. Here's one definition of the word myth: "A traditional story accepted as history; serves to explain the world view of a people." So there is no need for the story to be false by that standard, although it usually is. Traditional Greek, Christian, etc mythologies might be accepted as true by some people, but by reasonable objective standards they clearly aren't.
2. For an extensive discussion of different aspects of this effect see the
Bad Coriolis web page.
You can also access other interesting discussions about meteorology by using the
teaching menu from the home page of this site.
3. The email emphasises the whole story is top secret, but these facts have been revealed: the speed is believed to be cruise at Mach 3.5, and top speed in the Mach 4 range. Its a super-cruise stealth fighter, bomber, interceptor. The range is approximately a 4,000 nautical miles. While these stats are at the high end of newer fighter capabilities, they aren't totally silly, which makes the story more believable.
4. This is the plot of the movie: "An intelligent, next-generation drone fighter plane develops a mind of its own and an American pilot must go up against it." What? No aliens or terrorist involved at all? I'm surprised.
Sources of Further Information
There are several useful web sites with information on this subject. Before distributing anything which seems suspect I recommend checking it against one (or all) of these sites!