While the origin of this myth is obscure, it was undoubtedly a mom who just didn't want her child eating gum. It is the sort of thing, though, that can really stick in a child's mind (if not really his stomach) and conjure up images of an ever-growing rainbow ball of gum clogging up his insides. And it obviously is not true, said Rachel C. Vreeman, a fellow in children's health services research at Indiana University School of Medicine and Riley Children's Hospital.
"The truth about chewing gum is that while it's not something that is particularly well-digested, it passes right through your system," she said, laughing. "You don't have to worry about it sitting in there forever and ever."
You must drink eight glasses of water a day to stay healthy.
This is an example of something that sounds like a good idea, and maybe it is, but it is not backed up by any scientific evidence. The best the study authors could find was a recommendation of 6-8 glasses a day from a prominent nutritionist, but without any scientific references.
"I absolutely had thought that one was true as well until we looked at the literature," Dr. Vreeman said. "It's not that drinking water is harmful, it's more this myth that you have to drink a certain amount of water, that the fluid you take in has to be water." People can get a lot of their fluids from other sources, such as juice or milk and even from some of the foods they eat.
Shaving hair makes it grow back quicker and it will be darker and more coarse.
"I think that myth is so persistent just because of the optical illusion that happens when you really do shave," Dr. Vreeman said. The cut blunt edge of the hair might make it feel at first to be more coarse and thick. And because the hair hasn't been bleached by sun or lightened by exposure, it at first appears darker.
"And so regardless of what you tell people, that's what they think, that it is growing back faster, that it is darker and thicker and really it is just that optical illusion of what happens when we shave," Dr. Vreeman said.
We only use 10 percent of our brains.
This myth has been wrongly attributed to Albert Einstein, who was a physicist and not a neurologist anyway, and it might date back to the 1900s, to motivational literature seeking to get people to improve themselves, Dr. Vreeman said. That it was hokum was a revelation even to the doctors researching it.
"Even though it went against everything we had learned in medical school and anatomy class and so on, we never really thought to challenge that," she said.
It may be that medical technology just needed to catch up -- numerous brain functioning scans have since shown that there is no area of the brain that sits unused, the study noted. Still, the myth might live on simply because it offers people some hope.
"I think that one is so appealing because there is this idea that you have untapped potential," Dr. Vreeman said. "That you're not living up to your full potential and if you could just harness a little bit more of your brain power, maybe you could do that."
Reading in dim light will ruin your eyes.
Another myth undoubtedly hatched by some mom desperate to get her child to put down the book and go to sleep. This is a myth that might have a little bit of truth to it -- reading in dim light does cause some eyestrain, Dr. Vreeman said.
"Your eyes get tired and dry and maybe a little sore and so on from reading in the dark," she said. "Your eyes actually return to their full potential and the eye strain goes away once you return to full light."
You can tell the sex of an unborn child just by how high or low it appears to be riding in the mother's womb.
Another old wives' tale, probably perpetuated by old wives whose predictions were occasionally right. And there's a reason for that.
"You have a 50/50 chance of predicting correctly whether the baby is going to be a boy or a girl, assuming you don't have an ultrasound or things like that," Dr. Vreeman said. "It's hard to beat 50/50 odds, actually."
Then there are other medical myths for which the evidence is conflicting. Consider the well-worn 5-second rule -- that any fallen food picked up off the floor within five seconds is still safe to eat.
Two enterprising molecular biology students at Connecticut College dropped Skittles candy and apple slices on their dining hall floor and then tested them for bacteria colonization. The apple slices took up to a minute to gather bugs and the candy took up to five minutes. But Clemson University researcher Paul Dawson and his students dropped bologna and bread onto infected tile and carpet and found nearly all of the bacteria jumped to the food within five seconds.
The deciding factor for consumption, it appears, is how clean you think the floor is.
And there are some that appear to be medical myths that turn out to be true. Feed a cold, starve a fever has some basis in fact, at least for colds.
Researchers at Ohio State University, working in deer mice, found that those whose diets were reduced by 30 percent produced far fewer immune system cells needed to fight off infection. Fighting off infection appears to eat up fuel as well.
There also appears to be evidence to condemn the act of "double-dipping" -- reusing a chip you have already taken a bite from to plunge back into a bowl of dip, a practice that gained prominence through the antics of character George Costanza on the TV show Seinfeld. Once again, Dr. Dawson and his students took bacteria counts from dips that had been "double-dipped" and found that the bacteria levels were more than three times higher than those in normal use. Sorry, George. This is one myth that is true.