Conventional wisdom. Common knowledge. Whenever you hear something super interesting preceded by “they say,” don’t blindly believe it. Because when it comes to matters of biology, “they” apparently have no clue.
As many as 90 to 95 percent of the tales we tell about our bodies just aren’t true, estimates Rachel Vreeman, M.D., assistant professor of pediatrics at the Indiana University School of Medicine and coauthor of Don’t Put That in There!: And 69 Other Sex Myths Debunked.
And she should know—that’s just one of three books she and coauthor Aaron Carroll, M.D., have dedicated to uncovering the truth behind such body fables as whether your eyes will stay crossed (nope) to how long gum stays in your stomach (it’s closer to a day than to 7 years).
Check out the following list for body facts you may have heard from your buddies, your parents, the office know-it-all, or maybe even your doctor that just don’t stand up to scientific scrutiny.
1. YOUR URINE IS STERILE.
Sure, your pee can appear nearly crystal clear. But researchers at Stritch School of Medicine, Loyola University Chicago, found women’s urine harbors at least 85 species of bacteria.
Though they haven’t yet done the definitive test, there’s no reason men’s would be different, says study coauthor Alan Wolfe, Ph.D.
The findings might dissuade you from sipping your pee when water is scarce in an emergency scenario—though as Wolfe points out, if you’re so dehydrated you’re considering it, you probably have bigger problems than a few bacteria.
However, those little bugs do have an upside.
Some of the bacteria that hang out in your urine may be beneficial, and understanding what Wolfe calls the “urinary microbiota” may eventually help doctors treat infections and related issues, he says.
2. YOU WON’T GET SICK IF YOU FOLLOW THE 5-SECOND RULE.
Believe it or not, the so-called 5-second rule has been rigorously tested, Dr. Vreeman says.
The verdict: The amount of bacteria that end up on your snack depends not on how long it stayed on the ground, but on the type of food and the type of floor.
Moist, sticky baloney, for instance, will trap more bugs than dry, crusty bread. And slick tile, with bacteria sitting right on the surface, transfers more germs than wood or thick carpet, where bugs can burrow down a bit.
“Bacteria can live for weeks on the floor,” says Dr. Vreeman. “And if you drop food on the floor, the bacteria transfer to that food almost instantly.”
That’s the kind of bacteria that can make you sick.
If you eat the fallen food anyway and don’t get the heaves or diarrhea, you can thank your strong immune system or your clean-freak roommate who just mopped the floor—not your quick reflexes.
3. SHAVING CAUSES HAIR TO GROW BACK THICKER AND DARKER.
Looks can deceive: Running a razor over your hair near the root means you can see a larger portion of the shaft’s diameter when it grows back in, says Robert Dorin, D.O., a hair restoration expert and hair care specialist in New York City. So it may appear temporarily dark and lush.
In fact, shaving more often than necessary might hurt rather than help your next playoff beard, Dr. Dorin points out.
You may yank out some hairs at the shaft, causing micro-damage to cells that actually results in thinner regrowth.
4. BIGGER FEET MEANS A BIGGER PENIS.
Some myths start with a grain of truth, and when she first started researching it, Dr. Vreeman thought this might end up among them.
That’s because a gene called the Hox does play a role in the development of several appendages, including your penis, toes, and fingers.
That said, the gene doesn’t appear to make all your limbs grow bigger at the same rate. And the largest studies to date have shown no link between the two, Dr. Vreeman reports.
Case in point: A new U.K. review of 17 studies that examined the average penis size of more than 15,000 men found that foot size wasn’t consistently linked to penis size—and neither were other common “indicators” like weight or length of index finger.
If you’ve ever wondered how you stack up, Science Revealed the Average Penis Size.
5. WHEN YOU SLEEP WALK, YOU’RE A TICKING TIME BOMB.
Your snoozing girlfriend might act surprised or frightened if you startle her mid-stroll.
But don’t worry that you’ll give her a heart attack, stroke, or other serious health condition, says Men’s Health sleep advisor Christopher Winter, M.D.
Neither he nor Dr. Vreeman could find evidence of a single case where this has happened. Nor is anyone sure where the misconception arose, though perhaps it’s due to the difficulty sometimes involved with rousing a nighttime wanderer.
In fact, since sleepwalkers aren’t aware of their actions, sometimes waking them up can prevent them from falling or otherwise hurting themselves.
Your best bet: Play along with whatever scenario they’re enacting (“sure, honey, we’ll kill those dragons tomorrow”) and gently steer them back to bed, Dr. Winter says.
6. YOU LOSE MOST OF YOUR BODY HEAT THROUGH YOUR HEAD.
The rate at which heat dissipates depends on the surface area of a body part and little else—the more exposed skin, the quicker you cool down.
If you spread out the skin, your head is covered by about the same amount as an arm or a leg, Dr. Vreeman says.
Studies of lucky volunteers who entered cold chambers in everything from arctic survival suits to bathing suits confirm heat escapes approximately equally from every area.
This legend may live on due to circumstance—you’re a whole lot more likely to wear a coat, jeans, and no hat than you are to leave home in a stocking cap and boxers on a frigid day.
“If you’re still cold, and you’re not wearing a hat, by all means put on a hat,” Dr. Vreeman says. “But there’s nothing special about your head. There’s no reason why a hat would keep you more warm than, say, putting on pants.”
7. YOU ONLY USE 10 PERCENT OF YOUR BRAIN.
Sorry: You probably don’t have a hidden genius lurking underneath some lazy brain cells.
No one’s quite sure how this fiction formed, though it seems a motivational speaker may have simply declared it so, Dr. Vreeman says.
Related: Has Your Brain Already Peaked?
But it’s not hard to understand why it persists.
“We love this idea that we have untapped potential and if we just like really were able to engage more of our brain, who knows what might be possible?” she says.
Brain scans, though, fail to reveal a huge, dark expanse of neurons lying dormant—nearly all of your brain is working almost all the time. And brain damage almost anywhere in your white or grey matter causes noticeable effects, Dr. Vreeman says.
So while you might indeed be falling be short of your true cognitive capability, you’ll have to find another way to optimize it.