There are plenty of "facts" that everyone thinks are true just because of the sheer number of times they've heard them repeated. The illusory truth effect, also known as the reiteration effect, shows that repeated statements are easier to process than new ones and people tend to believe things that they hear repeated often.
This can lead to believing some pretty wild things that can be fairly obviously false if you stop to really think about it. Below are some excellent examples of this phenomenon.
Reddit user u/HaseebM1 asked:
That aquarium fish only live a few months.
My coworker was astonished that my fish had been alive for years and still had years left in them. When I told him it was because they had a big enough tank, a filter and water changes, he commented that that was way too much for a fish that won't live 3 months....big dumb circle of a conversation.
Too add onto this comment it is not true that fish will grow to the size of the tank. For example if you put a goldfish in a 10 gallon tank, yes, it will be smaller but, its organs don't stop growing and they die a slow horrible death.
Research the fish you get to make sure you can provide a good life for them!
Cutting an earthworm in half won't make two worms. It'll just give one worm a very good reason to be angry.
That brushing your teeth twice a day is all you need to prevent cavities. Actually, it's just as important to always have saliva in your mouth.
My dentist blamed my cavities on me sleeping in front on a fan with my mouth open.
That houseplants use a dangerous amount of oxygen at night and therefor shouldn't be put in (children's) bedrooms. They use less than one tenth of what they produce during the day. If they'd use more our planet would be in deep trouble and humans wouldn't have existed.
Deoxygenated blood is blue
I had a Biology high school teacher try to teach us that
That most Europeans thought the world was flat in 1491.
The Greek scholar Eratosthenes calculated the circumference of a spherical Earth around 200BC, to a surprisingly accurate degree. He's also credited with effectively inventing Geography.
'If you put a nail in a tree, in the future it will be higher up as the tree grows.'
I have seen this on far too many tv shows, this is not how trees work. Trees accumulate height at the top annually while the rest of the tree grows in diameter.
People think when you hit a higher tax bracket your entire income is taxed at the higher rate.
That different parts of your tongue are made for different tastes, I think the front is salty and the middle is sweet or something idk. I remember learning it in primary school, and Then when I grew older I asked my science teacher, she said it was false.
What? I remember doing that thing where you put q-tips into different flavored water (Sugar, Salt, Lemon, Grapefruit juice, tabasco,...) and then make a "map" of where you taste something...
...and it worked.
Guess a strong believe in something can make it seem real.
Every time I did that it never worked but every time I said "I can't tell the difference" I'd get ostracised by my peers.
That you are "left-brained" or "right-brained". This was an old theory (Galaburda-Geschwind model) which tried to argue that this is why women are more emotional whilst men are more rational. It's a load of horsepoop.
I'm always immediately wary with anyone who prides themselves on being "logical", or think that logic and emotions are somehow yin-yang type opposites, because nine times out of ten they equate "logical" with "infallible", and as a result they're the most insufferably smug know-it-all douchebags you'll ever meet.