What are some things everyone "knows" that are no longer true?

A few examples of things everyone “knows” that are no longer true.

“Don't crack your knuckles. You'll develop arthritis” (Cracking joints - Wikipedia). Scientific evidence doesn't support the idea.

• A 1990 study confirmed this in a controlled study of 300 knuckle crackers and 226 non-knuckle crackers (1), though it did conclude knuckle crackers ended up with swollen hands and lower grip strength.
• A prior 1975 study on 28 elderly residents in a home for the aged had also found no correlation between knuckle cracking and osteoarthritis (2).
• A quirky case study with an n of 1 consisted of the author, a rheumatologist, who for 50 years consistently and daily cracked twice a day the knuckles of only his left hand, leaving uncracked those on the right as controls. Fifty years later and left hand knuckles cracked some 36500 times, no arthritis in either hand and no apparent differences between the two hands (3).
• Subsequent reviews and studies only concluded more of the same, no arthritis from knuckle cracking (4, 5, 6).
• Use of better methodology and more sensitive technology found no evidence either that knuckle cracking led to hand swelling or lowered grip strength (7).
• Losing the ability to crack knuckles might even be emotionally distressing for some, according to a study on patients with end-stage renal disease (8). It appeared habitual knuckle crackers in this group of patients had lost their ability to do so, apparently a result of hyperparathyroidism, a secondary feature of their kidney disease. Surgical removal, parathyroidectomy, restored this ability, bringing back “great satisfaction from the emotional relief from what appeared to be habitual knuckle cracking”.

“Get cold, catch cold”. Hundreds of years old this superstition and yet controlled studies on volunteers repeatedly failed to prove exposure to cold led to increased risk of catching a 'cold' (9, 10, 11).

Five-second rule - Wikipedia, drop food or cutlery and they still remain safe enough to eat or use if picked up within five seconds. Several scientific studies have by now thoroughly debunked this myth.

“Shaving hair makes it come back thicker and faster”. Hmm, sounds like a surefire cure for baldness so bald people remain so by choice, eh?

Bibliography

1. Castellanos, Jorge, and David Axelrod. "Effect of habitual knuckle cracking on hand function." Annals of the rheumatic diseases 49.5 (1990): 308-309. http://ard.bmj.com/content/annrh...

2. Swezey, Robert L., and Stuart E. Swezey. "The consequences of habitual knuckle cracking." Western Journal of Medicine 122.5 (1975): 377. https://www.ncbi.nlm.nih.gov/pmc...

3. Unger, Donald L. "Does knuckle cracking lead to arthritis of the fingers?." Arthritis & Rheumatology 41.5 (1998): 949-950. Does knuckle cracking lead to arthritis of the fingers?

4. Gaetano, John. "Cracking the Cracked Knuckle: A Medical Student’s Take." The Journal of rheumatology 36.11 (2009): 2624-2624. http://citeseerx.ist.psu.edu/vie...;

5. Olszewski, Mariusz, and Rebecca Ortolano. "Knuckle cracking and hand osteoarthritis." The Journal of the American Board of Family Medicine 24.2 (2011): 169-174. Knuckle Cracking and Hand Osteoarthritis

6. Powers, Tye, Gary Kelsberg, and Sarah Safranek. "Does knuckle popping lead to arthritis?." (2016). https://mospace.umsystem.edu/xml...

7. Yildizgören, M. T., et al. "Effects of habitual knuckle cracking on metacarpal cartilage thickness and grip strength." Hand Surgery and Rehabilitation 36.1 (2017): 41-43. https://ac.els-cdn.com/S24681229...

8. Ross, Edward A., Jennifer L. Paugh-Miller, and Robert W. Nappo. "Knuckle cracking: secondary hyperparathyroidism and what your mother did not tell you." Clinical kidney journal 6.6 (2013): 671-673. https://pdfs.semanticscholar.org...

9. Andrewes, Christopher Howard. "Adventures Among Viruses. III. The Puzzle of the Common Cold." Reviews of infectious diseases 11.6 (1989): 1022-1028.

10. Dowling, Harry F., et al. "Transmission of the common cold to volunteers under controlled conditions. III. The effect of chilling of the subjects upon susceptibility." American journal of hygiene 68.1 (1958): 59-65.

11. Douglas Jr, R. Gordon, Keith M. Lindgren, and Robert B. Couch. "Exposure to cold environment and rhinovirus common cold: failure to demonstrate effect." New England Journal of Medicine 279.14 (1968): 742-747.

https://www.quora.com/What-are-some-things-everyone-knows-that-are-no-longer-true/answer/Tirumalai-Kamala