1. In the Middle Ages, people believed that men had two testicles because one produced boys and the other produced girls
Listen, it's important to remember that science was incredibly primitive until very recently - they simply didn't have the technology or know-how in the Middle Ages to truly understand anatomy or reproduction (and were probably more concerned with day-to-day worries regarding dying of starvation or catching the plague anyhow), but this misconception was a doozy: it was generally believed that men had two testicles for a very specific reason: one created "boy sperm," and the other created "girl sperm."
If you're a Middle Ages person, that seems to make enough sense - why would men have TWO testicles if all of their sperm had equal chances of being a boy or a girl? Logistically, it stood to reason that each gender had their own testicle. The problem came when people would ACT on that misconception - because it was typically more advantageous to have a boy than a girl for most, men would go to certain lengths to ensure their offspring was male. Like...removing the testicle associated with producing girls.
Yep - removing a testicle. In the Middle Ages. Meaning WITHOUT ANESTHESIA.
Of course, not everyone would go that far - some would simply tie off their left testicle with some twine to "choke off" the girl sperm during sexual intercourse (as the philosopher Aristotle believed). Regardless, the reality of how gender is determined (the chromosomal makeup of the fertilizing sperm) would not be known until the late 19th century.
2. In the Middle Ages, some people believed that being a redhead was a sign you were a vampire or a witch
Redheads have faced discrimination and unfair stereotyping through all ages of history - even in modern times (South Park dedicated an entire episode to questioning whether gingers had souls). But the type of misconceptions that existed around redheads / gingers in the Middle Ages goes pretty far beyond the pale - like, believing that being a redhead was a sign that you were a demonic entity like a vampire or a witch.
One of the most prominent sources of this misconception was the English translation of the Malleus Maleficarum - an extensive essay on the topic of witchcraft, the dangers it posed, and the necessity of the extermination of its practitioners - by Montague Summers. The Malleus Maleficarum was written by a Catholic clergyman in the 15th century - and quickly became a phenomenon, being the 2nd best-selling book throughout large portions of Europe for about 200 years (the only book to outsell it regularly was The Bible). In the English translation, an entire section was dedicated to the implication that red hair and green eyes were signs of a living demon:
Those whose hair is red, of a certain peculiar shade, are unmistakably vampires. It is significant that in ancient Egypt, as Manetho tells us, human sacrifices were offered at the grave of Osiris, and the victims were red-haired men who were burned, their ashes being scattered far and wide by winnowing-fans. It is held by some authorities that this was done to fertilize the fields and produce a bounteous harvest, red-hair symbolizing the golden wealth of the corn. But these men were called Typhonians, and were representatives not of Osiris but of his evil rival Typhon, whose hair was red.
Furthermore, ancient Romanian beliefs stated that someone with red hair (and blue eyes) were likely to be "strigoi" (which was basically their equivalent of a vampire - a supernatural being that fed on the blood of livestock or people and had transformational abilities).
Interestingly, red heads DO typically display some significant physical differences from the rest of the population - higher tolerance for certain types of pain, a resistance to anesthesia, and differences in melanin levels. But, as far as we can tell, they're not vampires.
3. In Ancient Egypt, it was believed that men menstruated just like women did...because of a common disease that led to blood in the urine
Disclaimer: men are incapable of menstruation. Never really thought I would have to write that out so explicitly, but I just wanted to be clear before moving forward.
There was a belief in Ancient Egypt that men menstruated, just as women did. Why, you ask? Because blood coming out of the nether regions of men was nearly as common as it was for women - but for entirely different reasons. Namely, because - given the lack of sanitation and reliance on the Nile River - it was incredibly common for men to contract a disease called "schistosomiasis."
"Schistosomiasis" is caused by parasitic flatworms that infect the host's urinary tract or intestines - and typically comes along with blood in the urine and/or stool, abdominal pain, and ultimately can lead to permanent organ damage. The parasites are commonly found in fresh water sources - and Ancient Egypt's reliance on the Nile for day-to-day life - from drinking water to bathing - made contracting the disease almost an inevitability, particularly given how little proper hygiene was understood at the time.
It gets worse though - not only was this disease commonly accepted as a natural part of life, it was even CELEBRATED: boys who found blood in their urine were often told this was a sign of maturity, a rite of passage similar to women receiving their first period. Except, obviously, the difference was that they had a painful and unnecessary parasite infecting their urinary tract.
4. Doctors believed that washing their hands or sterilizing their equipment was unnecessary
Here's something that'll make you glad you weren't born in the 19th century or earlier - up until the work of Joseph Lister, surgeons largely did not believe it was necessary to take any substantial hygienic measures before performing surgery. Now, they WERE aware of infections and post-surgical complications, but believed that these were caused by "bad air" (what was broadly known as the miasma theory - the idea that disease was caused by mysteriously polluted air that came from rotting meat). And as a result, surgeons would rarely take any antiseptic measures in preparing their own hands or instruments before a surgery...which very, very often led to deadly or painful post-surgical infections.
A famous example of this bad, bad idea in practice was the assassination of James Garfield - in 1881, Garfield was struck by two bullets from assassin Charles Guiteau, but only one remained lodged in his body. Regardless, the injury was not nearly as certainly fatal as you might think - doctors today theorize President Garfield would have survived and been out of the hospital in a matter of days if they had access to modern medical knowledge and technology...but also many believe that he would have survived back in 1881 if his doctors had simply not prodded his wound repeatedly with their unwashed and unsterilized hands.
Which they did. For WEEKS. Leading to sepsis and ultimately, his death. But if they had only thought that sterilization was a necessary precaution, he probably wouldn't have died.
And as a bit of a sidenote about OTHER bizarre misconceptions of the time, the doctors began feeding President Garfield RECTALLY about a month or so after the assassination attempt - literally shoving beef bouillon, egg yolks, milk, and other foods and liquids up his anus in an attempt to keep him nourished. Within a month, he had dropped about 100 lbs. in weight, and some theorize his state of utter malnourishment was a major contributor to his death.
So remember surgeons: wash your hands and don't shove eggs up your patients' butts.
5. It was believed that maggots would spontaneously generate from meat when left alone
Up until the mid-19th century and the work of Louis Pasteur, it was very commonly understood that life could spontaneously appear from or within non-living objects, with no parents or eggs or any traditional life-emitting things necessary. The most famous example was maggots spontaneously appearing in meat or flesh - it was shown that leaving a cut of meat in a jar and sealing it shut would eventually be covered in maggots. And since there was no way flies could have gotten into the jar, the only possible explanation was that the maggots somehow just self-generated in the meat.
Of course, this was insanely wrong - life cannot just APPEAR from out of nowhere for no discernible reason. In the example of the maggots in the meat, it was the fact that flies would lay their eggs in the meat prior to the meat being sealed in the jar. The eggs were so small and imperceptible, they would go unnoticed - and then the maggots would hatch, seemingly from out of thin air.
So no matter what Dr. Ian Malcolm tells you, life does not always find a way.
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