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Questionable scientific studies were performed for centuries until ethics groups were formed to restrict the types of experiments that could be carried out on live subjects. Fortunately, the fields of biomedicine and psychology have improved greatly when it comes to the types of experiments that can be done on humans and the importance of consent.

Nonetheless, shocking human trials were still performed up until relatively recent history, and cruel experiments involving animals are still carried out to this day. Click through the gallery to learn about some of the most heinous and unethical science experiments of all time.

▲Psychologist Harry Harlow is well-known for his experiments on monkeys in the early 20th century. One experiment involved separating baby monkeys from their mothers to study clinical depression.

▲After they had bonded with their mothers, Harlow would place the monkeys in total isolation for 10 weeks. The monkeys became psychotic after a handful of days and were unable to be treated.

▲At Utah State University, in the US, scientists genetically engineered goats that could produce milk containing the proteins found in spider silk.

▲As spider silk is several times stronger than steel, the hope was that these new spider-goat hybrids could produce silk that would later be made into biosteel polymers and manufactured into things like artificial ligaments and tendons.

▲During World War II, the Japanese Army performed chemical and biological warfare experiments on Chinese, Korean and Mongolian natives.

▲The experiments included live weapons testing, germ warfare attacks, forced pregnancy, and vivisections performed without anesthesia.

▲General of the US Army Douglas MacArthur pardoned the Japanese doctors participating in these war crimes, saying that they were assisting America in biological warfare research.

▲In Tuskegee, Alabama, United States, several hundred African-American male farmers who had contracted syphilis underwent a study of a medicine they were told would cure their disease.

▲However, even after penicillin was found to treat the disease, scientists denied the subjects the treatment as they wanted to observe the natural progression of the disease.

▲By the end of the experiment, many of the men had died from the disease itself or related complications. The leaking of the event led to changes in consent laws for scientific experimentation in the US.

▲Human limb regenerative powder was developed by the University of Pittsburgh's McGowan Institute of Regenerative Medicine in the United States.

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The catch is that the powder was developed from a pig's bladder, as detailed by the The Huffington Post. Apparently, the cells scraped from the bladder stimulate stem cells in the body to construct new tissue.

▲At the Salk Institute for Biological Studies in California, United States, researchers grew human brain cells out of stem cells injected into mice.

▲The human brain cells were found to successfully integrate into the developing nervous system of newborn mice, paving the way for future studies on new pharmaceutical therapies for neurodegenerative diseases, as outlined on Salk's website.

▲In the 1890s, the founder of psychoanalysis, Dr. Sigmund Freud, decided to treat his patient Emma Eckstein for what he determined to be hysteria.

▲He believed he could cure her by cauterizing her nose and had his surgeon-friend Wilhelm Fliess perform the surgery. She ended up developing a multitude of infections and was left permanently disfigured.

▲In medieval times, Holy Roman Emperor Frederick II was infamous for carrying out barbaric social experiments during his time in power.

▲In one experiment, he attempted to figure out if humans have a natural language by isolating newborn babies and not allowing them to have any social interaction. Instead of developing a language, they became savages and died in horrific conditions.

▲Psychologist and s** behavior expert Dr. John William Money had a theory that gender is learned rather than innate. He performed reassignment surgery on a baby boy.

▲The doctor convinced his parents to raise him as a girl, but the experiment failed when the child ended up identifying as male and not female.

▲Dr. Henry Cotton, a doctor at the New Jersey State Lunatic Asylum in the US, believed that diseased internal organs caused insanity.

▲In 1907, without getting consent from his patients first, the doctor extracted various body parts, including teeth and intestines, to study them.

▲Many of his patients ended up dying from the surgeries, which Dr. Cotton attributed to "end-stage psychosis," according to The New York Times.

▲Dr. Lauretta Bender of New York’s Creedmoor Hospital studied the effects of electroshock therapy on children in the 1960s.

▲She believed that it could be used to treat social issues and early signs of schizophrenia. She ended up applying the therapy to over 100 children before being forced to stop.

▲In the 1950s, the CIA sponsored a research project to study the effects of LSD and mind-altering chemicals.

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Unsuspecting individuals were lured by escort girls to a testing room where they were unknowingly given these substances and observed through a one-way glass.