More than 6 million people watched this season’s premiere of “American Horror Story: Freak Show.” Millions more will watch the season finale on January 21.

This season’s story line is built around a 1952 freak show, run by the mysterious Fräulein Elsa Mars, played by Oscar-winning actress Jessica Lange. The plot is fast moving. The action balances horror and camp. But what’s most compelling about “American Horror Story” are the characters — a number of whom are inspired by real people who lived with fascinating medical conditions.

Freak shows and sideshows had their heyday from around the time of the Civil War to the 1930s. Their stars were able to make a living and find acceptance among their peers. But the word “freak” is a sad misnomer, because as we will explore, these were real people who just so happened to have unusual, appearance-altering medical conditions.

The show’s conjoined twins, Dot and Bette Tattler, share a body, but have separate heads with two brains. They also have very different personalities. Bette is innocent, fame hungry, and friendly, while Dot is more distrusting, sullen, and cynical.

Dot and Bette are similar, physically, to real life Minnesota twins, Abigail and Brittany Hensel. Born in 1990, the Hensel twins share two legs and two arms. But they have separate brains, spinal cords, and hearts. Although both can control their shared limbs, they avoid a constant tug of war by having each operate only one side of their shared body.

Fun fact: They also had a short-lived reality TV show on TLC.

The Science

Identical twins develop from a single egg. Ordinarily, the egg separates after it is fertilized. There are two theories about how conjoined twins come about.

  • The egg doesn’t split completely and the twins remain connected.
  • The egg splits but the embryos rejoin and fuse together.

The point at which conjoined twins are connected varies. According to the American Pediatric Surgical Association (APSA), approximately 75 percent are joined at the chest, abdomen, or some portion of both.

The success of surgery to separate conjoined twins depends on where the twins are joined, what organs they share, and how those organs function. Surgery is excruciatingly complex. The Monroe Carell Jr. Children’s Hospital at VanderbiltUniversity provides a slide show of a successful surgery in which twin girls were separated. It’s a fascinating view of the complexity of the procedure.

According to the University of Maryland Medical Center, at least one twin survives around 75 percent of the time. That said, few surgeries are performed because of the risk involved, and in many cases, surgery isn’t an option because of shared vital organs.

How Common Is It?

Although estimates differ, the APSA says the incidence of conjoined twins in the United States is one in 50,000 to 100,000 live births. Sadly, the stillbirth rate is high, estimated at 40 to 60 percent.

Jimmy Darling, played by Evan Peters, performs as Lobster Boy in “American Horror Story: Freak Show.” Why the fishy nickname? Jimmy’s fingers are fused so they look like lobster claws. He moonlights as entertainment at Tupperware parties where the erotic manipulations of his special hands are deemed “a lifesaver for the American housewife.”

The real life “Lobster Boy” Grady Stiles was born in 1937 with fused fingers and toes. He was reported to represent the sixth generation in the Stiles family with syndactyly, a genetic tradition that continues in the family today.

The Science

Syndactyly is a condition characterized by fingers or toes that are fused together or webbed. Syndactyly occurs when fingers or toes fail to separate during the embryo’s development.

How Common Is It?

The Cincinnati Children’s Hospital Medical Center reports that syndactyly is common, occurring in about one out of every 2,500 to 3,000 newborns. In most instances, syndactyly is a genetic disorder and it’s often seen in tandem with other genetic conditions. Surgery is commonly used to divide the digits.

Kathy Bates plays Ethel Darling, a woman with a beard on her face and, usually, a bottle of whiskey in her hand. Her character is similar, aesthetically anyway, to one of America’s most well-known bearded ladies, Madame Devere, who was known for her 14-inch long beard. Devere was a Kentucky girl who married her manager and toured with circuses and sideshows, she died in 1912.

The Science

There are two main types of abnormal hair growth. Hirsutism refers to male-pattern hair growth on women and children. It can be caused by a number of underlying conditions. One of the most frequent causes is polycystic ovarian syndrome, in which women have an imbalance of female sex hormones.

The direct culprits are either high levels of androgen or hair follicles that are overly sensitive to androgen. Androgen is a hormone responsible for men’s physical characteristics and sex organs. Women have androgen, too, but in women it’s mostly converted to estrogen.

Hypertrichosis, informally called werewolf syndrome, is characterized by excessive hair growth over all or parts of the body. The condition can be genetic in origin or caused by certain medications. Hypertrichosis can affect people of both genders and all ages; infants can be born with the condition. Because hypertrichosis is not caused by excess androgen, hormone therapy is not effective.

How Common Is It?

According to the Cleveland Clinic, five to 10 percent of women in their childbearing years have hirsutism. The condition can usually be treated with hormones.

Pepper and Salty, portrayed in “American Horror Story” by Naomi Grossman and Christopher Neiman, have tiny heads and sloped foreheads. The characters are reminiscent of Schlitzie Surtees, a real life, legendary sideshow performer and actor who spent his life in the sideshows of circuses including Ringling Brothers, Barnum & Bailey, and Tom Mix Circus.

Described as an exuberant man who laughed often, Schlitzie was said to be a joy to his audience and colleagues. “American Horror Story” is a reminder that performers described as “freaks” were people with unfortunate medical conditions who were able to forge a life entertaining others.

The Science

These characters have microcephaly syndrome, a condition in which a person’s head size is significantly smaller than normal for their age and gender. As a baby grows in the womb and during infancy, the size of its head is determined by the size of the brain. When the brain doesn’t grow as it should, the child’s head will be smaller, but the person’s face will grow normally.

Some children with mild microcephaly have no developmental problems. But the incidence of mental retardation and neurological deficits, speech problems, abnormal reflexes, and loss of muscle control increase with the severity of the condition. Short stature or dwarfism often accompanies microcephaly.

Microcephaly can be caused by a number of factors, including:

  • chromosomal abnormalities
  • exposure to rubella infection (or German measles)
  • maternal alcoholism
  • environmental toxins

How Common Is It?

Boston Children’s Hospital reports that microcephaly affects about 25,000 children in the United States each year.