Conjoined twins have long been the subject of great curiosity due in part to the rarity of the condition. Not to mention, many different ways exist for siblings to become anatomically connected to one another. Fueled by that curiosity, a Soviet physiologist named Pyotr Anokhin - who studied the development and structure of human circulatory and nervous systems - made Masha and Dasha Krivoshlyapova the focus of many of his science experiments - experiments that were conducted for the majority of their lives.
These USSR-approved experiments on conjoined twins subjected the sisters to torture, isolation, and a life greatly lacking in typical human interactions. Adding to the complexity of Masha and Dasha Krivoshlyapova's situation, the sisters had diametrically clashing personalities - one showing the traits of a psychopath and the other an empath.
A Soviet physiologist named Pyotr Anokhin dedicated his career to conducting experiments that sought to pinpoint the different functions of the human circulatory and central nervous systems, specifically when a person was sleep-deprived, subjected to extreme temperature changes, or denied sustenance. So, when conjoined twins Masha and Dasha were born in Moscow in 1950 possessing one body that contained a joint circulatory system and two separate nervous systems, Anokhin saw them as the perfect subjects for his future experimentation.
Shortly after they were born, the twins were taken from their mother, who was told they had died of pneumonia just after birth. In reality, the sisters were transported to a nearby medical institute in Moscow where they became the subjects of extensive, life-altering experiments.
Since Masha and Dasha shared the same circulatory system but had separate nervous systems, scientists were particularly interested in testing each twin's reaction (or lack thereof) to the other twin's physical distress. The experiments they conducted involved tactics like covering one twin in ice while the other twin would be carefully observed for a reaction. Similar experiments were also conducted with extreme heat, painful stimuli, and even the injection of radioactive iodine.
The girls are also believed to have been electrocuted, had tubes inserted into their stomachs to measure gastric juices, and to have been deprived of sleep, particularly during their formative years, from birth to around age 12.
While the girls were living at the medical institute, the USSR Academy of Medical Sciences filmed many of their interactions with Masha and Dasha, from their infancy well into their early childhood years. The resulting film documents each of the twins being probed with various stimulus tools to measure the reaction of the 'unharmed' sister. The footage also shows them learning how to put on socks and use crutches for walking.
They were also reportedly injected with radioactive iodine to see if the substance would make its way from one sister's bloodstream into the other's. In another experiment, one twin was fed while the other starved, in an attempt to measure the gastric juices of both using tubes inserted into their stomachs.
Masha became noticeably disobedient as soon as Soviet scientists began trying to measure the motor skills of the twins as toddlers. While Dasha would lift the leg she controlled in response to a nurse's request to put socks on her, Masha would ignore the caregiver and even throw the sock away aggressively. As a result, Dasha quickly learned how to put socks on herself while Masha was unable - or unwilling - to concentrate on the task long enough to get it done.
As the sisters grew older, they became friends with a woman named Juliet Butler, who claims Masha would scream threats at Dasha while physically attacking her. Butler also noted that Masha often refused to let Dasha speak for herself in many of the trio's conversations, and alleged that Masha compulsively lied and showed narcissistic tendencies.