The Gibbons Twins
June and Jennifer Gibbons were twins who grew up near a Welsh army base. They developed speech late and when they finally did speak, they spoke mostly to each other in weird, garbled language. As they aged, their odd speech patterns became so foreign that no else could understand them. For the twins, that was just fine, because they only wanted each other's company anyway. They even mirrored each other's physical actions without conscious effort.
They were black children in a white community, regularly alienated and bullied. The girls withdrew even further. Psychologists separated them in an attempt to change their behavior, but the girls simply withered without each other.
Once they were reunited they spent all of their time together, playing and writing creative stories. They also took up arson, for which they were arrested and then committed to a high-security mental hospital for 14 years, in part because people found their twin-obsessed behavior so unsettling.
In 1993, the women were finally released to another clinic. Upon leaving the hospital grounds, Jennifer died from sudden inflammation of her heart, although no cause was ever determined.
June later revealed to a therapist that the two had made a pact that upon their release, one of them had to die. The two had finally found the constant presence of the other to be overwhelming — in order for one of them to truly live, the other had to perish.
There was no evidence of foul play, and June now lives alone, near her parents in Wales.