Human quadrupeds discovered in Turkey
The siblings, the subject of a new BBC documentary to be aired on March 17, suffer from a genetic abnormality that may prevent them from walking upright.
Two sisters and one son have only ever walked on two hands and two feet, while another daughter and son occasionally walk on two feet.
All five are mentally retarded and have problems with language as a result of a form of underdevelopment of the brain known as cerebellar ataxia.
However Humphrey told the Times their behaviour may be partly the result of their parents tolerating the behaviour in childhood.
They are aged between 18 and 34 and live in southern Turkey, athough the makers of the documentary have not disclosed their exact location.
"They walk like animals and that's very disturbing at first. But we were also very moved by this family's tremendous warmth and humanity," Jemima Harrison of Passionate Productions told the Times.