The five belonged to the Bowes-Lyon family of Queen Mother Elizabeth, who at 87 is the most popular member of British royalty.
The first cousins were listed as dead in the 1963 edition of Burke's Peerage, a guide to the British aristocracy, even though one - Katherine Bowes-Lyon - is still alive and the second died only last year. Two of the three second cousins are still alive.
Speculation immediately developed on two issues. The first was that there was an inherited mental illness in the family. "If five female cousins all have one mental problem, it's pretty obvious the cause is genetic," leading genealogist Hugh Peskett was quoted as saying.
But he and other genealogists agreed that whatever flaw existed appeared to be connected to the royal family by marriage, not blood. They trace the illness to the 21st Baron Clinton, who died in 1957 and was survived by two daughters. One daughter, Fenella, married the brother of Queen Mother Elizabeth, John Bowes-Lyon, and had two daughters, Katherine and Nerissa. The other daughter, Harriet, was the mother of Idonia, Rosemary Jean - who died in 1972, records show - and Etheldreda Fane.
The Fane and Bowes-Lyon sisters were all taken to the Royal Earlswood mental hospital in 1941, according to reports.
"The great relief is that the genes are obviously in the Clinton family and not in the royal family," Peskett said. "It is clearly a Clinton family gene which is wrong, rather than that of the Bowes-Lyon family."
The second issue is whether Nerissa and Katherine were reported dead in order to circumvent complicated inheritance rules, which prevented the present Lord Clinton from taking up his great-grandfather's title while they were alive.
Asked about such speculation, Lord Clinton said, "It is a theory, but it is not true. My great-aunt was very vague, and she probably made a mistake filling in the forms for Burke's. Some people are vague when they get that old. I was the senior in line in the family, so why shouldn't the title have come to me? There really is not a cover-up, there's nothing to hide."
Lord Clinton, speaking at the 20,000-acre family estate, said that his family never spoke of his mentally retarded relatives during his boyhood. He was 7 years old when his aunts were admitted to the hospital. Now 52, he said that he remembers visiting Etheldreda at the hospital about 15 years ago, but that he never met any of the others.
A spokesman at Buckingham Palace, the queen's London home, had said that the monarch "was aware" of the confinement of cousins Katherine and Nerissa Bowes-Lyon but that it was "a matter for the immediate (Bowes-Lyon) family."