Some children became self-conscious and reluctant to speak

In 1939, the plaintiffs - all orphans in state care - were tormented for six months by Iowa University researchers.

The study was testing the theory that children develop speech impediments because of psychological pressure.

In 2001, those involved learned the truth behind their treatment, and sued, originally seeking $13.5m in damages.

The six, now in their 70s and 80s, said the experiment had left them with psychological and emotional scars.

'Reluctant to speak'

Iowa's attorney general said in a statement that the settlement of $900,000 to five of the plaintiffs and $25,000 to one other was "fair and appropriate".

"For the plaintiffs, we hope and believe it will help provide closure relating to experiences from long ago and to memories going back almost 70 years," said Tom Miller.

The 1939 experiment was dubbed The Monster Study because of the researchers' methods.

Over a period of six months, Dr Wendell Johnson, a pioneer in speech pathology, tested his theory on 22 children from the Iowa Soldiers' Orphans' Home.

Some were subjected to prolonged harassment, while the remainder were used as a control group.

None of the children became stutterers but some became self-conscious and reluctant to speak, according to the study.

In 2001, a Californian newspaper revealed details of the secretive study, basing its story on the testimony of a former research assistant.