The Psychology of Forced Confessions:
Research conducted throughout the seventies by psychologist Gisli Gudjonnson and psychiatrist James McKeith demonstrated that some persons, particularly those of low intelligence, may be highly suggestible to certain forms of questioning - so that they make statements that can be taken to be confessions.
Steel was examined in March 1996 by one of the leading experts in this field - Mrs Olive Tunstall at the Royal Free Hospital in London.
Mrs Tunstall had read various reports and statements from the investigations of 1977 and 1979. She assessed Steelís intelligence, his educational attainments, personality and his suggestibility. Her conclusions, in summary, found that Steel is bordering on being mentally retarded, his reading age is that of a 10 year old, his personality is normal and his suggestibility falls within the "abnormal" range.
Mrs Tunstall concluded that Steel is "potentially vulnerable in the context of the police interviews". She considered that he may not have fully understood the caution put to him and was not properly aware of his rights.
What was most surprising was that Mrs Tunstall also added that that there is some evidence to suggest that the witness statements from the police do not provide a complete record of the interviews they had with Steel.
In an endpaper to her report, she analyses the police interviews by dividing the length of time taken by the number of pages of resulting transcript. She discovers some apparent discrepancies; some interviews yield less than a quarter the number of words per minute than others. The range is between 2.77 minutes of interview per page of transcript to 22 minutes per page of transcript.
This type of evidence was first admitted in the court in the Guildford Four case in 1989.
If you would like to know more about it, the authoritative work on the subject is "The Psychology of Interrogations, Confessions and Testimony" published by John Wiley & Sons, by Gisli H. Gudjonsson. In particular pages 289 - 293 and the footnote on page 297 might be helpful regarding admissibility of such evidence.