Twelve medical doctors in all examined Haigh in prison, some before and some after his trial.  They were particularly interested in his claims to have a compulsion to kill for blood.  Most often, such a compulsion is part of a sexual deviation and is incidental to the sexual frenzy itself.  Haigh gave no indication of such a perversion.  In fact, he seemed to have little interest in sex.

Haigh went through several examinations, including an electroencephalogram.  The results were normal.  Most of the doctors were of the opinion that he was sane and was merely malingering, or faking, his insanity.

Four psychiatrists examined him for the defense.  Not one was able to give the opinion that Haigh was not responsible for his actions.  Dr. Henry Yellowlees, when told of the opinion of his colleagues, came up with a different result.  He believed that Haigh was mentally ill, consistent with the description of paranoia, but even that diagnosis was not conclusively deemed a mental disease.  Nevertheless, this professional opinion was all that the defense had.

Yellowlees, 61, was a physician with a degree in psychological medicine.  During the war, he had been a consulting psychiatrist to the British Expeditionary Force in France.  He was also examiner in mental diseases for the University of London.  He visited the prison on five different occasions between July 1st and July 6th.  During three of those visits, he interviewed Haigh.  He also had examined Haigh's two confessions thoroughly, as well as looking over all other documents in the case.  To Yellowlees, it was obvious that Haigh had a "paranoid constitution"—the same mental disease as Hitler.

According to descriptions in the 'forties, such a condition results partly from heredity and partly from environment, in particular the early upbringing.  It is a preliminary stage to the "paranoid insanities."  Based in part on what Haigh had told to Dr. Matheson about his childhood and upbringing, Yellowlees explained how Haigh had been sheltered in a fanatical and paranoid religion and raised by a mother who gave a lot of credibility to dreams as tools of divination.  He was made to fear the wrath of God for every false step, and he was not allowed to have friends.  "The solitary schoolboy," he said, quoting Dr. Perry Smith, "is the potential paranoiac."  To the psychiatrist, this was not the picture of a stable home.  A youth raised in such a place is bound to escape into fantasy.

Yellowlees also noted how important it was that Haigh had been raised in one extreme form of religion and then had plunged into another extreme, which essentially was considered a sin within his primary religion.  "I think the change would appear to him an ideal way to escape."  He also mentioned the recurring dream Haigh had as a teenager of the bloody Christ.  "All along it was the question of blood that was troubling him."  He then went on to say that a person forming a paranoid personality develops a certain amount of secrecy, which Haigh assuredly did.  They develop a private mystic life, "which they treasure because it is apart from the cruel world."

Such a person then believes he is cleverer than others are and can get away with things.  That is the first stage of the paranoid personality.  He begins to live two lives.  He has to be part of society and also to avoid having his clever bluff called, so he becomes vain and takes delight in taking advantage of others for his own gain.

Yellowlees used as his point of reference a book written by Professor Tanzi on mental disorders.  There are various types of paranoia and the one that he felt fit Haigh was "the most rare and terrible" of the lot.  It was one of the "egocentric paranoias," sometimes referred to as "ambitious" or "mystical" paranoia.  The patient's fantasy world becomes his psychological home.  He views himself as omnipotent.  He is in touch with some outside force that guides him.  Yellowlees mentioned that Haigh had told him that he had been divinely guided by an interpretation of a verse in the Old Testament to drink his own urine.  He claimed to have followed that instruction quite regularly.  Paranoiacs are also uninterested in sex, because the sexual instinct is "sublimated" into self-worship, and Haigh apparently was consistent in that respect.  He believed that by killing these people he was fulfilling some destiny.  He knew that what he was doing was punishable by law, but he believed he was above the law.

"I think," said the physician, "that the absolute callous, cheerful, bland and the almost friendly indifference of the accused to the crimes which he freely admits having committed is unique in my experience."

While he did not think the blood dreams were invented, he thought that Haigh had exaggerated their effect on him.  He thought, too, that while Haigh had tasted blood, it was doubtful that he drank it as he claimed to do.  Yellowlees thought he was too lucid and intelligent not to know what he was doing.

Haigh wrote a note to him identifying the various unusual personalities throughout history, including Christ and Hitler, in an effort to get the doctor to understand the full scope of his abnormality.  He didn't bite.

What Yellowlees failed to find out is that Haigh had befriended an employee of Sussex psychiatric hospital and over the years had gathered a lot of information about mental illness.  He knew about the behavioral patterns, traits, and habits of various disorders.  The subject fascinated him and he never ceased to ask questions.  In the past, he had posed as many other things—a lawyer, an engineer, a doctor—so it would not be difficult for him to pose as a person suffering from a mental condition.  Most people were of the mind that he was doing precisely that—although not in a way that convinced most of those who examined him. 

Yellowlees' diagnosis was put to the test in court.