Straffen's defence called several of those who had seen him in earlier years to give evidence of his mental condition. The prosecution then called prison medical officers and psychiatrists to give evidence in rebuttal. Dr Thomas Munro, who was a specialist in mental deficiency and had seen Straffen, testified that he had said that to murder was wrong because it was breaking the law and because "it is one of the commandments". When Munro asked Straffen to name the other commandments, Straffen could remember only four.