While expert evidence may be deemed relevant when deciding the impact of any mental defect of the accused, it has generally been ruled that evidence cannot be adduced as to the credibility of witnesses.[11] In the care proceedings of Re S (a child) (adoption: psychological evidence) it was deemed wrong that a Judge should consider evidence that the mother in the proceedings was prone to lie to authorities, based on the results of a psychological test. Another clear example is illustrated in R v Robinson,[12] where the prosecution called an educational psychiatrist to effectively bolster the testimony of the complainant, by asserting that due to his educational difficulties he was unlikely to lie. The Court of Appeal overturned the appellant's conviction based on this evidence, with the court labeling such evidence as merely 'oath-helping'.[13] A limited exception to the rule was established in R v Lowery.[14] Here, two co-defendants were charged with murder, with the facts of the case such that one of them must have committed the offence.