Although there is a voluminous psychiatric literature concerned with obsessional personality disorder, there are very few psychological studies of the condition. In particular, whilst the thinking of anankasts has been classically and perceptively described by many psychiatric authorities, only a handful of psychological experiments exclusively concerned with anankastic cognition have been reported (cf. Skoog, 1964). It is suggested that this is largely because interest has centred on the content of obsessional behaviour at the expense of its form. The present study is a brief report of one among several investigations suggested by consideration of the formal qualities of obsessional thought as opposed to its “dynamic” or symbolic features. The hypothesis (Reed, 1968) is that the formal characteristics of anankastic cognition are directly related to functional impairment in the spontaneous organization and integration of experience. It is postulated that this failure is expressed in the over-structuring of input and in the maladaptive over-defining of categories and boundaries. From this may be derived the prediction that, given a classificatory or conceptual task, the anankast will be over-specific in his interpretation of the given class and therefore too strict in his acceptance of appropriate class members and attributes.