This paper examines the idea that an important dimension of human cognition is the amount of objective evidence required for perception of meaningful patterns. At the clinical extreme of this dimension are patients with hallucinations and delusions who experience perception with no external evidence and see connections between objectively unrelated events. Also, normal individuals exhibit considerable variation along this continuum. The theory proposed here predicts that normal subjects with low evidential criteria will be more likely to accept causal explanations, not only for everyday ''paranormal'' coincidences, but also for random contingencies in a laboratory experiment. This prediction was confirmed when 40 students completed a differential reinforcement of low rates (DRL) task designed to induce superstitious behaviour and were then questioned about their hypotheses concerning the contingencies for successful performance. Participants scoring high on the Magical Ideation scale (indicating greater belief in paranormal phenomena) tested fewer hypotheses during the task, and they ended up believing in more hypotheses regarding illusory contingencies than did their low-scoring peers. We proposed that a continuum of hypothesis-testing behaviour underlies the schizotypy continuum, with ''positive'' schizotypal traits reflecting a Type I error bias and ''negative'' traits a Type II error bias. Differential activation patterns within frontal-limbic networks are tentatively suggested as a physiological correlate of the behavioural continuum.

Cohen, in a now classic paper on statistical power, reviewed articles in the 1960 issue of one psychology journal and determined that the majority of studies had less than a 50-50 chance of detecting an effect that truly exists in the population, and thus of obtaining statistically significant results. Such low statistical power, Cohen concluded, was largely due to inadequate sample sizes. Subsequent reviews of research published in other experimental psychology journals found similar results. We provide a statistical power analysis of clinical neuropsychological research by reviewing a representative sample of 66 articles from the Journal of Clinical and Experimental Neuropsychology, the Journal of the International Neuropsychology Society, and Neuropsychology. The results show inadequate power, similar to that for experimental research, when Cohen's criterion for effect size is used. However, the results are encouraging in also showing that the field of clinical neuropsychology deals with larger effect sizes than are usually observed in experimental psychology and that the reviewed clinical neuropsychology research does have adequate power to detect these larger effect sizes. This review also reveals a prevailing failure to heed Cohen's recommendations that researchers should routinely report a priori power analyses, effect sizes and confidence intervals, and conduct fewer statistical tests.

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