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Age differences in syllogistic reasoning in relation to crystallized and fluid ability were studied in 278 adults from 19 to 96 years of age. Two reasoning tasks, the evaluation and the construction of conclusions for syllogisms of varying complexity and believability, a vocabulary test, and 3 tasks of working memory were administered. The magnitude of age-related variance on selected reasoning tasks was only partially reduced by statistically controlling measures of both working memory and vocabulary. Additional age-related effects on reasoning were found to be significantly associated with number of mental models and bias produced by conflict between belief and logic. A significant bias was also found toward acceptance of invalid syllogisms as valid, even when contents were abstract. These sources of error in logic are discussed in relation to Johnson-Laird's (1983) theory of mental models and Evans's (1989) account of bias in human reasoning.

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If two gratings with a sinusoidal luminance profile are crossed and if each has a different colour then they are seen to alternate; first one dominates and then the other. Part of the time both are seen. The rate of this monocular rivalry varies with the angle between the two gratings. If the two are lined up in parallel the percept is quite stable and remains so until their orientation is about 15° to 20° apart. The rate of alternation then increases rapidly reaching a maximum rate of 30 per minute. The orientational selectivity of the rivalry suggests that the orientational properties of the neurones in the visual cortex discovered by Hubel and Wiesel are involved. Acknowledgements. We wish to thank Professor Peter M. Milner for his keen interest in these experiments and for acting as a subject. F.W.C. is supported by a grant from the Medical Research Council. The work of A.S.G. during her visit to Cambridge was supported in part by a research grant GB-31029 from the National Science Foundation. E.R.H. was awarded a Wellcome Trust Fellowship while in Cambridge.