It has been suggested that category-specific impairments for natural objects may reflect that natural objects are more globally visually similar than artefacts and therefore more difficult to recognize following brain damage [Aphasiology 13 (1992) 169]. This account has been challenged by the finding that the 'normal' disadvantage claimed for natural objects may be reversed when items from the categories of natural objects and artefacts are matched for visual complexity, familiarity and name frequency [Neuropsychologia 37 (1999) 1263]. In the experiments reported here it was investigated whether category effects could be found on object decision tasks (deciding whether pictures represented real objects or not), when the stimulus material was matched across categories. In experiment 1, a disadvantage for natural objects was found on difficult object decision tasks whereas no category difference was found on easy object decision tasks. In experiment 2 an advantage for natural objects was found during object decisions performed under degraded viewing conditions (lateralized stimulus presentation). It is argued that these findings can be accounted for by assuming that natural objects are more globally visually similar than artefacts, but that this difference between categories affects performance in different ways depending on task characteristics. Thus, the greater overlap between natural objects may be a disadvantage when the demand on perceptual differentiation is high (as it is in difficult object decision tasks). However, when viewing conditions are degraded and performance tends to depend on global shape information (carried by low spatial frequency components), natural objects may fare better than artefacts because the global shape of natural objects reveals more of their identity than the global shapes of artefacts.