Why are autism and the fragile-X syndrome associated? Conceptual and methodological issues. Investigations of the association between autism and the fragile-X syndrome have yielded conflicting results with some studies indicating a strong correlation and others indicating no relation between the disorders. In this paper, we review the relevant research on this controversy and discuss the conceptual and methodological problems involved in such an inquiry. We conclude that autism and fragile X are associated and that this relation will prove fruitful in understanding the role of the X chromosome in a variety of behavior disorders and in unraveling various theoretical accounts on the etiology of autism. 