Six categories of content interrogatives (roughly, wh-forms) are distinguished in a representative sample of 50 languages, namely those eliciting individuals (x), locations (l), times (t), manners (m), quantities (q) and reasons (r), using categories recognized in Functional Discourse Grammar. The hypothesis is examined that, on the assumption that the named categories show increasing cognitive complexity, there will be parallel degrees of system, item and signal complexity, such that an implicational hierarchy can be formulated. It is demonstrated that item complexity (judged by phonemic weight) does gradually increase from x to r, with only time (t) deviating from this pattern. However, a correlation with system complexity is confirmed only for the extremes of the proposed hierarchy, and cognitive complexity is shown to have only limited impact upon signal complexity, i.e. morphosyntactic relatedness. Some suggestions are put forward to explain the imperfect corroboration of the hypothesis. © 2008 Elsevier B.V. All rights reserved.