A set of formalisms is developed, based on the marker hypothesis, ie, that natural languages are marked for complex syntactic structure at surface form. A comparison of the expressivity & restrictedness of these formalisms shows that (1) not all constraints are restrictive & (2) the marker hypothesis & its implicit function/content word distinction provide strong restrictions on the form of allowable grammars. These restrictions may in turn provide evidence about its actual psychological reality & salience. In particular, the class of strongly marked languages can be demonstrated not to admit all finite languages & thus not be subject to the hangman's noose of E. M. Gold's (1967) learnability proofs. It is conjectured that these languages may provide a computable method of inferring human-like languages. 33 References. Adapted from the source document