Many Mayan languages make use of a special dependent verb form (the Agent Focus, or AF verb form), which alternates with the normal transitive verb form (the synthetic verb form) of main clauses when the subject of a transitive verb is focused, questioned or relativized. It has been a centerpiece of research in Mayan morphosyntax over the last forty years, due to its puzzling formal and distributional properties. In this dissertation I show how a usage-oriented approach to the phenomenon can provide important insights into this area of grammar which resists any categorical explanation. I use Yucatec Maya as the lens through which to examine the phenomenon. I first show that the AF alternation is a subtype of resumptive pronoun (RP)/gap alternation. This sets the AF alternation in typological context, providing a basis for analyzing the variation, both within and across Mayan languages, as well as from a cross-linguistic perspective. The difference between Yucatec, and other types of RP/gap alternating languages lies in the locus of the alternation. Rather than involve the presence or absence of an independent pronoun, the alternation in Yucatec relates to two verb forms: one which carries a morphologically dependent subject pronoun (the synthetic verb), the other (the AF verb) which does not. I extend this analysis to a range of head-marking languages, where similar phenomena have been documented. Just like Mayan languages, such languages make use of special verb forms in dependent clauses in A-bar dependencies (wh-questions, relative clauses and clefts), which are distinguished (minimally) from their main clause counterparts by the absence of person marking inflection. And just as in Mayan, the distributions of the verbal alternants are conditioned by the same factors that are implicated in RP/gap distributions cross linguistically, providing support for the notion that they are all exemplars of the same basic phenomenon. I propose that the historical origins of these special verb forms can be traced to the emergence of head marking. Drawing on a wide range of cross-linguistic and historical data, I argue that the special verbs that occur in A-bar dependencies are byproducts of the frequency-sensitive gramaticalization process by which independent pronouns become pronominal inflection on verbs. I show that the relatively low frequency of adjacent pronoun-verb combinations in extraction contexts (where gaps are more frequent than resumptive pronouns) can give rise to asymmetric patterns of pronoun grammaticalization, and thus lead to the emergence of these morphological alternations. The asymmetric frequency distributions of gaps and RPs (within and across languages) in turn can be explained by processing preferences. I present three experiments which show that Yucatec speakers are more likely to use the resumptive verb form in embedded environments, and where the antecedent is indefinite. Specifically, these studies indicate the need to bring discourse-level processing principles into the account of what have often been taken to be autonomously sentence-internal phenomena: factors such as distance and the referential salience of the antecedent have been shown to influence referential form choice in discourse, suggesting that the same cognitive principles lie behind both types of variation. More generally, the Yucatec studies demonstrate that production preferences in Yucatec relative clauses reflect patterns of RP/gap distributions that have been attested "across grammars". The Highest Subject Restriction (the ban on subject RPs in local dependencies), which is apparently a categorical constraint in many languages, is reflected probabilistically in Yucatec in terms of production preferences. The definiteness restriction (RPs are obligatory with indefinite antecedents), which has been reported categorically in other languages, is also visible probabilistically in Yucatec production. This lends some statistically robust support to the view that that typological patterns can arise via the conventionalization of processing preferences. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]