The incorporation of empirical and theoretical knowledge from the language acquisition literature into formal diachronic syntax has been largely exiguous despite the growth of the latter as a subfield in formal linguistics over the past thirty years. In this dissertation, it is shown that a greater knowledge of diachronic phenomena, particularly those that are not cross-linguistically attested, may be achieved upon combining what is known about the inner workings of the language acquisition device (LAD) with surfacing studies that show and hypothesize about different deviant structures in child language. In this work, several individual syntactic phenomena, both synchronic and diachronic, are addressed and united in order to explain what is referred to here as the Galician complementizer agreement phenomenon (Galician C-AGR). By reviewing the limited historical data available within the purview of empirical data on cliticization in Western Iberian children and theories regarding cliticization patterns in this language family, it is shown that several of the proposed steps can be accounted for if one assumes that endogenous grammar change seen in child language data is one of the sources of diachronic change. [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.]