In Bosnian-Croatian-Serbian (BCS), the gentive (G) and dative/locative (DL) cases exhibit adjectival long-form allomorphy (ALFA). The genitive -"og" -"oga" and the DL -"om" -"ome" -"omu" stand in free variation, inasmuch as when one form is substituted for another the truth value of an utterance remains unchanged. Some sociolinguists (particularly Milroy & Gordon 2003) have held the view that while morphosyntactic free variation is possible it is much harder to analyze from the sociolinguistic perspective. While it is undoubtedly true that phonological variables occur much more frequently than so-called "higher-level" variables, many valuable contributions to sociolinguistics (Labov 1969, Wolfram 1969, Rickford et al. 1991, Mufwene 1998, Poplack and Tagliamonte 2001, and many others) have focused on morphosyntactic variables, and consequently have revealed important patterns of morphosyntactic variation in speech communities. Surprisingly, this sort of research has rarely been conducted on the South Slavic languages, especially BCS, which exhibits an incredible amount of morphosyntactic variation, given that Bosnian, Croatian, and Serbian have shared dialect continua for centuries and are mutually intelligible, but now stand at odds with one another due to changes in language policy stemming from the breakup of the Former Yugoslavia (Greenberg 2004). This dissertation is a fundamental rejection of the notion that is still being proffered (at least in Croatian grammars, cf. Baric 2007, Raguz 2010) that there are contextually based rules or tendencies for ALFA usage in Contemporary BCS. While previous grammars may have stated the facts accurately for their synchronic period of BCS, many contemporary grammars have essentially "piggy-backed" on these prior grammars (e.g Shirokov & Gudkov 1977, Browne & Alt 2004), which viewed these ALFAs as adding a sense of "definiteness", and subsequently have neglected the possibility of language change. Alexander (2006) is tst to propose that ALFAs are more common in Croatian than in Bosnian or Serbian. My sociolinguistic questionnaire and analysis of the Croatian Language Repository, the Croatian National Corpus, and Wikipedia have all verified that ALFAs are more common in Croatian, although they can be heard rarely in Serbian (only for possessives and demonstratives), and slightly more frequently in Bosnian. Furthermore, the empirical studies conducted in this investigation have informed the diachronic origin of ALFAs. It can certainly be said that ALFAs occur infrequently with descriptive adjectives, but exhibit strong patterns of variation for possessives and demonstratives. Moreover, for monosyllabic possessives and demonstratives (which are still monosyllabic after the addition of G and DL base forms -"og" and -"om"), when a preposition precedes the adjectival phrase the ALFAs are overwhelmingly preferred. I link this tendency to pronominal prepositional phrases which require long forms, e.g. "o meni", but *"o mi". This can be viewed as a "two-syllable" (or "two-footed") constraint, which then spread to all prepositional phrases in the language via analogy. This "two-syllable" rule can be viewed as a restatement of "Wackernagel's position", which can be either prosodic or syntactic (Serbian vs. Croatian, for example), in syllabic terms, although it is possible that this process could have begun on the moric level o v as , but *"o vas". In the absence of a noun this element either had already took on, or developed an anaphoric relationship to a following noun, and thus was reinterpreted as a marker of definiteness. Subsequent analogies spread ALFAs to the descriptive, although the "two-syllable" constraint would never have been violated for descriptive, given that all descriptive roots after the addition of ALFA bases would have at minimum two syllables already. Finally, the lack of ALFAs in Western Stokavian (one of the historical predecessors of modern-day Croatian) pre-19th century and their early attestation in 15th century Eastern Stokavian (the historical predecessor of modern-day Serbian) points to a Serbian import of ALFAs to Croatian lands. Ivic (1972) has demonstrated that the political and economic repercussions of the Ottoman occupation of Serbia led many Serbs to seek asylum with their Christian neighbors. As repeated migrations of Serbs brought to Croatian dialects characteristic Serbian features, such as: the neo-Stokavian stress retraction, the da complementizer, etc., the linguistic conformance behind the Vienna Literary Agreement of 1850 would have viewed the presence of these features as "unifying" elements. Therefore, the statement made by Greenberg (2004) that the ALFAs were reintroduced as "puristic" elements in Croatian post-1991 can be considered an ironic twist in Croatian language policy, inasmuch as, from the diachronic perspective, ALFAs have been shown to be characteristic of 15th century Serbian, and then later Croatian. [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://www.proquest.com/en-US/products/dissertations/individuals.shtml.]