This paper analyses the alternation between complement clauses with and without complementizer (syndetic and asyndetic), in historical Spanish (15th–18th century). While previous studies have shown that this syntactic alternation was regulated by the degree of integration of the clauses, its stylistic distribution is understudied. In this paper we investigate whether the syndetic/asyndetic alternation is governed by socio-stylistic factors (discourse traditions, audience and speaker design). The analysis of the data from the corpus CODEA+2015, is carried out by using regression models. Our results indicate that the selection of asyndetic complements was predicted by the type of audience (reactive style-shift), employed especially when addressing superiors. Also, asyndetic complements were favored by the use of deferential request verbs such as suplicar (‘to beg’) that represents the petitioner as an inferior (proactive style-shift). Furthermore, the stylistic choice for an asyndetic complement was determined at the sentential level (“micro” discourse traditions), when the writer is also the performer of the speech-act. Thus, we show that in choosing alternative variants, writers are concerned with expressing grammatical meanings but also social identity. Additionally, the diachronic analysis indicates that while the proactive style-shift effect grows stronger over time, the relevance of the reactive style-shift declines, showing that socio-stylistic predictors, similarly to linguistic predictors, can be affected diachronic fluctuations. Our paper evinces the relevance of a multidimensional and diachronic approach to the study of syntactic variation, demonstrating how a monolithic view of the textual dimension can hide fine-grained socio-stylistic effects.