Recent studies have proposed that the Auxiliary Selection Hierarchy (ASH) captures how systems of perfect auxiliary selection develop diachronically. The central claim of the ASH is that the selection of either BE or HAVE as a perfect auxiliary is driven by verb meaning. Moreover, it predicts certain patterns of language change. In the change of the Spanish auxiliary selection system from "ser" to "aver", verbs denoting states were the first to switch from the selection of "ser" to "aver". However, certain verbs denoting change of state, and especially motion, stopped selecting ser as auxiliary only in the seventeenth century. While the ASH makes the right predictions, it fails to explain why exactly these effects are observed.

My study proposes that in addition to the meaning of lexical verbs, the notion of constructional meaning is important. Grammaticalisation studies have established a common pattern of the development of perfect constructions: starting from a resultative meaning to denoting actions reaching into the present to a semantics of present relevance. Crucially, in Spanish only the "aver"-perfect has undergone this process. This implies that the frequent collocation of (telic) change-of-state verbs with the "ser"-perfect needs to be explained by the resultative semantics of that construction. By contrast, the "aver"-perfect experienced a semantic diversification favouring its use with verbs of diverse semantic classes.

Moreover, this study postulates an influence of entrenchment processes, and thus, frequency effects, on the development of the auxiliary system in Spanish. Certain verbs, which chose "ser" as perfect auxiliary until the seventeenth century, display a high discourse frequency. Assuming entrenchment effects allows for explanations of why, within the semantic verb classes, some verbs stopped selecting ser later than others. The study therefore takes a data-driven, quantitative approach to determine the complex interplay between the influence of verb meaning, constructional meaning, and entrenchment effects on the history of auxiliary selection in Spanish.