Abstract This paper extends a previous claim that auxiliary selection in the present perfect of monadic verbs reflects a pattern grounded in lexico-aspectual properties of individual verbs though mediated through their syntax. A novel argument in support of this conclusion is made, based on the cross-contextual typology of auxiliaries, showing that the cross-linguistic distribution of auxiliaries in the present perfect cannot be understood in isolation of other constructions requiring an auxiliary. In particular, it is argued that two sets of constraints restrict the mapping from lexico-aspectual features to a particular auxiliary ( have or be ). One set restricts the mapping from lexical-aspectual features to argument structure at the lexicon-syntax interface, the other restricts the mapping from argument structure to the marked member of the auxiliary pair ( have ). The analysis succeeds in predicting exactly 15 possible languages or patterns of auxiliary selection with monadic verbs, a variety of which are discussed throughout the paper. While the fundamental syntactic distinction made by the Unaccusative Hypothesis is confirmed by the formal analysis, it is argued that the present perfect imposes restrictions of its own on the argumental status of any verb entering the construction, on a par with constructions such as passives and resultatives.